Method for providing a labeled single isomeric chemical entity targeting vector based on the use of a symmetrical diene FIELD The present invention relates to a method for providing a labeled single isomeric chemical entity targeting vectors, the targeting vectors obtained and the uses of the targeting vectors. The labeled single isomeric chemical entity targeting vectors can be used in therapy such as radiotherapy, diagnostics, imaging, and other photochemistry methods. BACKGROUND Labeled targeting vectors based on click chemistry between dienes and dienophiles are used both for imaging purposes such as diagnostics and other photochemistry imaging methods and in therapy. Such targeting vectors have for instance been labeled with radiolabels than can be applied in diagnostics and/or in therapy. The specific use depends on the identity of the radiolabeling used because different radionuclides provide for different purposes. The specific use moreover depends on the specific target that the vector is directed at. Several combinations of radiolabels and vectors are applied presently in diagnosis, therapy, theranostic and imaging. Different chemical entities connecting the radiolabeled entity with the target directed entity exists, the present invention is based on click-chemistry wherein a diene and a corresponding dienophile is ligated thereby bridging the radiolabel and the target directed entity. The term “click chemistry” refers to a class of reactions that are fast, simple to use, versatile, chemoselective, and give high product yields. These reactions have found application in a wide variety of research areas, including materials science, polymer chemistry, and pharmaceutical sciences. Radiochemistry is one of the fields that showed the true potential of click chemistries as for example disclosed in Zeng et al, Journal of Nuclear Medicine, 54, 829-832, 2013. Essentially, the selectivity, ease, rapidity, and modularity of click ligations make them nearly ideally suited for the construction of radiotracers, a process that usually involves working with biomolecules in aqueous conditions with fast decaying radioisotopes. Among the different click chemistries, one of the most suited and utilized for radiolabeling is the tetrazine ligation. The tetrazine ligation is a click reaction which is characterized by the formation of covalent bonds between a 1,2,4,5-tetrazines (Tz) and typically a trans-cyclooctene (TCO). The reaction is initiated by an inverse electron-demand Diels-Alder reaction, followed by a retro-Diels-Alder reaction, driven by the expulsion of N ₂ . The tetrazine ligation is among the fastest known chemical ligations, with second order rate constants up to 10 ⁶ M ^-1 s ^-1 in acetonitrile at 25 ^o C. This stands in contrast to other known click reactions, such as the Staudinger ligation (0.003 M ^-1 s ^-1 ) and strain- promoted azide-alkyne cycloaddition (SPAAC) (0.1 M ^-1 s ^-1 ). For this reason, and because of its specificity, the Tz ligation is ideally suited for synthon-based labeling. One major drawback of the conventional ligation of a Tz with a TCO is that the ligation takes place without any regio-specificity. Moreover, the conventional ligation of a Tz with a TCO gives rise to several tautomeric forms. This means that conventional ligation of a Tz with a TCO typically results in complex reaction mixtures of at least sixteen unique isomeric products (Scheme 1). Such mixtures of isomeric products cannot be applied directly for pharmaceutical purposes because the particular isomeric form of the compound influences the pharmacokinetics of the therapeutic agent as and any potential toxicological effects of the individual isomeric forms cannot be distinguished. While the resulting multiple dihydropyridazines that are obtained from conventional ligation may slowly auto-oxidize, over the course of several hours - up to several days, to the corresponding pyridazines and thereby slowly reducing the number of tautomeric forms of the product (WO2012/121746), the number of regio-isomeric and stereo-isomeric products are not reduced by this slow oxidation transformation. Means for speeding up the transformation from dihydropyridines to pyridazines have been described (Karaki Fumika et al, Tetrahedron, 97, 132411, 2021; Keinänen et al., ACS Medicinal Chemistry Letters, 7, 62-66, 2015), however, in none of reported cases was the oxidation completed within two hours, which is required if the pyridazines are to be used as radiopharmaceuticals. WO2017/059397, WO2020/242948, Syvänen et al., ACS Chemical Neuroscience, 11, 4460-4468, 2020, and WO2012/121746 discloses ligations between tetrazines and TCO’s, which will inevitably provide several isomeric chemical entities Alternatively, pyridazines can be prepared via the ligation of a Tz to a strained cyclic alkyne, however this reactions suffers from slow second order rate constants. Radiopharmaceuticals are increasingly used in theranostic, especially within oncology, both for diagnostic imaging and for targeted radionuclide therapy. Positron emission tomography (PET) is the gold standard in nuclear imaging with better resolution and quantification than other modalities.2,200,800 clinical PET scans were performed in 2019 in the US alone. Targeted radionuclide therapy is more effective at treating cancer than many state-of-the-art chemotherapies. It also has the advantage over external beam radiotherapy (e.g. “gamma knife”) in that it offers a way to confine the delivered dose to the tumor and its immediate surrounding area, which makes particular sense in the radiotherapy of micrometastatic disease. The combination of both diagnostic imaging and targeted radiotherapy can be used in “theranostics”, a concept with powerful application in personalized medicine, with respect to patient selection, dose-finding and therapy response monitoring. A theranostic pair is two radionuclides, which can be substituted with each other, without changing the pharmacokinetics of the radiopharmaceutical, but shifting their application between diagnostic imaging and radionuclide therapy. The two most widely used Diagnostic Imaging methods are the nuclear based PET and SPECT. Both methods rely on the combination of radionuclides with vectors that specifically target cancer cells. In imaging, such radiolabeled vectors are referred to as “radiotracers”. Radiotracers are accumulated in tumor lesions, the location of which can then be visualized by detecting the emitted radiation. PET is strongly favored in oncology, while SPECT is dominant in cardiology and for producing bone scans and certain other specialized organ scans. Globally, the ratio of SPECT cameras to PET cameras used in hospitals is circa 5:1. Single-photon emission computed tomography (SPECT) is the older of the two methods. SPECT imaging employs radionuclides emitting gamma photons, typically in the 100-200 keV range. A series of 2D projection images of radiotracer distribution in the body are acquired by one of more gamma cameras from multiple angles. These projection images are then assembled to produce a 3D image. Positron emission tomography (PET) is currently considered the most advanced form of nuclear imaging. The key use of PET in oncology is diagnosis and treatment monitoring, especially of metastatic cancer. Compared to previous modalities, notably SPECT, PET offers improved resolution and sensitivity, and generally higher quality images. These properties are especially relevant in the detection, and subsequent treatment, of very small metastases. New innovations, notably total-body PET dramatically increase sensitivity and detects more metastases and are expected to further strengthen the advantageous position of PET in the modern clinic. PET relies on the use of radionuclides that emit positrons upon their decay. These positrons travel a limited distance, and then undergo annihilation with an electron in the surrounding medium. This produces two annihilation photons, each of 511 keV, which are emitted in opposite directions. These photons can be detected by a PET scanner. The most optimal radionuclide for PET is fluorine-18 ( ¹⁸ F). With a decay half-life of 110 minutes and 97% positrons emitted per decay, ¹⁸ F is close to ideal for clinical PET applications. This holds true especially for small molecular and peptide-based radiopharmaceuticals, which represent the vast majority of relevant PET tracers. Of equal importance, ¹⁸ F can be practically produced in enormous quantities (>300 doses per production) on standard biomedical cyclotrons, which are readily available throughout most of the world, with more than 200 present in Europe alone. Accordingly, ¹⁸ F does not share the concerns for sufficient supply associated with its closest competitor, the generator-produced radiometal gallium-68 ( ⁶⁸ Ga). In addition, the lower positron energy of ¹⁸ F provides higher resolution images. Compared to the current diagnostic radionuclide of most widespread use, the SPECT radionuclide technetium-99m, ¹⁸ F offers the highest quality images through its status as a PET radionuclide. Accordingly, ¹⁸ F is poised as the key diagnostic radionuclide of the future. Radioactive iodine is widely used in SPECT imaging. Traditionally, iodine-131 ( ¹³¹ I) was used, but nowadays, clinical SPECT scans are typically done with iodine-123 ( ¹²³ I). This is due to the shorter half-life of ¹²³ I (T½ = 13.2 hours) which together with its lack of beta minus emission offers a more favorable radiotoxicity profile than ¹³¹ I. Its gamma photon energy of 159 keV is excellently suited for clinical SPECT imaging. Due to the intrinsic accumulation of iodine in the thyroid, ¹²³ I in its free form is widely used for imaging thyroid disease. As a component of SPECT radiotracers, ¹²³ I is for example used in the imaging agents MIBG (oncology) and ioflupane (CNS). ¹²³ I forms a theranostic pair with the clinically used beta minus emitting therapeutic radionuclide ¹³¹ I and the investigational Auger electron radiotherapeutic ¹²⁵ I. Iodine-123 is produced in a cyclotron by proton irradiation of xenon in a capsule and is commercially available. Iodine-124 ( ¹²⁴ I) can be used for PET imaging. It is usually produced in a cyclotron by bombardment of enriched tellurium-124. However, the imaging characteristics of ¹²⁴ I are not ideal. It has a complex decay scheme with many high energy γ-emissions. Only 23% of its decay leads to positron emissions. There are, however, challenges to introduce above mentioned radionuclides into molecules which limits the practical use of radionuclide-based therapy, diagnostic, and imaging. Most of the PET, SPECT and therapeutic radionuclides mentioned above are radiometals. Conventional method of introducing radiometals into the vector molecules is the use of chelator groups that form coordinate bonds with the radiometal atom. The radiolabeling procedure typically involves mixing the radiolabeling precursor (vector with a chelator group attached) with radiometal ions and heating the mixture to allow the chelation reaction to proceed. Although chelation of radiometals is conceptually simple, it has a number of drawbacks, namely: - the radiolabeled product often cannot be separated from the unlabeled precursor, because the difference in physico-chemical properties is not significant; - chelation reaction is sensitive to trace metal impurities in solutions used for the radiolabeling, which makes upscaling problematic; - heating, which is necessary to overcome the activation barrier of the chelation reaction, may degrade temperature-sensitive vectors. Compared to its radiometal alternatives ¹⁸ F is a halogen and requires covalent bonding to targeting vectors. This stands in the contrast to the chelator-based labelling techniques utilized for radiometals. Covalent bonds are currently typically formed via direct nucleophilic displacement of a leaving group, such as triflate. The conditions for such chemistry are harsh, lengthy and poorly scalable, and therefore incompatible with many vectors, notably the peptide class, which is growing in importance. Small molecular radiopharmaceuticals containing radioiodine are typically prepared using either electrophilic destannylation or iodine-iodine exchange radiochemistry. The former is a mild, versatile and practical reaction, in which radioactive iodide is oxidized to a positively charged iodine species, which then replaces a leaving group, typically stannyl, in an aromatic substitution reaction. This reaction occurs at room temperature in often quantitative yield. Iodine-iodine isotopic exchange is used when high molar activity is not a concern and when substrates can withstand harsh conditions. The exchange occurs at elevated temperature with acid and copper as catalysts. ¹⁸ F has long established itself as the best-in-class radionuclide for diagnostic PET imaging, while iodine radioisotopes ¹²³ I, ¹²⁴ I, ¹²⁵ I and ¹³¹ I are useful for SPECT imaging, PET imaging, Auger therapy and beta-therapy, respectively. Moreover, all three elements can be introduced into aryl rings forming fluoro/iodo/astatoaryl moieties with maximum structural similarity, which is essential for theranostic pairs. Nevertheless, there are no reports of combined use of ¹⁸ F with radioiodine ¹²³ I, ¹²⁴ I, ¹²⁵ I, ¹³¹ I, for theranostic applications. One obstacle to the development of ¹⁸ F/ ¹²³ I, ¹²⁴ I, ¹²⁵ I, ¹³¹ I, theranostic pairs is the harsh labeling chemistry, as described above, which prevents direct regioselective labeling of biomolecular targeting vectors with these radionuclides. Another obstacle is that not all targeting vectors contain aryl moieties, so these must be introduced as prosthetic groups, also known as synthons. Due to these challenges, synthon-based methods have been investigated for the preparation of radiopharmaceuticals. These methods involve the direct labeling of a separate intermediate compound (“synthon”), which is subsequently conjugated to the vector under mild conditions. Thus, the vector is not subjected to the harsh conditions of direct radiolabeling, although it does need to be modified by a chemical tag complementary to the radiolabeled synthon. What becomes crucial then, is the chemistry used to conjugate the synthon with the vector. This chemistry must be specific, compatible with pharmaceutically relevant aqueous media and high yielding, to avoid loss of radioactivity and minimize the need for subsequent purification. Critically, it must also be extremely fast, as procedures for preparing radiopharmaceuticals occur in nano- to micromolar concentrations and must proceed to completion or near completion within minutes, due to radionuclide decay. In light of that, click-chemistry has emerged as a strategic approach to radiolabel an array of targeting vectors such as mAbs, nanomedicines, peptides or small molecules. Targeted radionuclide therapy (TRT) can be based on beta-emitters, Auger electron emitters and alpha-emitters. Beta-particle emitting radionuclides (such as ⁹⁰ Y, ¹⁷⁷ Lu, ¹³¹ I) decay via the emission of high-energy electrons (beta particles) which travel distances in the tissue of up to about 12 mm. The decay energy is deposited within the largest volume of tissue of the three therapy types mentioned here. Beta-emitters are thus suitable for the treatment of medium-sized tumors, where most of the dose will be absorbed by the cancer cells. For micrometastases or heterogeneous tumors however, even with perfect concentration of the radionuclide in and around the tumor, a large fraction of the irradiation dose is absorbed by surrounding healthy cells. Therefore, beta-emitters are not optimal for the treatment of micrometastases or heterogeneous tumors. This is an important drawback of beta-emitters, because micrometastases are one of the major causes of cancer recurrence and cancer mortality. Alpha-emitters (such as ²¹² Pb) decay with the emission of alpha particles. Alpha- particles are much heavier than beta-particles, and their tracks are straight and short - on the order of 30-100 µm, in the order of the diameter of a handful of mammalian cells. Thus, all energy from a decay is delivered to just a few neighboring cells. Alpha- radiation possesses greater cytotoxicity, compared to beta-radiation and can be delivered to micro metastases in a highly focused manner. In a telling example of the advantages of alpha therapy, a patient with metastatic prostate cancer resistant to the flagship beta-therapy agent ¹⁷⁷ Lu-PSMA achieved complete remission after three cycles of the alpha-therapy agent ²²⁵ Ac-PSMA. Auger electron radiotherapy (AeRT) employs radionuclides that upon decay by electron capture (EC) or internal conversion (IC) emit a shower of extremely short ranged electrons. With advanced drug delivery technology, these specialized radionuclides can be delivered to the nuclei of cancer cells. Here, the emitted Auger electrons destroy the DNA and kill the cancer cells. Notably, the short range of the Auger electrons ensures that their energy is deposited mainly within the targeted cell, allowing for extremely localized therapy. Both ¹²³ I and ¹²⁵ I have high Auger electron yields and are suitable for AeRT. Iodine-131 ( ¹³¹ I) is a beta particle emitter that is widely used in clinical radionuclide therapy. It has a decay half-life of 8.0 days and a main emission of beta particles with a maximal energy of 606 keV at 90% abundance. These beta-particles have a maximum range in tissue of about 2 mm, enabling ¹³¹ I to treat small to medium sized tumor lesions. It is widely used for thyroid ablation due to its intrinsic accumulation in thyroid tissue. In addition, a therapeutic variant of MIBG is available, radiolabeled with ¹³¹ I, and ¹³¹ I is used in radioimmunotherapy. It forms theranostic pairs with ¹²³ I (SPECT) and ¹²⁴ I (PET). Both iodine-123 and iodine-125 have substantial emission of Auger electrons, about 10 and 20 electrons, respectively. This makes them suitable for Auger electron radiotherapy, a currently investigational form of radionuclide therapy. However, in order for click-chemistry such as the Tz-TCO ligation to furnish end- products that are viable in a regulatory and commercial context, it is critical that only a single ligation product is produced. This is not possible using available methods because the ligation conventionally yields a large number of isomeric products that are impossible to separate, barring it from clinical translation due to toxicity concerns and unpredictable pharmacokinetics. The present invention provides a method wherein certain combinations of chemical entities with complementary inverse electron demand Diels-Alder cycloaddition reactivity, which upon ligation, followed by a rapid oxidation, will form a single compound. This means that only one isomeric product is obtained and accordingly no separation of isomeric products is required. The method advantageously enables radiolabeling, for example with ¹⁸ F, ¹²³ I, ¹²⁴ I, ¹²⁵ I or ¹³¹ I, of any tracer in unmatched efficiency and practicality. SUMMARY The present invention provides a method for providing labeled single isomeric chemical entity targeting vectors. The method applies click chemistry wherein one chemical entity which is conjugated to a label is clicked together with a second chemical entity with complementary inverse electron demand Diels-Alder cycloaddition reactivity which is conjugated to a targeting vector followed by a rapid oxidation, to form a single isomeric compound. The advantage of the method is that one single isomeric end-product, within a minimum period of time will be provided, and thereby easing clinical translation and production costs. The method for providing a labeled single isomeric chemical entity targeting vector comprises the following steps: a) labeling a first chemical entity having inverse electron demand Diels- Alder cycloaddition reactivity and being conjugated to a pharmaceutic agent, an imaging agent, or a therapeutic agent, with a labeling agent; wherein the first chemical entity is selected from the group consisting of a symmetrical substituted diene wherein at least one of the symmetry planes pass through the nitrogen-nitrogen bonds of at least one tetrazine ring, an enantiomerically pure dienophile, or a cis,5,6-disubstituted cis,5,6-disubstituted dienophile; and b) ligating the labeled first chemical entity obtained in step a) with a second chemical entity having complementary inverse electron demand Diels-Alder cycloaddition reactivity and being conjugated to a targeting vector; wherein the second chemical entity is selected from the group consisting of a symmetrical substituted diene wherein at least one of the symmetry planes pass through the nitrogen-nitrogen bonds of at least one tetrazine ring, an enantiomerically pure dienophile, or a cis,5,6-disubstituted cis,5,6-disubstituted dienophile; wherein the reaction kinetics for the inverse electron demand Diels-Alder cycloaddition between the first and second chemical entities has a minimum second order rate constant of 500 M ^-1 s ^-1 in phosphate-buffered saline at 25 °C, as determined by stopped-flow spectrophotometry, and wherein the first and second chemical entities having complementary inverse electron demand Diels-Alder cycloaddition reactivity being ligated are selected from one of the following combinations: i) a symmetrical substituted diene wherein at least one of the symmetry planes pass through the nitrogen-nitrogen bonds of at least one tetrazine ring and an enantiomerically pure dienophile ii) a symmetrical substituted diene wherein at least one of the symmetry planes pass through the nitrogen-nitrogen bonds of at least one tetrazine ring and a cis,5,6-disubstituted dienophile; c) oxidizing the ligated labeled targeting vector obtained from step b) at a temperature ranging from 15 °C to 50 °C for up to 60 minutes by adding from 1 to 100 equivalents of chloranil, fluoranil, DDQ or NaNO ₂ . This labeling agent can be any agent that is useful as a marker, an imaging agent, a therapeutic agent or a theranostic agent and includes radionuclides and fluorescent entities. The targeting vector can be any suitable vector directed at a specific target and includes antibodies, nanobodies, polymers, nanomedicines, cells, proteins, peptides, and small molecules. Several examples of symmetrical substituted dienes, enantiomerically pure dienophiles and cis,5,6-disubstituted dienophiles suitable for this method is disclosed herein. Suitable dienes include for example tetrazines. Suitable dienophiles include for example trans-cycloheptenes (TCH’s), trans-cyclooctenes (TCO’s) and trans- cyclononenes (TCN’s). The method of the present invention also include an embodiment wherein the first chemical entity and/or the second chemical entity is obtained from specific pre- cursors. These precursors include precursors for obtaining symmetrical substituted dienes, precursors for obtaining enantiomerically pure dienophiles, and precursors for obtaining cis,5,6-disubstituted dienophiles, respectively. The single isomeric chemical entity targeting vectors provided by the method of the present invention are particularly suitable for use in theranostic, therapy, radiotherapy, diagnostic and imaging. BRIEF DESCRIPTION OF DRAWINGS Figure 1: Scheme showing the synthesis of symmetrical tetrazines. Figure 2: Scheme showing an alternative synthesis of symmetrical tetrazines. Figure 3: X-ray crystal structure of (S,Z)-cyclooct-4-en-1-yl (1R,4S)-4,7,7-trimethyl- 3-oxo-2-oxabicyclo[2.2.1]heptane-1-carboxylate. Figure 4: Radio-HPLC of [ ¹⁸ F]I at end of deprotection. Figure 5: Scheme of click reaction performed with 20 and table with data from click reaction performed with 20. Figure 6 a and b. HPLC-MS analysis after oxidation of the tetrazine-TCO pyridazine tested in Example 9. Figure 7: Structures of vectors tested in Example 9. Figure 8: Table showing the results of the oxidation of vectors from Example 9. DETAILED DESCRIPTION OF THE INVENTION The present invention provides in a first aspect a method for providing a labeled single isomeric chemical entity targeting vector. The method applies specific combinations between a diene and a dienophile with complementary inverse electron demand Diels-Alder cycloaddition reactivity, which upon ligation, followed by oxidation, will form compounds of a single isomeric form. Either the diene or the dienophile is conjugated to an agent of interest such as a pharmaceutic agent, an imaging agent, or a therapeutic agent and labeled with a labeling agent. The compatible diene or dienophile, respectively, is conjugated to a targeting vector of interest. The ligation between the diene and the dienophile is based in inverse electron demand Diels-Alder cycloaddition reactivity, and accordingly, the diene and the dienophile to be ligated must have complementary inverse electron demand Diels- Alder cycloaddition reactivity. Moreover, the ligation between the diene and the dienophile should have reaction kinetics with a minimum rate constant of 500 M ^-1 s ^-1 in PBS at 25 °C, determined by stopped-flow spectrophotometry, in order to be of relevance to the present method. Ligations with rate constants below 500 M ^-1 s ^-1 in phosphate-buffered saline (PBS) at 25 °C, determined by stopped-flow spectrophotometry, will take too long time to provide the labeled targeting vectors because the radioactive labeling agent often have a limited period of time for use on imaging and/or therapy after the ligation step. Second order rate constant can be measured by different means, but is typically measured by stopped-flow spectrophotometry as for example described in (Chance, Rev. Sci. Instrum.1951, 22, 619– 627). Herein, the method described in Battisti et al. J. Med. Chem.2021, 64, 20, 15297–15312 was applied. In order to provide a labeled chemical entity targeting vector that will result in only a single isomeric form when ligating the diene and the dienophile with complementary Diels-Alder cycloaddition reactivity, a combination of two requirements is necessary. The first requirement relates to selecting the structures of the diene and of the dienophile to be ligated. In order to prevent tautomer forms of the targeted vector composition a structural symmetrical substituted diene wherein at least one of the symmetry planes pass through the nitrogen-nitrogen bonds of at least one tetrazine ring should be selected (hereinafter simply referred to as a “symmetrical” substituted diene or in some embodiments “symmetrical” tetrazine). It has been found here that two ligation options are available for combining the symmetrical substituted diene and the dienophile in order to provide a combined (i.e., clicked) product that upon oxidation will provide a targeting vector in just one isomeric form: combination option i) ligation between a symmetrical substituted diene and an enantiomerically pure dienophile; combination option ii) ligation between a symmetrical substituted diene and a cis,5,6- disubstituted dienophile. The second requirement relates to the oxidation step. It is shown herein that not all oxidants are suitable for this method either because the oxidation is not efficient enough when working within the required time frame available or give side products or because the oxidation impacts on the structure of the targeting vector, thereby potentially preventing the labeled targeting vector from binding to its target. The oxidation efficiency of the present oxidation step is at least 90% i.e., at least 90% of the labeled and clicked targeting vector should be in a single isomeric form after the oxidation step. Oxidation conditions providing less than 90% of the product being in a single isomeric form, will not be of sufficient purity for use in therapy/imaging/diagnosis and it will require additional toxicological studies. It is shown herein that a suitable oxidation is performed at a temperature ranging from 15 °C to 50 °C for up to 60 minutes by adding from 1 to 100 equivalents of an oxidant selected from the group comprising chloranil, fluoranil, DDQ and NaNO ₂ . Accordingly, the method for providing a labeled single isomeric chemical entity targeting vector comprises: a) labeling a first chemical entity having inverse electron demand Diels- Alder cycloaddition reactivity and being conjugated to a pharmaceutic agent, an imaging agent, or a therapeutic agent, with a labeling agent; wherein the first chemical entity is selected from the group consisting of a symmetrical substituted diene wherein at least one of the symmetry planes pass through the nitrogen-nitrogen bonds of at least one tetrazine ring, an enantiomerically pure dienophile, or a cis,5,6-disubstituted dienophile; and b) ligating the labeled first chemical entity obtained in step a) with a second chemical entity having complementary inverse electron demand Diels-Alder cycloaddition reactivity and being conjugated to a targeting vector; wherein the second chemical entity is selected from the group consisting of a symmetrical substituted diene wherein at least one of the symmetry planes pass through the nitrogen-nitrogen bonds of at least one tetrazine ring, an enantiomerically pure dienophile, or a cis,5,6-disubstituted dienophile, wherein the reaction kinetics for the inverse electron demand Diels-Alder cycloaddition between the first and second chemical entities has a minimum rate constant of 500 M ^-1 s ^-1 in PBS at 25 °C, determined by stopped-flow spectrophotometry, and wherein the first and second chemical entities having complementary inverse electron demand Diels-Alder cycloaddition reactivity being ligated are selected from one of the following combinations: i) a symmetrical substituted diene and an enantiomerically pure dienophile ii) a symmetrical substituted diene and a cis,5,6-disubstituted dienophile; c) oxidizing the ligated labeled targeting vector obtained from step b) at a temperature ranging from 15 °C to 50 °C for up to 60 minutes by adding from 1 to 100 equivalents of a quinone oxidant, such as chloranil, fluoranil, DDQ or NaNO ₂ . The pharmaceutic agent, imaging agent or therapeutic agent that the first chemical entity is conjugated to is in some embodiments identical with the labeling agent. This may for instance be the case when the labeling agent is an agent that can be applied both as a label and as a therapeutic or imaging agent. In some embodiments, the labeling agent is a radionuclide. Some radionuclides can be applied both in imaging, in diagnostics and/or in therapy and in the present examples, the same radionuclide have been applied as labeling agent as well as imaging or therapeutic agent. Labeling of a diene or dienophile with a radionuclide will normally not provide 100% labeling efficiency with the radionuclide, some of the products labeled will inevitably be labeled with a stable isotope of the corresponding radionuclide element. When using a symmetrical substituted diene as a starting point as the entity to be radiolabeled, it will only be possible to provide a radiolabeled single isomeric chemical entity targeting vector if the radiolabel exists in both a radioactive and in a stable form because the symmetrical substituted diene will comprise two targets for the radionuclide/the stable isotope and labeling with radionuclide that does not exist in a stable form may include products wherein only one of the targets are labeled which will not provide a structural symmetric diene. In contrast, the diene will still be structurally symmetrical if labeled with one radionuclide and one stable form of the same isotope. Some of the radionuclides that are of interest in therapy and imaging are, however, not obtainable in a stable form. Thus, no corresponding element can label the symmetric position of the labeling target and this would inevitably result in more than one isomeric form of the final product. Accordingly, no symmetric diene can be obtained if the labeling agent is ²¹¹ At, ²²³ Ra or ²²⁵ Ac. The method enables labeling such as radiolabeling, for example with ¹⁸ F, radioiodine ( ¹²³ I, ¹²⁴ I, ¹²⁵ I or ¹³¹ I) and many other labels, of any targeting vector in unmatched efficiency and practicality. The ground-breaking nature of the method is the possibility of forming a single end-product, within 60, often within much less than 60 minutes such as within 1-20 minutes, and thereby easing clinical translation. In contrast, conventional tetrazine ligations result in multiple products and as a result need massive and unmanageable toxicological packages or tedious and time-consuming separation. DEFINITIONS A symmetrical tetrazine means, in the context of the present invention, any tetrazine that as a “cold” reference or after radioactive labeling/deprotection shows one or more symmetry planes in the chemical structure. One of the symmetry planes pass through the nitrogen-nitrogen bonds of the tetrazine ring(s). A cold reference means, in the context of the present invention, a compound that is labeled with a non-radioactive isotope of an atom, where a radioactive isotope of the same atom is required in order to provide a radiolabeled version of the same compound. The term cold reference moreover includes, in the context of the present invention a compound that comprises one or more protective group(s) that will be replaced by the labeling agent upon labeling. Enantiomerically pure dienophile or enantiomerically pure TCH/TCO/TCN means, in the context of the present invention, any dienophile or any TCH, TCO and TCN, respectively, that has been isolated/synthesized as a single stereoisomer. Cis,5,6-disubstituted-dienophile or cis,5,6-disubstituted-TCO means, in the context of the present invention, any dienophile or any TCO, that after click and oxidation forms a meso compound. TCH means, in the context of the present invention, any 7-membered ring with at least one double bond in trans-configuration able to react as a dienophile in an inverse electron demand Diels-Alder cycloaddition. TCO means, in the context of the present invention, any 8-membered ring with at least one double bond in a trans-configuration able to react as a dienophile in an inverse electron demand Diels-Alder cycloaddition. TCN means, in the context of the present invention, any 9-membered ring with at least one double bond in a trans-configuration able to react as a dienophile in an inverse electron demand Diels-Alder cycloaddition. It is well known within click chemistry to use dienes such as tetrazines that are clicked with dienophiles such as trans-cycloheptenes (TCH’s), trans-cyclooctene (TCO’s) or a trans-cyclononenes (TCN’s) with complementary inverse electron demand Diels- Alder cycloaddition reactivity. In a preferred embodiment, the method for providing a labeled single isomeric chemical entity targeting vector the diene is a tetrazine and the dienophile is a trans-cycloheptene (TCH), a trans-cyclooctene (TCO) or a trans- cyclononene (TCN). The labeled single isomeric chemical entity targeting vectors obtainable by the method according to the present invention, can be applied for various purposes depending on the characteristics of the agent applied as a label. Labeling agents that are suitable for the method includes radiolabels and fluorescent labels. In a preferred embodiment, the labeling agent applied in step a) in the method for providing a labeled single isomeric chemical entity targeting vector is a radionuclide or a stable isotope of a corresponding element. The characteristics and accordingly the use of the different radionuclides normally applied are well known in the art. Radionuclide labeling agents and stable isotopes of a corresponding element that are suitable for use as a labeling agent in step a) in the method for providing a labeled single isomeric chemical entity targeting vector includes: ¹ H, ² H, ³ H, ¹¹ C, ¹² C, ¹³ C, ¹⁴ C ¹³ N, ¹⁴ N, ¹⁵ N ¹⁸ F, ¹⁹ F, ¹²³ I, ¹²⁴ I, ¹²⁵ I, ¹²⁷ I, ¹³¹ I, ¹⁵ O, ¹⁶ O, ¹⁷ O, ¹⁸ O, ⁴³ Sc, ⁴⁴ Sc, ⁴⁵ Sc, ⁴⁵ Ti, ⁴⁶ Ti, ⁴⁷ Ti, ⁴⁸ Ti, ⁴⁹ Ti, ⁵⁰ Ti, ⁵⁵ Co, ⁵⁸ mCo, ⁵⁹ Co, ⁶⁰ Cu, ⁶¹ Cu, ⁶³ Cu, ⁶⁴ Cu, ⁶⁵ Cu, ⁶⁷ Cu, ⁶⁷ Ga, ⁶⁸ Ga, ⁶⁹ Ga, ⁷¹ Ga, ⁷⁶ Br, ⁷⁷ Br, ⁷⁹ Br, ⁸⁰ mBr, ⁸¹ Br, ⁷² As, ⁷⁵ As, ⁸⁶ Y, ⁸⁹ Y, ⁹⁰ Y, ⁸⁹ Zr, ⁹⁰ Zr, ⁹¹ Zr, ⁹² Zr, ⁹⁴ Zr, ¹⁴⁹ Tb, ¹⁵² Tb, ¹⁵⁹ Tb, ¹⁶¹ Tb, ¹¹¹ In, ¹¹³ In, ¹¹⁴ mIn, ¹¹⁵ mIn, ¹⁷⁵ Lu, ¹⁷⁷ Lu, ¹⁸⁵ Re, ¹⁸⁶ Re, ¹⁸⁸ Re, ²⁰¹ Tl, ²⁰³ Tl, ²⁰⁵ Tl, ²⁰⁶ Pb, ²⁰⁷ Pb, ²⁰⁸ Pb, ²¹² Pb, ²⁰⁹ Bi, ²¹² Bi, ^{2 13} Bi, ³¹ P, ³² P, ³³ P, ³² S, ³⁵ S ⁴⁵ Sc, ⁴⁷ Sc, ⁸⁴ Sr, ⁸⁶ Sr, ⁸⁷ Sr, ⁸⁸ Sr, ⁸⁹ Sr, ¹⁶⁵ Ho, ¹⁶⁶ Ho, ¹⁵⁶ Dy, ¹⁵⁸ Dy, ¹⁶⁰ Dy, ¹⁶¹ Dy, ¹⁶² Dy, ¹⁶³ Dy, ¹⁶⁴ Dy, ¹⁶⁵ Dy , ²²⁷ Th, ²³² Th, ⁵¹ Cr, ⁵² Cr, ⁵³ Cr, ⁵⁴ Cr, ⁷³ Se, ⁷⁴ Se, ⁷⁵ Se, ⁷⁶ Se, ⁷⁷ Se, ⁷⁸ Se, ⁸⁰ Se, ⁸² Se, ⁹⁴ Tc, ^99m Tc , ¹⁰³ Rh, ¹⁰³ mRh, ¹¹⁹ Sb, ¹²¹ Sb, ¹²³ Sb, ¹³⁵ La, ¹³⁸ La, ¹³⁹ La, ¹⁶² Er, ¹⁶⁴ Er, ¹⁶⁵ Er, ¹⁶⁶ Er, ¹⁶⁷ Er, ¹⁶⁸ Er, ¹⁷⁰ Er, ¹⁹³ mPt, ¹⁹⁵ mPt, ¹⁹² Pt, ¹⁹⁴ Pt, ¹⁹⁵ Pt, ¹⁹⁶ Pt, ¹⁹⁸ Pt. In a preferred embodiment, the radionuclide labeling agents is selected from the group consisting of: ¹¹ C, ¹³ N, ¹⁵ O, ¹⁸ F, ⁴³ Sc, ⁴⁴ Sc, ⁴⁵ Ti, ⁵⁵ Co, ⁶⁰ Cu, ⁶¹ Cu, ⁶⁴ Cu, ⁶⁸ Ga, ⁷⁶ Br, ⁷² As, ⁸⁶ Y, ⁸⁹ Zr, ⁹⁰ Y, ¹⁴⁹ Tb, ¹⁵² Tb; and the stable isotopes of the corresponding element is selected from the group consisting of: ¹² C, ¹³ C, ¹⁴ N, ¹⁵ N, ¹⁶ O, ¹⁷ O, ¹⁸ O, ¹⁹ F, ⁴⁵ Sc, ⁴⁶ Ti, ⁴⁷ Ti, ⁴⁸ Ti, ⁴⁹ Ti, ⁵⁰ Ti, ⁵⁹ Co, ⁶³ Cu, ⁶⁵ Cu, ⁶⁹ Ga, ⁷¹ Ga, ⁷⁵ As, ⁷⁹ Br, ⁸¹ Br, ⁸⁹ Y, ⁹⁰ Zr, ⁹¹ Zr, ⁹² Zr, ⁹⁴ Zr, ¹⁵⁹ Tb. These radionuclides and their stable isotopes of the corresponding elements are particularly useful in Positron Emission Tomography (PET). In another preferred embodiment, the radionuclide labeling agents is selected from the group consisting of: ⁶⁴ Cu, ⁶⁷ Cu, ⁶⁷ Ga, ¹¹¹ In, ¹³¹ I, ¹⁷⁷ Lu, ¹⁸⁶ Re, ²⁰¹ Tl, ²¹² Pb, ²¹³ Bi; and the stable isotope of the corresponding element is selected from the group consisting of: ⁶³ Cu, ⁶⁵ Cu, ⁶⁹ Ga, ⁷¹ Ga, ¹¹³ In, ¹²⁷ I, ¹⁷⁵ Lu, ¹⁸⁵ Re, ²⁰³ Tl, ²⁰⁵ Tl, ²⁰⁶ Pb, ²⁰⁷ Pb, ²⁰⁸ Pb, ²⁰⁹ Bi. These radionuclides and their stable isotopes of the corresponding elements are particularly useful in Single Photon Emission Computed Tomography (SPECT). In another preferred embodiment, the radionuclide labeling agents is selected from the group consisting of: ³² P, ³³ P, ⁴⁷ Sc, ⁶⁴ Cu, ⁶⁷ Cu, ⁸⁹ Sr, ⁹⁰ Y, ¹⁶⁶ Ho, ¹⁶¹ Tb, ¹⁶⁵ Dy, ¹⁷⁷ Lu, ¹⁸⁶ Re, ¹⁸⁸ Re; and the stable isotope of the corresponding element is selected from the group consisting of: ³¹ P, ⁴⁵ Sc, ⁶³ Cu, ⁶⁵ Cu, ⁸⁴ Sr, ⁸⁶ Sr, ⁸⁷ Sr, ⁸⁸ Sr, ⁸⁹ Y, ¹⁶⁵ Ho, ¹⁵⁹ Tb, ¹⁵⁶ Dy, ¹⁵⁸ Dy, ¹⁶⁰ Dy, ¹⁶¹ Dy, ¹⁶² Dy, ¹⁶³ Dy, ¹⁶⁴ Dy, ¹⁷⁵ Lu, ¹⁸⁵ Re. These radionuclides are beta- particle emitters and these radionuclides along with their stable isotopes of the corresponding element are applied in therapy for instance in relation to the treatment of various tumorous diseases. In another preferred embodiment, the radionuclide labeling agents is selected from the group consisting of: ¹⁴⁹ Tb, ²¹² Pb, ²¹² Bi, ²¹³ Bi, ²²⁷ Th; and the stable isotope of the corresponding element is selected from the group consisting of: ¹⁵⁹ Tb, ²⁰⁶ Pb, ²⁰⁷ Pb, ²⁰⁸ Pb, ²⁰⁹ Bi, ²³² Th. These radionuclides are alpha-particle emitters and these radionuclides along with their stable isotopes of the corresponding element are applied in therapy for instance in relation to the treatment of various tumorous diseases. In another preferred embodiment, the radionuclide labeling agents is selected from the group consisting of: ⁵¹ Cr, ⁵⁸ mCo, ⁶⁴ Cu, ⁶⁷ Ga, ⁷³ Se, ⁷⁵ Se, ⁷⁷ Br, ⁸⁰ mBr, ⁹⁴ Tc, ^99m Tc, ¹⁰³ mRh, ¹¹¹ In, ¹¹⁴ mIn, ¹¹⁵ mIn, ¹¹⁹ Sb, ¹²³ I, ¹²⁴ I, ¹²⁵ I, ¹³⁵ La, ¹⁶⁵ Er, ¹⁹³ mPt, ¹⁹⁵ mPt; and the stable isotope of the corresponding element is selected from the group consisting of: ⁵² Cr, ⁵³ Cr, ⁵⁴ Cr, ⁵⁹ Co, ⁶³ Cu, ⁶⁵ Cu, ⁶⁹ Ga, ⁷¹ Ga, ⁷⁴ Se, ⁷⁶ Se, ⁷⁷ Se, ⁷⁸ Se, ⁸⁰ Se, ⁸² Se, ⁷⁹ Br, ⁸¹ Br, ¹⁰³ Rh, ¹¹³ In, ¹²¹ Sb, ¹²³ Sb, ¹²⁷ I, ¹³⁸ La, ¹³⁹ La, ¹⁶² Er, ¹⁶⁴ Er, ¹⁶⁶ Er, ¹⁶⁷ Er, ¹⁶⁸ Er, ¹⁷⁰ Er, ¹⁹² Pt, ¹⁹⁴ Pt, ¹⁹⁵ Pt, ¹⁹⁶ Pt, ¹⁹⁸ Pt. These radionuclides emit electrons via the Auger effect with low kinetic energy. These radionuclides along with their stable isotopes of the corresponding element are applied in Auger therapy for instance in relation to highly targeted treatment of various tumorous diseases. In another preferred embodiment, the radionuclide labeling agents is selected from the group consisting of: ³ H, ¹⁴ C and ³⁵ S and the stable isotope of the corresponding element is selected from the group consisting of: ¹ H, ² H, ¹² C, ¹³ C, ³² S. These radionuclides are applied to in vitro studies such as displacement and titration assays. In another preferred embodiment, the radionuclide labeling agents is selected from the group consisting of: ¹¹ C, ¹³ N, ¹⁸ F, ¹²³ I, ¹²⁵ I, ¹³¹ I; and the stable isotope of the corresponding element is selected from the group consisting of: ¹² C, ¹⁴ N, ¹⁹ F, ¹²⁷ I. These radionuclides along with their stable isotopes of the corresponding element, are among the most frequently used radionuclides in therapy and imaging presently. The targeting vector that is conjugated to either the diene or to the dienophile mentioned in step b) in the for providing a labeled single isomeric chemical entity targeting vector can be any kind of targeting vector that is suitable for use in therapy, imaging, or diagnostics. Such commonly used targeting vectors that are suitable in the present method include antibodies, nanobodies, polymers, nanomedicines, cells, proteins, peptides, and small molecules. Commonly applied targeting vectors, that are suitable in the present method includes: peptides such as Octreotide, Octreotate, AE105; small molecules such as FAPI derivatives and PSMA derivatives. In a preferred embodiment, the targeting vector applied in the method for providing a labeled single isomeric chemical entity targeting vector is selected from the group comprising: Octreotide, Octreotate, AE105, FAPI derivatives and PSMA derivatives. The oxidizing step c) in the method for providing a labeled single isomeric chemical entity targeting vector is carried out at a certain temperature and time, by adding a specific oxidant to the ligated compound obtained in step b). These conditions ensure that the efficiency of the oxidation step is ≥90% thereby meeting the speed required for therapeutic, diagnostic or imaging use of the labeled single isomeric chemical entity targeting vector. The time required to obtain an oxidation efficiency of ≥90% depends on the specific compound being oxidized, temperature, oxidant equivalents and on the oxidation agent. With the conditions applied in the present method, the ≥90% oxidation efficiency will be obtained within 60 minutes, such as from 0 - 50 minutes, from 0 - 40 minutes, from 0 - 30 minutes, from 0 - 20 minutes, from 0 - 10 minutes, or from 0 - 5 minutes. In a preferred embodiment, the oxidation efficiency obtained is ≥90% in 0 - 20 minutes. The temperature for the oxidation step is 15 ⁰C – 50 ⁰C, such as 15 ⁰C – 45 ⁰C, 15 ⁰C - 40⁰C, 15 ⁰C – 35 ⁰C, 20 ⁰C – 30 ⁰C, or at approximately 20 ⁰C - 25 ⁰C. The preferred temperature is room temperature such as between 20 ⁰C - 25 ⁰C. The oxidant should be a quinone oxidant, selected from chloranil, fluoranil, DDQ, or NaNO ₂ . It has surprisingly been found herein, that using other types of oxidants will not provide the desired single isomeric form of the labeled chemical entity targeting vector or will negatively impact the structure of the targeting vector. The oxidant is added to the ligated labeled compound obtained from step b) in the method for providing a labeled single isomeric chemical entity targeting vector from 1 to 100 equivalents of the product obtained in step b), such as from 10 to 90, 20 to 80, 30 to 70, 40 to 60 or 50 equivalents of the product obtained in step b). Preferably 1 to 10, such as 1 equivalent oxidant is added to the labeled compound obtained from step b). In one embodiment, the oxidant is solid phase supported. Any commonly available solid support would be applicable, for instance oxidants supported by alumina, silica gel, polymer, montmorillonite, zeolite or a nanomaterial. The advantages of using a solid supported oxidants in general include easy removal from reactions by filtration, excess reagents can be used to drive reactions to completion without introducing difficulties in purification, easy to handle, recycling of recovered reagents is economical, and efficient. The conventional ligation between a tetrazine and a TCO will result in a number of different tautomers, and enantiomers as schematically shown in Scheme 1. Scheme 1. Overview of all isomeric forms after conventional tetrazine ligation, inlcuding tautomers, enantiomers and regioisomers. The below scheme 2 is an example of a current state-of-the-art tetrazine ligation. A reaction which gives too many isomeric products:

Scheme 2. Explicit example of current state-of-the-art tetrazine ligation. (all isomeric forms are omitted, see Scheme 1 for overview of the isomeric forms. Multiple isomers (at least 6 isomers) can be formed in this reaction, which all have very similar polarities and are accordingly very difficult to separate for instance by HPLC. Herein, we describe a method, which allows for the use of a labeled first chemical entity such as radiolabelled symmetrically tetrazine-based synthons in the radiolabelling of dienophils, such as trans-cycloheptenes (TCH), trans-cyclooctene (TCO) and trans-cyclononene (TCN) functionalized vectors and vice versa, which upon subsequent chemical oxidation yields a single final compound within short time, such as within 0 - 60 minutes (Figure 5). The method comprises two steps: a ligation step followed by an oxidation step.. The final single isomeric form of the radiolabeled diene-dienophile targeting vector which is the outcome of the method can be reached via different alternative starting points for the first step in the method i.e. the ligation step (referred to as combination i), ii), respectively) followed by the second step which is an oxidation step. In a preferred embodiment, the method for providing a labeled single isomeric chemical entity targeting vector comprises: a) labeling a first chemical entity having inverse electron demand Diels-Alder cycloaddition reactivity and being conjugated to a pharmaceutic agent, an imaging agent, or a therapeutic agent, with a labeling agent; wherein the first chemical entity is selected from the group consisting of a symmetrical tetrazine wherein at least one of the symmetry planes pass through the nitrogen- nitrogen bonds of at least one tetrazine ring, an enantiomerically pure trans- cycloheptene (TCH), an enantiomerically pure trans-cyclooctene (TCO), an enantiomerically pure trans-cyclononene (TCN), and a cis,5,6-disubstituted trans-cyclooctene (TCO), b) ligating the labeled first chemical entity obtained in step a) with a second chemical entity having complementary inverse electron demand Diels-Alder cycloaddition reactivity and being conjugated to a targeting vector; wherein the second chemical entity is selected from the group consisting of a symmetrical tetrazine wherein at least one of the symmetry planes pass through the nitrogen-nitrogen bonds of at least one tetrazine ring, an enantiomerically pure trans-cycloheptene (TCH), an enantiomerically pure trans-cyclooctene (TCO), an enantiomerically pure trans-cyclononene (TCN and a cis,5,6-disubstituted trans-cyclooctene (TCO), wherein the reaction kinetics for the inverse electron demand Diels-Alder cycloaddition between the first and second chemical entities has a minimum rate constant of 500 M ^-1 s ^-1 in PBS at 25 °C, determined by stopped-flow spectrophotometry, and wherein the first and second chemical entities having complementary inverse electron demand Diels-Alder cycloaddition reactivity being ligated are selected from one of the following combinations: i) a symmetrical tetrazine and an enantiomerically pure TCO, TCN or TCH a symmetrical tetrazine and a cis,5,6-disubstituted TCO c) oxidizing the ligated labeled targeting vector obtained from step b) at a temperature ranging from 20 °C to 30 °C for up to 20 minutes by adding from 1 to 100 equivalents of chloranil, fluoranil, DDQ or NaNO ₂ . Ligation combination i): In ligation combination i), the starting entities to be ligated is a symmetrical substituted diene wherein at least one of the symmetry planes pass through the nitrogen-nitrogen bonds of at least one tetrazine ring, such as a symmetrical tetrazine, and an enantiomerically pure dienophile, such as an enantiomerically pure TCH, TCO, or TCN. Using a symmetrical substituted diene, such as a symmetrical tetrazine, in combination with an enantiomerically pure dienophile, such as an enantiomerically pure TCH, TCO, or TCN, reduces the number of formed click-products, by eliminating all enantiomeric products. The formed tautomeric dihydropyridazines will be subsequently oxidized to the corresponding pyridazine, resulting in a single product. The ‘R’ substituents on the diene, such as a tetrazine, employed in this method will be functionalized on one side with ¹⁸ F or ¹²³ I, ¹²⁴ I, ¹²⁵ I, ¹³¹ I and the opposite site with 1 ⁹ F, or ¹²⁷ I, respectively. Due to the fact that both these isotopes of either fluorine or iodine, respectively, are chemically identical, the single product, formed via this method, is still considered, from a chemical perspective, a single entity. The below scheme 2 is an illustration of a ligation in accordance with ligation combination i) here exemplified in using a symmetrical tetrazine and a TCO conjugated to a targeting vector: Scheme 3. The targeting vector is an antibody, a nanobody, a polymer, a nanomedicine, a cell, a protein, a peptide, or a small molecule. Ligation combination ii): In ligation combination ii), the starting entities to be ligated is either a symmetrical substituted diene wherein at least one of the symmetry planes pass through the nitrogen-nitrogen bonds of at least one tetrazine ring, such as a symmetrical tetrazine, and a cis,5,6-disubstituted dienophile, such as a cis,5,6-disubstituted TCO. Using a cis,5,6-disubstituted dienophile, such as a cis,5,6-disubstituted TCO, and a symmetrical tetrazine, reduces the number of formed click-products, by eliminating all enantiomeric products. The formed tautomeric entities, such as dihydropyridazines, will be subsequently oxidized to the corresponding single isomeric form, such as a pyridazine, resulting in a single product. The ‘R’ substituents on the dienes employed in this method will typically be functionalized with ¹⁸ F, ¹²³ I, ¹²⁴ I, ¹²⁵ I, or ¹³¹ I. The below scheme 3 is an illustration of a ligation in accordance with ligation combination ii) here exemplified in using an unsymmetrical tetrazine and a TCO conjugated to a targeting vector: Scheme 4. The targeting vector is an antibody, a nanobody, a polymer, a nanomedicine, a cell, a protein, a peptide, or a small molecule. Symmetrical tetrazines of formula Tz1 are examples of preferred dienes suitable for both ligation combination i) and ii), respectively: wherein R and R ₁ is wherein the curly sign indicates the link to the tetrazine; and where R ₂ is -H or (i) an isotope labeling agent directly connected to the aromatic ring; or (ii) an isotope labeling agent connected to the aromatic ring via a linker, said linker being selected from the group consisting of (CH ₂ ) _n , -LO(CH ₂ ) _n , -LNH(CH ₂ ) _n , - LCONH(CH ₂ ) _n , -LNHCO(CH ₂ ) _n , where L is -(CH ₂ ) _m or -O(CH ₂ CH ₂ O) _m , where n and m are independently selected from 1-25; or (iii) an isotope labeling agent that is chelated through a chelator selected from: 1,4,7,10-tetraazacyclododecane- N,N',N',N"-tetraacetic acid (DOTA), N,N'-bis(2-hydroxy-5- (carboxyethyl)benzyl)ethylenediamine N,N'-diacetic acid (HBED-CC), 14,7- triazacyclononane-1,4,7-triacetic acid (NOTA), 2-(4.7-bis(carboxymethyl)-1,4,7- triazonan-1-yl)pentanedioic acid (NODAGA), 2-(4,7,10-tris(carboxymethyl)-1,4,7,10- tetraazacyclododecan-1- yl)pentanedioic acid (DOTAGA), 14,7-triazacyclononane phosphinic acid (TRAP), 14,7-triazacyclononane-1-methyl(2-carboxyethyl)phosphinic acid-4,7-bis(methyl(2-hydroxymethyl)phosphinic acid (NOPO), 3,6,9,15- tetraazabicyclo9.3.1.pentadeca-1 (15),11,13-triene-3,6,9- triacetic acid (PCTA), N'- (5-acetyl (hydroxy)aminopentyl-N-(5-(4-(5- aminopentyl)(hydroxy)amino-4- oxobutanoyl)amino)pentyl-N- hydroxysuccinamide (DFO), diethylenetriaminepentaacetic acid (DTPA), trans-cyclohexyl- diethylenetriaminepentaacetic acid (CHX-DTPA), 1-oxa-4,7,10-triazacyclododecane- 4,7,10-triacetic acid (OXO-Do3A), p-isothiocyanatobenzyl-DTPA (SCN-BZ-DTPA), 1- (p-isothiocyanatobenzyl)-3-methyl-DTPA (1B3M), 2-(p-isothiocyanatobenzyl)-4- methyl-DTPA (1M3B), and 1-(2)-methyl-4-isocyanatobenzyl-DTPA (MX-DTPA) connected to the aromatic ring through a linker, said linker being selected from the group consisting of (CH ₂ ) _n , -LO(CH ₂ ) _n , -LNH(CH ₂ ) _n , -LCONH(CH ₂ ) _n , -LNHCO(CH ₂ ) _n , where L is -(CH ₂ ) _m or -O(CH ₂ CH ₂ O) _m, and n and m are independently selected from 1-25; wherein, when R ₂ is either (i) or (ii) the isotope labeling agent is selected from the group consisting of: ¹ H, ² H, ³ H, ¹¹ C, ¹² C, ¹³ C, ¹³ N, ¹⁴ N, ¹⁵ N ¹⁸ F, ¹⁹ F, ¹²³ I, ¹²⁴ I, ¹²⁵ I, ¹²⁷ I, ¹³¹ I, ²¹¹ At, ¹⁵ O, ¹⁶ O, ¹⁷ O, ¹⁸ O, ³² S, ³⁵ S, ⁴³ Sc, ⁴⁴ Sc, ⁴⁵ Sc, ⁴⁵ Ti, ⁴⁶ Ti, ⁴⁷ Ti, ⁴⁸ Ti, ⁴⁹ Ti, ⁵⁰ Ti, ⁵⁵ Co, ⁵⁸ mCo, ⁵⁹ Co, ⁶⁰ Cu, ⁶¹ Cu, ⁶³ Cu, ⁶⁴ Cu, ⁶⁵ Cu, ⁶⁷ Cu, ⁶⁷ Ga, ⁶⁸ Ga, ⁶⁹ Ga, ⁷¹ Ga, ⁷⁶ Br, ⁷⁷ Br, ⁷⁹ Br, ⁸⁰ mBr, ⁸¹ Br, ⁷² As, ⁷⁵ As, ⁸⁶ Y, ⁸⁹ Y, ⁹⁰ Y, ⁸⁹ Zr, ⁹⁰ Zr, ⁹¹ Zr, ⁹² Zr, ⁹⁴ Zr, ¹⁴⁹ Tb, ¹⁵² Tb, ¹⁵⁹ Tb, ¹⁶¹ Tb, ¹¹¹ In, ¹¹³ In, ¹¹⁴ mIn, ¹¹⁵ mIn, ¹⁷⁵ Lu, ¹⁷⁷ Lu, ¹⁸⁵ Re, ¹⁸⁶ Re, ¹⁸⁸ Re, ²⁰¹ Tl, ²⁰³ Tl, ²⁰⁵ Tl, ²⁰⁶ Pb, ²⁰⁷ Pb, ²⁰⁸ Pb, ²¹² Pb, ²⁰⁹ Bi, ²¹² Bi, ^{2 13} Bi, ³¹ P, ³² P, ³³ P, ⁴⁵ Sc, ⁴⁷ Sc, ⁸⁴ Sr, ⁸⁶ Sr, ⁸⁷ Sr, ⁸⁸ Sr, ⁸⁹ Sr, ¹⁶⁵ Ho, ¹⁶⁶ Ho, ¹⁵⁶ Dy, ¹⁵⁸ Dy, ¹⁶⁰ Dy, ¹⁶¹ Dy, ¹⁶² Dy, ¹⁶³ Dy, ¹⁶⁴ Dy, ¹⁶⁵ Dy , ²²⁷ Th, ²³² Th, ⁵¹ Cr, ⁵² Cr, ⁵³ Cr, ⁵⁴ Cr, ⁷³ Se, ⁷⁴ Se, ⁷⁵ Se, ⁷⁶ Se, ⁷⁷ Se, ⁷⁸ Se, ⁸⁰ Se, ⁸² Se, ⁹⁴ Tc, ^99m Tc, ¹⁰³ Rh, ¹⁰³ mRh, ¹¹⁹ Sb, ¹²¹ Sb, ¹²³ Sb, ¹³⁵ La, ¹³⁸ La, ¹³⁹ La, ¹⁶² Er, ¹⁶⁴ Er, ¹⁶⁵ Er, ¹⁶⁶ Er, ¹⁶⁷ Er, ¹⁶⁸ Er, ¹⁷⁰ Er, ¹⁹³ mPt, ¹⁹⁵ mPt, ¹⁹² Pt, ¹⁹⁴ Pt, ¹⁹⁵ Pt, ¹⁹⁶ Pt, ¹⁹⁸ Pt, and wherein X and Y are independently selected from: -CH and -N- ; and wherein R ₃ is independently selected from H or a moiety selected from the group consisting of a hydroxy group, a sulfonamide, a carboxyl group, a sulfonyl group, amine, a substituted amine with 1-5 polyethylene glycol unit(s), a −(O−CH ₂ −CH ₂ ) _{1- 5} −OCH ₂ -COOH, Methyl, Ethyl, Propyl, optionally substituted heteroaryl, and optionally substituted arylalkyl; wherein optionally substituted in relation to said substituted amine means one or more substituents selected from, a halogen, a hydroxy group, a sulfonamide, a carboxyl group, a sulfonyl group, amine, (C1-C10) alkyl, (C2- C10)alkenyl, (C2-C10)alkynyl, (C1-C10)alkylene, (C1-C10)alkoxy, (C2- C10)dialkylamino, (C1-C10)alkylthio, (C2-C10)heteroalkyl, (C2-C10)heteroalkylene, (C3-30 C10)cycloalkyl, (C3-C10)heterocycloalkyl, (C3-10)cycloalkylene, (C3- C10)heterocycloalkylene, (C1-C10)haloalkyl, (C1-C10)perhaloalkyl, (C2-C10)- alkenyloxy, (C3-C10)-alkynyloxy, aryloxy, arylalkyloxy, heteroaryloxy, heteroarylalkyloxy, (C1-C6)alkyloxy-(C1-C4)alkyl, optionally substituted aryl, optionally substituted heteroaryl, and optionally substituted arylalkyl; wherein optionally substituted means one or more substituents selected from a halogen, a hydroxy group, a sulfonamide, a carboxyl group, a sulfonyl group, amine, a substituted amine with 1-5 polyethylene glycol unit(s), a −(O−CH ₂ −CH ₂ ) _1-5 −OCH ₂ -COOH, H, Methyl, Ethyl, Propyl, optionally substituted heteroaryl, and optionally substituted arylalkyl; wherein optionally substituted in relation to said substituted amine means one or more substituents selected from a halogen, a hydroxy group, a sulfonamide, a carboxyl group, a sulfonyl group, and amine; and wherein R and R ₁ are identical or differs only in the isotope number of the labelling agent. The below tetrazines are preferred tetrazines of Formula Tz1 for use in ligation combinations i) and ii) in step b) of the method for providing a labeled single isomeric chemical entity targeting vector:

The below trans-cycloheptenes (TCH’s), trans-cyclooctenes (TCO’s), and a trans- cyclononenes (TCNs) are preferred enantiomerically pure dienophiles for use in ligation combination i) in step b) of the method for providing a labeled single isomeric chemical entity targeting vector: Wherein X is O, NH, S, or CH ₂ ; and wherein the linker is selected from the group comprising: -(CH ₂ ) _n - (CH ₂ ) _n NH, (CH ₂ ) _n CO, (CH ₂ ) _n O, (CH ₂ CH ₂ O) _n (CH ₂ CH ₂ O) _n CH ₂ CH ₂ NH, (CH ₂ CH ₂ O) _n CH ₂ CH ₂ CO, -CO(CH) ₂ - CO(CH ₂ ) _n NH, CO(CH ₂ ) _n CO, CO(CH ₂ ) _n O, CO(CH ₂ CH ₂ O) _n CO(CH ₂ CH ₂ O) _n CH ₂ CH ₂ NH, CO(CH ₂ CH ₂ O) _n CH ₂ CH ₂ CO, COO(CH) ₂ - COO(CH ₂ ) _n NH, COO(CH ₂ ) _n CO, COO(CH ₂ ) _n O, COO(CH ₂ CH ₂ O) _n COO(CH ₂ CH ₂ O) _n CH ₂ CH ₂ NH, COO(CH ₂ CH ₂ O) _n CH ₂ CH ₂ CO, CONH(CH) ₂ - CONH(CH ₂ ) _n NH, CONH(CH ₂ ) _n CO, CONH(CH ₂ ) _n O, CONH(CH ₂ CH2O) _n CONH(CH ₂ CH ₂ O) _n CH ₂ CH ₂ NH, CONH(CH ₂ CH ₂ O)nCH ₂ CH ₂ CO, -CONHPhCO, -COOPhCO, -COPhCO, CONHCHMCO, (CH ₂ ) _n NHCHMCO, (CH ₂ ) _n OCONHCHMCO, (CH ₂ ) _n NHCHMCO, (CH ₂ ) _n NHCOCHMNH, (CH ₂ )OCOCHMNH, (CH ₂ CH ₂ O) _n CH ₂ CH ₂ NHCHMCO, (CH ₂ CH ₂ O) _n CH ₂ CH ₂ CONHCHMCO, (CH ₂ CH ₂ O) _n CH ₂ CH ₂ NHCHMCO, (CH ₂ CH ₂ O) _n CH ₂ CH ₂ NHCOCHMNH, (CH ₂ CH ₂ O) _n COCHMNH, where n is 0-25 and where M is a side chain selected from the group consisting of side chains of the natural amino acids: H, CH ₃ , CH ₂ SH, CH ₂ COOH, CH ₂ CH ₂ COOH, CH ₂ C ₆ H ₅ , CH ₂ C ₃ H ₃ N ₂ , CH(CH ₃ )CH ₂ CH ₃ , (CH ₂ ) ₄ NH ₂ , CH ₂ CH(CH ₃ ) ₂ , CH ₂ CH ₂ SCH ₃ , CH ₂ CONH ₂ , (CH ₂ ) ₄ NHCOC ₄ H ₅ NCH ₃ , CH ₂ CH ₂ CH ₂ , CH ₂ CH ₂ CONH ₂ , (CH ₂ ) ₃ NH-C(NH)NH ₂ , CH ₂ OH, CH(OH)CH ₃ , CH ₂ SeH, CH(CH ₃ ) ₂ , CH ₂ C ₈ H ₆ N, CH ₂ C ₆ H ₄ OH; and where targeting vector is an antibody, a nanobody, a polymer, a nanomedicine, a cell, a protein, a peptide, or a small molecule. The below trans-cyclooctenes (TCO’s) are preferred cis,5,6-disubstituted dienophiles for use in ligating combination ii) in step b) of the method for providing a labeled single isomeric chemical entity targeting vector: Wherein X is -O, NH, S, or CH2; -and wherein the linker is selected from the group comprising: -(CH ₂ ) _n - (CH ₂ ) _n NH, (CH ₂ ) _n CO, (CH ₂ ) _n O, (CH ₂ CH ₂ O) _n (CH ₂ CH ₂ O) _n CH ₂ CH ₂ NH, (CH ₂ CH ₂ O) _n CH ₂ CH ₂ CO, -CO(CH) ₂ - CO(CH ₂ ) _n NH, CO(CH ₂ ) _n CO, CO(CH ₂ ) _n O, CO(CH ₂ CH ₂ O) _n CO(CH ₂ CH ₂ O) _n CH ₂ CH ₂ NH, CO(CH ₂ CH ₂ O) _n CH ₂ CH ₂ CO, COO(CH) ₂ - COO(CH ₂ ) _n NH, COO(CH ₂ ) _n CO, COO(CH ₂ ) _n O, COO(CH ₂ CH ₂ O) _n COO(CH ₂ CH ₂ O) _n CH ₂ CH ₂ NH, COO(CH ₂ CH ₂ O) _n CH ₂ CH ₂ CO, CONH(CH) ₂ - CONH(CH ₂ ) _n NH, CONH(CH ₂ ) _n CO, CONH(CH ₂ ) _n O, CONH(CH ₂ CH2O) _n CONH(CH ₂ CH ₂ O) _n CH ₂ CH ₂ NH, CONH(CH ₂ CH ₂ O)nCH ₂ CH ₂ CO, -CONHPhCO, -COOPhCO, -COPhCO, CONHCHMCO, (CH ₂ ) _n NHCHMCO, (CH ₂ ) _n OCONHCHMCO, (CH ₂ ) _n NHCHMCO, (CH ₂ ) _n NHCOCHMNH, (CH ₂ )OCOCHMNH, (CH ₂ CH ₂ O) _n CH ₂ CH ₂ NHCHMCO, (CH ₂ CH ₂ O) _n CH ₂ CH ₂ CONHCHMCO, (CH ₂ CH ₂ O) _n CH ₂ CH ₂ NHCHMCO, (CH ₂ CH ₂ O) _n CH ₂ CH ₂ NHCOCHMNH, (CH ₂ CH ₂ O) _n COCHMNH, where n is 0-25 and where M is a side chain selected from the group consisting of side chains of the natural amino acids: H, CH ₃ , CH ₂ SH, CH ₂ COOH, CH ₂ CH ₂ COOH, CH ₂ C ₆ H ₅ , CH ₂ C ₃ H ₃ N ₂ , CH(CH ₃ )CH ₂ CH ₃ , (CH ₂ ) ₄ NH ₂ , CH ₂ CH(CH ₃ ) ₂ , CH ₂ CH ₂ SCH ₃ , CH ₂ CONH ₂ , (CH ₂ ) ₄ NHCOC ₄ H ₅ NCH ₃ , CH ₂ CH ₂ CH ₂ , CH ₂ CH ₂ CONH ₂ , (CH ₂ ) ₃ NH-C(NH)NH ₂ , CH ₂ OH, CH(OH)CH ₃ , CH ₂ SeH, CH(CH ₃ ) ₂ , CH ₂ C ₈ H ₆ N, CH ₂ C ₆ H ₄ OH; and where the targeting vector is an antibody, a nanobody, a polymer, a nanomedicine, a cell, a protein, a peptide, or a small molecule. Examples of Enantiopure TCOs TCH, TCN that are suitable for ligation combination Wherein the linker is selected from the group comprising: -(CH ₂ ) _n - (CH ₂ ) _n NH, (CH ₂ ) _n CO, (CH ₂ ) _n O, (CH ₂ CH ₂ O) _n (CH ₂ CH ₂ O) _n CH ₂ CH ₂ NH, (CH ₂ CH ₂ O) _n CH ₂ CH ₂ CO, - CO(CH) ₂ - CO(CH ₂ ) _n NH, CO(CH ₂ ) _n CO, CO(CH ₂ ) _n O, CO(CH ₂ CH ₂ O) _n CO(CH ₂ CH ₂ O) _n CH ₂ CH ₂ NH, CO(CH ₂ CH ₂ O) _n CH ₂ CH ₂ CO, COO(CH) ₂ - COO(CH ₂ ) _n NH, COO(CH ₂ ) _n CO, COO(CH ₂ ) _n O, COO(CH ₂ CH ₂ O) _n COO(CH ₂ CH ₂ O) _n CH ₂ CH ₂ NH, COO(CH ₂ CH ₂ O) _n CH ₂ CH ₂ CO, CONH(CH) ₂ - CONH(CH ₂ ) _n NH, CONH(CH ₂ ) _n CO, CONH(CH ₂ ) _n O, CONH(CH ₂ CH2O) _n CONH(CH ₂ CH ₂ O) _n CH ₂ CH ₂ NH, CONH(CH ₂ CH ₂ O)nCH ₂ CH ₂ CO, -CONHPhCO, -COOPhCO, -COPhCO, CONHCHMCO, (CH ₂ ) _n NHCHMCO, (CH ₂ ) _n OCONHCHMCO, (CH ₂ ) _n NHCHMCO, (CH ₂ ) _n NHCOCHMNH, (CH ₂ )OCOCHMNH, (CH ₂ CH ₂ O) _n CH ₂ CH ₂ NHCHMCO, (CH ₂ CH ₂ O) _n CH ₂ CH ₂ CONHCHMCO, (CH ₂ CH ₂ O) _n CH ₂ CH ₂ NHCHMCO, (CH ₂ CH ₂ O) _n CH ₂ CH ₂ NHCOCHMNH, (CH ₂ CH ₂ O) _n COCHMNH, where n is 0-25 and where M is a side chain selected from the group consisting of side chains of the natural amino acids: H, CH ₃ , CH ₂ SH, CH ₂ COOH, CH ₂ CH ₂ COOH, CH ₂ C ₆ H ₅ , CH ₂ C ₃ H ₃ N ₂ , CH(CH ₃ )CH ₂ CH ₃ , (CH ₂ ) ₄ NH ₂ , CH ₂ CH(CH ₃ ) ₂ , CH ₂ CH ₂ SCH ₃ , CH ₂ CONH ₂ , (CH ₂ ) ₄ NHCOC ₄ H ₅ NCH ₃ , CH ₂ CH ₂ CH ₂ , CH ₂ CH ₂ CONH ₂ , (CH ₂ ) ₃ NH-C(NH)NH ₂ , CH ₂ OH, CH(OH)CH ₃ , CH ₂ SeH, CH(CH ₃ ) ₂ , CH ₂ C ₈ H ₆ N, CH ₂ C ₆ H ₄ OH; and where the targeting vector is an antibody, a nanobody, a polymer, a nanomedicine, a cell, a protein, a peptide, or a small molecule. Examples of Cis,5,6-disubstituted-TCOs suitable for ligation ii) Wherein X is -O, NH, S, or CH _2; -and wherein the linker is selected from the group comprising: -(CH ₂ ) _n - (CH ₂ ) _n NH, (CH ₂ ) _n CO, (CH ₂ ) _n O, (CH ₂ CH ₂ O) _n (CH ₂ CH ₂ O) _n CH ₂ CH ₂ NH, (CH ₂ CH ₂ O) _n CH ₂ CH ₂ CO, -CO(CH) ₂ - CO(CH ₂ ) _n NH, CO(CH ₂ ) _n CO, CO(CH ₂ ) _n O, CO(CH ₂ CH ₂ O) _n CO(CH ₂ CH ₂ O) _n CH ₂ CH ₂ NH, CO(CH ₂ CH ₂ O) _n CH ₂ CH ₂ CO, COO(CH) ₂ - COO(CH ₂ ) _n NH, COO(CH ₂ ) _n CO, COO(CH ₂ ) _n O, COO(CH ₂ CH ₂ O) _n COO(CH ₂ CH ₂ O) _n CH ₂ CH ₂ NH, COO(CH ₂ CH ₂ O) _n CH ₂ CH ₂ CO, CONH(CH) ₂ - CONH(CH ₂ ) _n NH, CONH(CH ₂ ) _n CO, CONH(CH ₂ ) _n O, CONH(CH ₂ CH2O) _n CONH(CH ₂ CH ₂ O) _n CH ₂ CH ₂ NH, CONH(CH ₂ CH ₂ O)nCH ₂ CH ₂ CO, -CONHPhCO, -COOPhCO, -COPhCO, CONHCHMCO, (CH ₂ ) _n NHCHMCO, (CH ₂ ) _n OCONHCHMCO, (CH ₂ ) _n NHCHMCO, (CH ₂ ) _n NHCOCHMNH, (CH ₂ )OCOCHMNH, (CH ₂ CH ₂ O) _n CH ₂ CH ₂ NHCHMCO, (CH ₂ CH ₂ O) _n CH ₂ CH ₂ CONHCHMCO, (CH ₂ CH ₂ O) _n CH ₂ CH ₂ NHCHMCO, (CH ₂ CH ₂ O) _n CH ₂ CH ₂ NHCOCHMNH, (CH ₂ CH ₂ O) _n COCHMNH, where n is 0-25 and where M is a side chain selected from the group consisting of side chains of the natural amino acids: H, CH ₃ , CH ₂ SH, CH ₂ COOH, CH ₂ CH ₂ COOH, CH ₂ C ₆ H ₅ , CH ₂ C ₃ H ₃ N ₂ , CH(CH ₃ )CH ₂ CH ₃ , (CH ₂ ) ₄ NH ₂ , CH ₂ CH(CH ₃ ) ₂ , CH ₂ CH ₂ SCH ₃ , CH ₂ CONH ₂ , (CH ₂ ) ₄ NHCOC ₄ H ₅ NCH ₃ , CH ₂ CH ₂ CH ₂ , CH ₂ CH ₂ CONH ₂ , (CH ₂ ) ₃ NH-C(NH)NH ₂ , CH ₂ OH, CH(OH)CH ₃ , CH ₂ SeH, CH(CH ₃ ) ₂ , CH ₂ C ₈ H ₆ N, CH ₂ C ₆ H ₄ OH;; and where the targeting vector is an antibody, a nanobody, a polymer, a nanomedicine, a cell, a protein, a peptide, or a small molecule. Step c) in the method for providing a labeled single isomeric chemical entity targeting vector is an oxidation step. Even though auto-oxidation of the ligated entity targeting vector, such as a pyridazine, obtained in step b) of the method occurs spontaneously, this process is extremely slow and can last from several hours up to several days. Step c) in the method provides a fast way for oxidizing the pyridazine compound wherein only a single isomer form is obtained at least within 60 minutes, such as within 0-20 minutes. In order to facilitate this process, the dihydropyridazines are oxidized by either a standard, or solid-supported oxidant, preferably solid-supported. The oxidizing step can be performed at a temperature ranging from 15 to 50 °C, such as at 20-30 °C, preferably at room temperature, for approximately 10 to 60 minutes, preferably for less than 20 minutes. To facilitate the oxidation adding 1 to 100 equivalents, preferably 1, of an oxidant to the ligated compound obtained from the ligation step. The oxidant needs to be selective for the oxidation of the dihydropyrazine to pyridazine (95% efficiency). The targeting vector must not be chemically modified by the oxidant. The oxidant is a quinone oxidant selected from the group comprising: chloranil, fluoranil, DDQ, NaNO _2. Precursors that are useful in providing some of the dienes and dienophiles suitable for the method for providing a labeled single isomeric chemical entity targeting vector have also been provided herein. The following structures are the preferred precursors of symmetrical substituted dienes for use in the method for providing a labeled single isomeric chemical entity targeting vector for ligating combination i) and ii):

The following structures are the preferred precursors of enantiomerically pure dienophiles for use in the method for providing a labeled single isomeric chemical entity targeting vector for ligating combination i): wherein the targeting vector is an antibody, a nanobody, a polymer, a nanomedicine, a cell, a protein, a peptide, or a small molecule. The following structures are the preferred precursors of cis,5,6-disubstituted dienophile for use in the method for providing a labeled single isomeric chemical entity targeting vector for ligating combination ii): wherein the targeting vector is an antibody, a nanobody, a polymer, a nanomedicine, a cell, a protein, a peptide, or a small molecule. The labeled single isomeric chemical entity targeting vector provided by the method for providing a labeled single isomeric chemical entity targeting vectors can be used in therapy, radiotherapy, theranostics, diagnostics, or imaging, depending on the labeling agent, or the pharmaceutical agent, or imaging agent or therapeutic agent and on the targeting vector. Preferably, the targeting vector is coupled to the linker via a nitrogen on the targeting vector. Alternatively, the targeting vector is preferable coupled to the linker via a carbonyl on the targeting vector. In a preferred embodiment, the labeled single isomeric chemical entity targeting vector provided by the method for providing a labeled single isomeric chemical entity targeting vectors is used in therapy. In another preferred embodiment, the labeled single isomeric chemical entity targeting vector provided by the method for providing a labeled single isomeric chemical entity targeting vectors is used in radiotherapy. In another preferred embodiment, the labeled single isomeric chemical entity targeting vector provided by the method for providing a labeled single isomeric chemical entity targeting vectors is used in theranostics. In another preferred embodiment, the labeled single isomeric chemical entity targeting vector provided by the method for providing a labeled single isomeric chemical entity targeting vectors is used in diagnostics. In another preferred embodiment, the labeled single isomeric chemical entity targeting vector provided by the method for providing a labeled single isomeric chemical entity targeting vectors is used in imaging. The following Examples describes (1) the synthesis of tetrazines and TCOs representative for use in step a) and b) of the present method for providing a labeled single isomeric chemical entity targeting vector and (2) click reactions and oxidations between such compounds, yielding a single isomeric pyridazine. EXAMPLES General All reagents and solvents were dried prior to use according to standard methods. Commercial reagents were used without further purification. Analytical TLC was performed using silica gel 60 F254 (Merck) with detection by UV absorption and/or by charring following immersion in a 7% ethanolic solution of sulfuric acid or KMnO ₄ - solution (1.5 g of KMnO ₄ , 10 g K ₂ CO ₃ , and 1.25 mL 10% NaOH in 200 mL water). Purification of compounds was carried out by column chromatography on silica gel (40-60 μm, 60 Å) or employing a CombiFlash NextGen 300+ (Teledyne ISCO). ¹ H and ¹³ C NMR spectra were recorded on Brucker (400 and 600 MHz instruments), using Chloroform-d, Methanol-d ₄ or DMSO-d ₆ as deuterated solvent and with the residual solvent as the internal reference. For all NMR experiences the deuterated solvent signal was used as the internal lock. Chemical shifts are reported in δ parts per million (ppm). Coupling constants (J values) are given in Hertz (Hz). Multiplicities of ¹ H NMR signals are reported as follows: s, singlet; d, doublet; dd, doublet of doublets; ddd, doublet of doublets of doublets; dt, doublet of triplets; t, triplet; q, quartet; m, multiplet; br, broad signal. NMR spectra of all compounds are reprocessed in MestReNova software (version 12.0.22023) from original FID’s files. Mass spectra analysis was performed using MS-Acquity-A: Waters Acquity UPLC with QDa- detector. Purification by preparative HPLC was performed on Agilent 1260 infinity system, column SymmetryPrep-C18, 17 mL/min H ₂ O-MeCN gradient 50-100% 15 min with 0.1% trifluoroacetic acid. All final compounds were >95% pure as determined by analytical HPLC. Analytical HPLC method: (Thermo Fisher® UltiMate 3000) with a C- 18 column (Luna® 5u C18(2) 100Å, 150 x 4.6 mm), eluents: A: H2O with 0.1% TFA, B: MeCN with 0.1% TFA. Gradient from 100% A -> 100% B over 15minutes, back to 100% A over 4 minutes, flow rate 1.5 mL/min. Detection by UV-absorption at λ = 254 nm on a UVD 170U detector. Example 1 Synthesis of symmetrical tetrazines and their precursors Compound I and XXIV Figure 1. shows the synthesis of symmetrical tetrazines. Reagents and conditions: i) NH ₂ (CH ₂ ) ₂ R, MeCN, 12 h, rt; ii) Boc ₂ O, Et ₃ N, DCM, 12 h, rt; iii) Zn(OTf) _2, NH ₂ NH ₂ , ^. H ₂ O, EtOH, 65 ºC, 24 h; iv) HCl, dioxane, rt, 4 h; v) t-Butyl bromoacetate, Et ₃ N, DMF, 50 ˚C, 12 h; vi) TFA, DCM, rt, 2 h; vii) MsCl, Et ₃ N, DMAP, DCM, rt, 12 h. Figure 2. Shows an alternative synthesis of symmetrical tetrazines. Reagents and conditions: i) NH ₂ (CH ₂ ) ₂ OH, MeCN, 12 h, rt; ii) Boc ₂ O, Et ₃ N, DCM, 12 h, rt; iii) Zn(OTf) _2, NH ₂ NH ₂ ^. H ₂ O, EtOH, 65 ºC, 24 h; iv) HCl, dioxane, rt, 4 h; v) t-Butyl bromoacetate, Et ₃ N, DMF, 50 ˚C, 12 h; vi) DAST, DCM, -78 ºC to rt, 4 h; vii) TFA, DCM, rt, 2 h; viii) MsCl, Et ₃ N, DMAP, DCM, rt, 12 h. Synthesis of 4-(((2-hydroxyethyl)amino)methyl)benzonitrile (3) To a solution of ethanolamine in DCM (60 mL) was added dropwise a solution of 4- (bromomethyl)benzonitrile (4 gr, 20.20 mmol) in DCM (20 mL). The reaction was stirred at rt for 1 h. The organic phase was then washed with water (3 x 30 mL), dried and concentrated under reduced pressure to give 3.55 g (99%) of the desired product as a white solid. Rf = 0.21 (DCM/MeOH 95/5); ¹ H NMR (400 MHz, CDCl ₃ ) δ 8.01 – 7.52 (m, 2H), 7.47 – 7.30 (m, 2), 4.05 – 3.76 (m, 2H), 3.74 – 3.43 (m, 2H), 2.73 (dtd, J = 10.7, 6.0, 1.7 Hz, 2H), 2.47 (s, 1H); ¹³ C NMR (101 MHz, CDCl ₃ ) δ 145.76, 132.22, 128.67, 118.88, 110.71, 60.91, 53.07, 50.78. Synthesis of tert-butyl (4-cyanobenzyl)(2-hydroxyethyl)carbamate (4) To a solution of 4-(((2-hydroxyethyl)amino)methyl)benzonitrile (1.62 g, 9.19 mmol) ^{and Et} _{3N (2.56 mL, 18.39 mmol) in DCM (30 mL) was added Boc2O (2.10 gr, 9.65} mmol). The reaction was stirred at room temperature for 12 h. The solution was then washed with water (50 mL) and K ₂ CO ₃ saturated solution (50 mL), dried over anhydrous Na2SO4, filtered and concentrated under reduced pressure to afford 2.36 g (93%) of the desired product as a crude (mixture of rotamers). Rf. = 0.4 (heptane/EtOAc 50/50); ¹ H NMR (600 MHz, CDCl ₃ ) δ 7.63 (d, J = 7.9 Hz, 2H), 4.55 – 4.52 (m, 2H), 3.74 (s, 2H), 3.51 – 2.94 (m, 2H), 2.69 (s, 1H), 1.76 – 0.53 (m, 9H); ¹³ C NMR (151 MHz, CDCl ₃ ) δ 156.91, 144.11, 132.43, 128.02, 127.50, 118.72, 111.19, 81.02, 62.17, 61.46, 52.10, 51.17, 50.37, 49.51, 28.32. Synthesis of 4-(((2-fluoroethyl)amino)methyl)benzonitrile (5) To a solution of 4-(bromomethyl)benzonitrile (0.78 g, 4.00 mmol) in CH ₃ CN (40 mL) was added K ₂ CO ₃ (0.33 g, 24.0 mmol) and 2-fluoroethylamine hydrochloride (0.16 g, 16.0 mmol). The mixture was stirred at room temperature overnight. The solvent was removed under reduced pressure, and the residue was diluted with water (20 mL), extracted with EtOAc. The combined organic layer was washed with brine, dried over MgSO ₄ , filtered and concentrated under reduced pressure. The crude product was purified by flash column chromatography using EtOAc (Heptane/EtOAc 50/50) in heptane to afford 0.54 g (76%) of the desired product as a colorless oil. Rf = 0.24 (Heptane/EtOAc 40/60). ¹ H NMR (400 MHz, CDCl ₃ ) δ 7.55 (d, J = 8.2 Hz, 2H), 7.40 (d, J = 8.0 Hz, 2H), 4.63 – 4.48 (m, 1H), 4.47 – 4.37 (m, 1H), 3.84 (s, 2H), 2.93 – 2.84 (m, 1H), 2.84 – 2.72 (m, 1H), 1.65 (s, 1H). ¹³ C NMR (101 MHz, CDCl ₃ ) δ 145.6, 132.3, 128.6, 118.9, 110.9, 83.5 (d, J = 165.5 Hz), 53.1, 49.1 (d, J = 19.7 Hz). Synthesis of tert-butyl 4-cyanobenzyl(2-fluoroethyl)carbamate (6) To a solution of 4-(((2-fluoroethyl)amino)methyl)benzonitrile (540 mg, 3.03 mmol) and Et ₃ N (1.27 mL, 9.09 mmol) in CH ₂ Cl ₂ (40 mL) was added Boc ₂ O (790 mg, 3.63 mmol) and the mixture was stirred at room temperature for 12 h. The solution was washed with water and saturated K ₂ CO ₃ solution, dried over Na ₂ SO _4, filtered and concentrated under reduced pressure. The crude product was purified by flash column chromatography using (Heptane/EtOAc 70/30) to afford 0.710 g (84%) of the desired product as a colorless oil (mixture of rotamers). Rf = 0.42 (Heptane/EtOAc 80/20). ¹ H NMR (400 MHz, CDCl ₃ ) δ 7.55 (d, J = 7.8 Hz, 2H), 7.27 (d, J = 7.8 Hz, 2H), 4.79 – 4.10 (m, 4H), 3.62 – 3.28 (m, 2H), 1.96 – 1.05 (m, 9H). ¹³ C NMR (101 MHz, CDCl ₃ ) δ 155.4, 144.2, 143.8, 132.4, 128.1, 127.5, 118.7, 111.1, 83.2 (d, J = 168.2 Hz), 82.7 (d, J = 170.5 Hz), 52.1, 51.2, 47.7, 28.3. Synthesis of di-tert-butyl (((1,2,4,5-tetrazine-3,6-diyl)bis(4,1- phenylene))bis(methylene))bis((2-fluoroethyl)carbamate) (7) and tert-butyl (4- (6-(4-(((tert-butoxycarbonyl)(2-fluoroethyl)amino)methyl)phe nyl)-1,2,4,5- tetrazin-3-yl)benzyl)(2-hydroxyethyl)carbamate (8) To a suspension of tert-butyl 4-cyanobenzyl(2-fluoroethyl)carbamate (1.1 gr, 3.95 mmol), tert-butyl (4-cyanobenzyl)(2-hydroxyethyl)carbamate (0.27 gr, 0.99 mmol) and Zn(OTf) ₂ (0.72 gr, 1.98 mmol) in EtOH (30 mL) was added hydrazine monohydrate (3.83 mL, 79 mmol). The mixture was allowed to stir at 70 ºC for 22 hours, and when the reaction is completed, is cooled at room temperature. The volatiles were removed under reduced pressure and the residue solubilized in EtOH (40 mL). A solution of NaNO ₂ (5.52 g, 80.00 mmol ) in water (20 mL) was added to the crude reaction followed by dropwise addition of HCl (2M) until gas evolution ceased and a pH of 2-3 was achieved producing a red mixture. The crude reaction was extracted with DCM (3 x 40 mL) and washed with brine (3 x 20 mL). The organic phase was collected, dried over MgSO ₄ , filtered and concentrated under reduced pressure. Purification by flash chromatography (DCM/MeOH 98/2) afforded 0.300 g (26%) of di-tert-butyl (((1,2,4,5-tetrazine-3,6-diyl)bis(4,1-phenylene))bis(methyle ne))bis((2- fluoroethyl)carbamate) as a red oil (mixture of rotamers). Rf = 0.45 (DCM/MeOH 98/2); ¹ H NMR (600 MHz, CDCl ₃ ) δ 8.63 (d, J = 8.0 Hz, 4H), 7.50 (d, J = 7.0 Hz, 4H), 5.11 – 4.38 (m, 8H), 3.97 – 3.26 (m, 4H), 1.95 – 0.57 (m, 18H); ¹³ C NMR (151 MHz, CDCl ₃ ) δ 163.77, 155.61, 155.57, 143.70, 130.81, 128.51, 128.21, 127.87, 83.18 (d, J = 167.9 Hz), 82.61 (d, J = 169.3 Hz), 72.44, 65.78, 52.09, 51.09, 47.67 (d, J = 19.9 Hz), 47.19 (d, J = 21.0 Hz), 28.38. 0.08 g (7%) of a second more polar fraction corresponding to (4-(6-(4-(((tert- butoxycarbonyl)(2-fluoroethyl)amino)methyl)phenyl)-1,2,4,5-t etrazin-3-yl)benzyl)(2- hydroxyethyl)carbamate was isolated as a red solid (mixture of rotamers). Rf = 0.22 (DCM/MeOH 98/2); ¹ H NMR (400 MHz, CDCl ₃ ) δ 8.54 (d, J = 7.9 Hz, 4H), 7.40 (d, J = 8.1 Hz, 4H), 4.79 – 4.34 (m, 8H), 3.52 – 3.25 (m, 5H), 1.38 (d, J = 8.8 Hz, 18H). Synthesis of N,N'-(((1,2,4,5-tetrazine-3,6-diyl)bis(4,1- phenylene))bis(methylene))bis(2-fluoroethan-1-amine) (9) Di-tert-butyl (((1,2,4,5-tetrazine-3,6-diyl)bis(4,1-phenylene))bis(methyle ne))bis((2- fluoroethyl) carbamate) (0.15 gr, 0.25 mmol) was treated with a solution of HCl (4 M) in dioxane (1 mL). A precipitate was formed. Filtration afforded 0.1 gr (83%) of the desired product as hydrochloride salt. ¹ H NMR (600 MHz, DMSO) δ 9.70 (s, 4H), 8.61 (d, J = 8.0 Hz, 4H), 7.89 (d, J = 8.0 Hz, 4H), 4.86 (t, J = 4.6 Hz, 2H), 4.78 (t, J = 4.6 Hz, 2H), 4.37 (s, 4H), 3.40 - 3.25 (m, 4H); ¹³ C NMR (151 MHz, DMSO) δ 163.63, 136.83, 132.81, 131.66, 128.28, 80.09 (d, J = 165.1 Hz), 50.24, 47.21 (d, J = 19.8 Hz). Synthesis of di-tert-butyl 2,2'-((((1,2,4,5-tetrazine-3,6-diyl)bis(4,1- phenylene))bis(methylene))bis((2-fluoroethyl)azanediyl))diac etate (10) To a suspension of the hydrochloride salt of N,N'-(((1,2,4,5-tetrazine-3,6-diyl)bis(4,1- phenylene))bis(methylene))bis(2-fluoroethan-1-amine) (0.09 gr, 0.20 mmol) in anhydrous DMF (3 mL) was added Et ₃ N (0.13 mL, 1.00 mmol). The reaction was stirred until all the precipitate went in solution. Tert-butyl bromo acetate (0.12 mL, 0.80 mmol) was added and the reaction stirred for 6 h at 50 ˚C. The mixture was cooled to room temperature and diluted with EtOAc (30 mL). The organic phase was washed with water (2 x 15 mL) and brine (2 x 15 mL). The organic phase was collected, dried over MgSO ₄ , filtered and concentrated under reduced pressure to give 0.11 gr (91%) of the desired compound as a red oil. Rf = 0.48 (Heptane/EtOAc 70/30); ¹ H NMR (400 MHz, CDCl ₃ ) δ 8.62 (d, J = 8.4 Hz, 4H), 7.65 (d, J = 8.2 Hz, 4H), 4.64 (t, J = 5.0 Hz, 2H), 4.53 (t, J = 5.0 Hz, 2H), 4.04 (s, 4H), 3.42 (s, 4H), 3.14 (t, J = 5.0 Hz, 2H), 3.07 (t, J = 5.0 Hz, 2H), 1.51 (s, 18H); ¹³ C NMR (101 MHz, CDCl ₃ ) δ 170.58, 163.82, 144.39, 130.80, 129.63, 128.02, 83.08 (d, J = 167.6 Hz), 81.19, 58.48, 55.69, 53.70 (d, J = 20.0 Hz), 28.22. Synthesis of 2,2'-((((1,2,4,5-tetrazine-3,6-diyl)bis(4,1- phenylene))bis(methylene))bis((2-fluoroethyl)azanediyl))diac etic acid (I) To a solution of di-tert-butyl 2,2'-((((1,2,4,5-tetrazine-3,6-diyl)bis(4,1-phenyl ene))bis(methylene))bis((2-fluoroethyl)azanediyl))diacetate (0.12 g, 0.19 mmol) in 5 mL of CH ₂ Cl ₂ was added 2 mL of TFA. The mixture was stirred at room temperature for 4 h. The solvent was then removed under reduced pressure. Purification by preparative HPLC afforded 0.70 g (50%) of the desired compound (TFA salt) as a red solid. ¹ H NMR (600 MHz, DMSO) δ 8.51 (d, J = 8.2 Hz, 4H), 7.67 (d, J = 7.9 Hz, 4H), 4.60 (t, J = 5.0 Hz, 2H), 4.53 (t, J = 5.0 Hz, 2H), 4.02 (s, 4H), 3.46 (s, 4H), 3.06 (s, 2H), 3.02 (s, 2H); ¹³ C NMR (151 MHz, DMSO) δ 172.57, 163.68, 144.56, 131.23, 130.11, 128.01, 82.92 (d, J = 165.3 Hz), 58.08, 54.66, 53.64 (d, J = 19.8 Hz). Synthesis of 2-((4-(6-(4-(((2-fluoroethyl)amino)methyl)phenyl)-1,2,4,5-te trazin-3- yl)benzyl)amino)ethan-1-ol (11) (4-(6-(4-(((Tert-butoxycarbonyl)(2-fluoroethyl)amino)methyl) phenyl)-1,2,4,5-tetrazin- 3-yl)benzyl)(2-hydroxyethyl)carbamate (0.06 gr, 0.1 mmol) was treated with a solution of HCl (4 M) in dioxane (1 mL). A precipitate was formed. Filtration afforded 0.035 gr (75%) of the desired product as hydrochloride salt. ¹ H NMR (600 MHz, DMSO) δ 9.64 (s, 4H), 8.61 (d, J = 8.3 Hz, 4H), 8.02 – 7.53 (m, 4H), 5.28 (s, 1H), 4.85 (t, J = 4.6 Hz, 1H), 4.77 (t, J = 4.6 Hz, 1H), 4.37 (s, 2H), 4.34 (s, 2H), 3.73 (s, 2H), 3.40 (t, J = 4.8 Hz, 1H), 3.35 (t, J = 4.8 Hz, 1H), 3.04 (s, 2H); ¹³ C NMR (151 MHz, DMSO) δ 163.63, 137.00, 136.86, 132.81, 132.74, 131.64, 128.29, 128.27, 80.12 (d, J = 165.3 Hz), 56.84, 50.27, 50.01, 49.12, 47.24 (d, J = 19.9 Hz). Synthesis of tert-butyl N-(4-(6-(4-(((2-(tert-butoxy)-2-oxoethyl)(2- fluoroethyl)amino) methyl)phenyl)-1,2,4,5-tetrazin-3-yl)benzyl)-N-(2- hydroxyethyl)glycinate (12) To a suspension of the hydrochloride salt of 2-((4-(6-(4-(((2- fluoroethyl)amino)methyl)phenyl)-1,2,4,5-tetrazin-3-yl)benzy l)amino)ethan-1-ol (0.03 gr, 0.077 mmol) in anhydrous DMF (3 mL) was added Et ₃ N (0.05 mL, 0.38 mmol). The reaction was stirred until all the precipitate went in solution. Tert-butyl bromo acetate (0.04 mL, 0.3 mmol) was added and the reaction stirred for 6 h at 50 ˚C. The mixture was cooled to room temperature and diluted with EtOAc (30 mL). The organic phase was washed with water (2 x 15 mL) and brine (2 x 15 mL). The organic phase was collected, dried over MgSO ₄ , filtered and concentrated under reduced pressure to give 0.03 gr (64%) of the desired compound as a red oil. Rf = 0.25 (Heptane/EtOAc 70/30); ¹ H NMR (400 MHz, CDCl ₃ ) δ 9.44 – 8.40 (m, 2H), 7.61 (dd, J = 9.6, 8.2 Hz, 2H), 4.62 (t, J = 5.0 Hz, 1H), 4.50 (t, J = 5.0 Hz, 1H), 4.02 (s, 2H), 3.96 (s, 2H), 3.63 (s, 2H), 3.39 (s, 2H), 3.29 (s, 2H), 3.23 (s, 1H), 3.11 (t, J = 5.0 Hz, 1H), 3.04 (t, J = 5.0 Hz, 1H), 2.91 (t, J = 5.1 Hz, 2H), 1.48 (s, 9H), 1.46 (s, 9H); ¹³ C NMR (151 MHz, CDCl ₃ ) δ 170.99, 170.60, 163.87, 163.75, 144.47, 143.63, 131.04, 130.76, 129.75, 129.64, 128.15, 128.05, 83.09 (d, J = 167.8 Hz), 81.71, 81.20, 59.12, 58.56, 58.48, 56.99, 55.71, 55.52, 53.70 (d, J = 20.1 Hz), 28.22, 28.12. Synthesis of tert-butyl N-(4-(6-(4-(((2-(tert-butoxy)-2-oxoethyl)(2- chloroethyl)amino) methyl)phenyl)-1,2,4,5-tetrazin-3-yl)benzyl)-N-(2- fluoroethyl)glycinate (XXIV) To a solution of compound tert-butyl N-(4-(6-(4-(((2-(tert-butoxy)-2-oxoethyl)(2- fluoroethyl)amino) methyl)phenyl)-1,2,4,5-tetrazin-3-yl)benzyl)-N-(2- hydroxyethyl)glycinate (0.04 g, 0.065 mmol) and DIPEA (0.034 mL, 0.19 mmol) in CH ₂ Cl ₂ (10 mL) were added mesyl chloride (0.01 g, 0.019 mmol) and DMAP (0.001 g, 0.01 mmol). The reaction was stirred at room temperature for 12 h. The solvent was removed under reduced pressure. Purification by flash chromatography (80/20 Heptane/EtOAc) afforded 0.020 (48%) of the desired product as a red solid. Rf = 0.55 (Heptane/EtOAc 70/30); ¹ H NMR (400 MHz, CDCl ₃ ) δ 8.60 (d, J = 8.1 Hz, 4H), 7.62 (dd, J = 8.4, 3.0 Hz, 4H), 4.62 (t, J = 5.0 Hz, 1H), 4.50 (t, J = 5.0 Hz, 1H), 4.11 – 3.89 (m, 4H), 3.57 (t, J = 6.8 Hz, 2H), 3.39 (s, 2H), 3.36 (s, 2H), 3.15-3.10 (m, 3H), 3.04 (t, J = 5.0 Hz, 1H), 1.48 (s, 18H); ¹³ C NMR (101 MHz, CDCl ₃ ) δ 170.51, 163.84, 163.81, 130.87, 130.79, 129.64, 129.56, 128.05, 128.03, 83.08 (d, J = 167.8 Hz), 81.35, 81.19, 58.48, 58.12, 55.96, 55.70, 55.50, 53.70 (d, J = 20.0 Hz), 28.23. MS Synthesis of di-tert-butyl 2,2'-((((1,2,4,5-tetrazine-3,6-diyl)bis(4,1-phenylene)) bis(methylene))bis((2-hydroxyethyl)azanediyl))diacetate (13) To a suspension of tert-butyl (4-cyanobenzyl)(2-hydroxyethyl)carbamate (4 gr, 14.47 mmol) and Zn(OTf) ₂ (2.63 gr, 7.23 mmol) in EtOH (40 mL) was added hydrazine monohydrate (14.04 mL, 289 mmol). The mixture was allowed to stir at 70 ºC for 22 hours, and when the reaction is completed, is cooled at room temperature. The volatiles were removed under reduced pressure and the residue solubilized in EtOH (80 mL). A solution of NaNO ₂ (19.97 g, 289.00 mmol ) in water (50 mL) was added to the crude reaction followed by dropwise addition of HCl (2M) until gas evolution ceased and a pH of 2-3 was achieved producing a red mixture. The crude reaction was extracted with DCM (3 x 60 mL) and washed with brine (3 x 50 mL). The organic phase was collected, dried over MgSO ₄ , filtered and concentrated under reduced pressure. Purification by flash chromatography (DCM/MeOH 95/5) afforded 1.2 g (28%) of the desired product as a red solid (mixture of rotamers). Rf = 0.21 (DCM/MeOH 95/5); ¹ H NMR (600 MHz, CDCl ₃ ) δ 8.63 (d, J = 7.9 Hz, 4H), 7.49 (d, J = 7.9 Hz, 4H), 4.62 (s, 4H), 3.79 (s, 4H), 3.60 – 3.37 (m, 4H), 3.01 (s, 2H), 1.48 (s, 18H); ¹³ C NMR (151 MHz, CDCl ₃ ) δ 163.74, 157.21, 156.07, 143.55, 130.84, 128.25, 127.88, 80.91, 62.19, 61.44, 52.12, 51.13, 50.28, 49.41, 28.39. Synthesis of di-tert-butyl 2,2'-((((1,2,4,5-tetrazine-3,6-diyl)bis(4,1-phenylene)) bis(methylene))bis((2-hydroxyethyl)azanediyl))diacetate (14) (4-(6-(4-(((tert-Butoxycarbonyl)(2-fluoroethyl)amino)methyl) phenyl)-1,2,4,5-tetrazin- 3-yl)benzyl)(2-hydroxyethyl)carbamate (0.9 gr, 1.55 mmol) was treated with a solution of HCl (4 M) in dioxane (15 mL). A precipitate was formed. Filtration afforded 0.68 gr (97%) of the desired product as hydrochloride salt. ¹ H NMR (400 MHz, DMSO) δ 9.41 (s, 4H), 8.59 (d, J = 8.4 Hz, 4H), 7.89 (d, J = 8.4 Hz, 4H), 5.29 (s, 2H), 4.33 (s, 4H), 3.74 (s, 4H), 3.04 (s, 4H); ¹³ C NMR (101 MHz, DMSO) δ 163.62, 137.03, 132.71, 131.65, 128.23, 56.84, 49.98, 49.14. Synthesis of di-tert-butyl 2,2'-((((1,2,4,5-tetrazine-3,6-diyl)bis(4,1- phenylene))bis(methylene))bis((2-hydroxyethyl)azanediyl))dia cetate (15) To a suspension of the hydrochloride salt of di-tert-butyl 2,2'-((((1,2,4,5-tetrazine-3,6- diyl)bis(4,1-phenylene)) bis(methylene))bis((2-hydroxyethyl)azanediyl))diacetate (0.25 gr, 0.55 mmol) in anhydrous DMF (10 mL) was added Et ₃ N (0.38 mL, 2.75 mmol). The reaction was stirred until all the precipitate went in solution. Tert-butyl bromo acetate (0.32 mL, 2.20 mmol) was added and the reaction stirred for 6 h at 50 ˚C. The mixture was cooled to room temperature and diluted with EtOAc (40 mL). The organic phase was washed with water (2 x 15 mL) and brine (2 x 15 mL). The organic phase was collected, dried over MgSO ₄ , filtered and concentrated under reduced pressure to give 0.31 gr (91%) of the desired compound as a red oil. Rf = 0.23 (Heptane/EtOAc 60/40); ¹ H NMR (400 MHz, CDCl ₃ ) δ 8.28 (d, J = 8.0 Hz, 4H), 7.27 (d, J = 8.0 Hz, 4H), 3.62 (s, 4H), 3.31 (t, J = 5.2 Hz, 4H), 2.96 (s, 4H), 2.58 (t, J = 5.1 Hz, 4H), 1.14 (s, 18H); ¹³ C NMR (101 MHz, CDCl ₃ ) δ 171.14, 163.75, 143.96, 130.90, 129.66, 128.11, 81.56, 59.21, 58.57, 56.95, 55.67. Synthesis of di-tert-butyl 2,2'-((((1,2,4,5-tetrazine-3,6-diyl)bis(4,1- phenylene))bis(methylene))bis((2-fluoroethyl)azanediyl))diac etate (10) and di- tert-butyl 2,2'-((((1,2,4,5-tetrazine-3,6-diyl)bis(4,1- phenylene))bis(methylene))bis((2-hydroxyethyl)azanediyl))dia cetate (12) To a solution of di-tert-butyl 2,2'-((((1,2,4,5-tetrazine-3,6-diyl)bis(4,1- phenylene))bis(methylene))bis((2-hydroxyethyl)azanediyl))dia cetate (0.24 g, 0.39 mmol) in anhydrous THF (15 mL) at -78 ˚C was added DAST (0.055 mL, 0.39 mmol). The resulting mixture was stirred for 1 hour at -78 ˚C and additional 3 hours at room temperature. Subsequently the reaction was quenched with NaHCO ₃ saturated solution (10 mL) and stirred for 30 minutes. The reaction mixture was extracted with DCM (3 x 30 mL) and washed with brine (3 x 30 mL). The organic phase was collected, dried over MgSO ₄ , filtered and concentrated under reduced pressure. Purification by flash chromatography (DCM/MeOH 95/5) afforded 0.12 g (50%) of di- tert-butyl 2,2'-((((1,2,4,5-tetrazine-3,6-diyl)bis(4,1-phenylene))bis(m ethylene))bis((2- fluoroethyl)azanediyl))diacetate. A more polar fraction corresponding to di-tert-butyl 2,2'-((((1,2,4,5-tetrazine-3,6-diyl)bis(4,1-phenylene))bis(m ethylene))bis((2- hydroxyethyl)azanediyl))diacetate was isolated in 0.04 g (14%). Example 2 Synthesis of enantiopure TCO Scheme 5. Synthesis of isomer-free TCO. i) mCPBA, THF, H ₂ O, 0 °C→RT, 17 h, 47%; ii) LiAlH ₄ , THF, 0 °C→RT, 12 h, 99%; iii) a) Et ₃ N, DMAP, CH ₂ Cl ₂ , 0 °C→RT, 12 h, 51%; iv) Crystallization from pentane, 41%; v) NaOH, THF, H ₂ O, reflux, 2 h, 52%; vi) AgNO ₃ , hV, rt, 8 h, 44%. (Z)‐9‐Oxabicyclo[6.1.0]non‐4‐ene (16) Cis,cis-1,5-cyclooctadiene (22.0 g, 203.36 mmol, 1.00 equiv.) and dry CH ₂ Cl ₂ (300 mL) were added to a 500 mL round-bottom flask. The mixture was cooled to 0 °C with an ice bath and mCPBA (45.57 g, 203.36 mmol, 1.00 equiv.) was added portion wise to give a white suspension. The mixture was allowed to reach room temperature and left stirring overnight. The mixture was filtered and washed with NaHCO ₃ saturated solution (3 x 100 mL) and NaCl saturated solution (1 x 100 mL). The organic layer was collected, dried with MgSO ₄ , filtered and concentrated under reduced pressure. Purification by flash chromatography (n-heptane/EtOAc, 90:10) yielded (Z)‐9‐ oxabicyclo[6.1.0]non‐4‐ene (11.82 g, 95.16 mmol, 47%) as a colorless oil. ¹ H NMR (600 MHz, CDCl3) δ 5.69 – 5.48 (m, 2H), 3.15 – 2.91 (m, 2H), 2.55 – 2.35 (m, 2H), 2.21 – 2.08 (m, 2H), 2.08 – 1.86 (m, 4H); ¹³ C NMR (151 MHz, CDCl ₃ ) δ 129.00, 56.87, 28.25, 23.82. (Z)‐Cyclooct‐4‐enol (17) Lithium aluminum hydride tablets (3.26 g, 85.93 mmol, 3.00 equiv.) were added to an oven-dried 500 mL three-necked round-bottom flask. The flask was sealed and flushed with argon. The flask was cooled to 0 °C using an ice-bath and dry THF (120 mL) was added slowly while vigorously stirring to give a grey suspension.1,2-Epoxy- 5-cyclooctene (3.56 g, 28.64 mmol, 1.00 equiv.) in dry THF (10 mL) was added dropwise and the mixture was allowed to reach room temperature and stirred overnight. The mixture was cooled to 0 °C in an ice bath and quenched with EtOAc (120 mL). A saturated solution of Rochelle salt (100 mL) was added, and the mixture was stirred vigorously for 10 minutes. The mixture was transferred to a separatory funnel and the organic layer was collected. The aqueous layer was extracted with DCM (3 x 150 mL). The combined organic layers were washed with H ₂ O (200 mL), dried over MgSO ₄ , filtered and concentrated under reduced pressure to give (Z)‐ Cyclooct‐4‐enol (3.49 g, 28.45 mmol, 99%). ¹ H NMR (600 MHz, CDCl ₃ ) δ 5.75 – 5.63 (m, 1H), 5.61 – 5.52 (m, 1H), 3.86 – 3.75 (m, 1H), 2.36 – 2.24 (m, 1H), 2.20 – 2.04 (m, 3H), 1.97 (s, 1H), 1.93 – 1.88 (m, 1H), 1.86 – 1.81 (m, 1H), 1.75 – 1.68 (m, 1H), 1.67 – 1.59 (m, 1H), 1.56 – 1.46 (m, 2H); ¹³ C NMR (151 MHz, CDCl ₃ ) δ 130.23, 129.63, 72.85, 37.75, 36.36, 25.75, 24.97, 22.88. (Z)-cyclooct-4-en-1-yl (1R,4S)-4,7,7-trimethyl-3-oxo-2-oxabicyclo[2.2.1]heptane- 1-carboxylate (Epimers mixture) (18) (±)-(Z)-Cyclooct-4-enol (4.5 g, 35.65 mmol) was dissolved in dry CH ₂ Cl ₂ (100 mL), to which was added DMAP (0.87 g, 7.13 mmol) and Et ₃ N (14.9 mL, 106.97 mmol). The solution was cooled to 0°C and (1S)-(-)-camphanic chloride (8.5 g, 38.22 mmol) was added portion wise to the mixture. The resulting solution was allowed to stir at room temperature for 17 hours. The mixture was washed with NaHCO ₃ saturated solution (3 x 100 mL) and NaCl saturated solution (1 x 100 mL). The organic layer was collected, dried with MgSO ₄ , filtered and concentrated under reduced pressure to give 5.63 g (51%) of a mixture of the desired epimers. Recrystallization from pentane afforded 2.31 g (41%) of (Z)-cyclooct-4-en-1-yl (1R,4S)-4,7,7-trimethyl-3-oxo-2- oxabicyclo[2.2.1]heptane-1-carboxylate as crystals (needles). ¹ H NMR (400 MHz, CDCl ₃ ) δ 5.75 – 5.58 (m, 2H), 5.05 – 4.94 (m, 1H), 2.46 – 2.29 (m, 2H), 2.28 – 2.08 (m, 3H), 2.06 – 1.84 (m, 4H), 1.84 – 1.74 (m, 1H), 1.74 – 1.33 (m, 4H), 1.10 (s, 3H), 1.04 (s, 3H,). 0.95 (s, 3H); ¹³ C NMR (101 MHz, CDCl ₃ ) δ 178.46, 166.97, 130.05, 129.99, 129.59, 129.56, 91.23, 54.96, 54.21, 33.87, 33.85, 33.81, 30.69, 29.12, 29.10, 25.67, 25.65, 24.92, 24.88, 22.37, 22.35, 17.01, 16.95, 16.88, 9.84. Figure 3. shows the X-ray crystal structures of (S,Z)-cyclooct-4-en-1-yl (1R,4S)-4,7,7- trimethyl-3-oxo-2-oxabicyclo[2.2.1]heptane-1-carboxylate (18). (S,Z)-cyclooct-4-en-1-ol (19) (S,Z)-cyclooct-4-en-1-yl (1R,4S)-4,7,7-trimethyl-3-oxo-2-oxabicyclo[2.2.1]heptane-1- carboxylate (0.26 g, 0.85 mmol) was dissolved in THF (10 mL), to which was added a solution of NaOH (0.2 g, 4.24 mmol) in H ₂ O (1 mL). The reaction was vigorously stirred at reflux for 2 hours. The mixture was quenched with H ₂ O (10 mL) and the aqueous layer was extracted with DCM (3 x 30 mL). The combined organic layers were washed with H ₂ O (20 mL), dried over MgSO ₄ , filtered and concentrated under reduced pressure to give (S,Z)‐Cyclooct4enol (0.056 g, 52%) as a colorless oil. ¹ H NMR (600 MHz, CDCl ₃ ) δ 5.75 – 5.63 (m, 1H), 5.61 – 5.52 (m, 1H), 3.86 – 3.75 (m, 1H), 2.36 – 2.24 (m, 1H), 2.20 – 2.04 (m, 3H), 1.97 (s, 1H), 1.93 – 1.88 (m, 1H), 1.86 – 1.81 (m, 1H), 1.75 – 1.68 (m, 1H), 1.67 – 1.59 (m, 1H), 1.56 – 1.46 (m, 2H); ¹³ C NMR (151 MHz, CDCl ₃ ) δ 130.23, 129.63, 72.85, 37.75, 36.36, 25.75, 24.97, 22.88. (S,E)-cyclooct-4-en-1-ol (20) A flash cartridge (220g, screw top, luer lock end fittings, Cat# FCSTLL-220-6) was packed with 8 cm silica (15-40µm) on the bottom and silver nitrate impregnated silica until the top. The column was flushed with 9:1 diethyl ether/n-heptane (500 mL) and the column was protected from light with aluminium foil. The cooling fence and UV lamps were turned on and after 10 minutes no detection of silver leakage was observed. Methyl benzoate (1 mL), (S,Z)-cyclooct-4-en-1-ol (1 g) and an additional 50 mL 9:1 diethyl ether/n-heptane solution were added to a round-bottom flask. The mixture was then added to the quartz flask. The pump was turned on (flowrate = 100 mL/min) and the photoreactor was turned on and photoisomerization was conducted for 8 hours. After 8 hours, the photoreactor was turned off and the column was dried by a stream of air. The silica was removed from the column and washed with 400 mL ammonia and 400 mL DCM. The mixture was stirred for 30 minutes, filtered and the organic layer was collected. The organic layer was washed with brine, dried with MgSO4, filtered and concentrated to give 0.44 g (44%) of the product as a yellowish oil (mixture of axial and equatorial isomers). Major equatorial ¹ H NMR (400 MHz, CDCl ₃ ) δ 5.57 (ddd, J = 15.3, 10.6, 4.1 Hz, 1H), 5.38 (ddd, J = 15.7, 10.9, 3.6 Hz, 1H), 3.50 – 3.40 (m, 1H), 2.32 (tq, J = 17.3, 5.1 Hz, 3H), 2.01 – 1.87 (m, 4H), 1.74 – 1.50 (m, 3H).Minor axial ¹ H NMR (400 MHz, CDCl ₃ ) δ 6.02 – 5.19 (m, 2H), 4.48 – 3.85 (m, 1H), 2.37 (dddd, J = 15.2, 10.5, 7.0, 4.8 Hz, 1H), 2.30 – 1.96 (m, 4H), 2.00 – 1.75 (m, 3H), 1.65 (dddd, J = 14.4, 12.8, 5.0, 1.7 Hz, 1H), 1.26 (dddd, J = 13.6, 10.9, 3.5, 0.9 Hz, 1H). Example 3 Ligation combination i) and oxidation Scheme 6. (S)-1,4-di(pyridin-2-yl)-5,6,7,8,9,10-hexahydrocycloocta[d]p yridazin-7-ol. 3,6-di(Pyridin-2-yl)-1,2,4,5-tetrazine was obtained as reported in Polezhaev, A. V.; Maciulis, N. A.; Chen, C.-H.; Pink, M.; Lord, R. L.; Caulton, K. G. Tetrazine Assists Reduction of Water by Phosphines: Application in the Mitsunobu Reaction. Chemistry – A European Journal 2016, 22, 13985-13998. (S,E)-cyclooct-4-en-1-ol (20 mg, 0.18 mmol) was dissolved in mixture of MeCN/H ₂ O (4/1 v/v, 5 mL), to which was added 21 (37 mg, 0.18 mmol). The reaction was stirred for 10 minutes at rt. p-Chloranil (80 mg, 0.36 mmol) was added and the reaction was stirred for 10 min. The solution was concentrated under reduced pressure and purified by flash chromatography to give 45 mg (0.13 mmol) of the desired product. ¹ H NMR (400 MHz, CDCl ₃ ) δ 8.71 (dd, J = 5.0, 1.5 Hz, 2H), 8.25 – 7.70 (m, 4H), 7.60 – 7.32 (m, 2H), 4.06 – 3.61 (m, 1H), 3.30 – 2.92 (m, 4H), 2.19 (ddt, J = 13.5, 8.7, 4.1 Hz, 1H), 2.03 – 1.84 (m, 3H), 1.83 – 1.57 (m, 2H); ¹³ C NMR (101 MHz, CDCl ₃ ) δ 158.77, 158.58, 156.68, 148.53, 148.49, 140.68, 140.39, 136.95, 136.88, 125.13, 125.06, 123.47, 123.41, 71.98, 39.00, 35.98, 26.98, 26.19, 23.91, 23.45. Example 4 Synthesis of cis,5,6-disubstituted-TCOs Scheme 7. Synthesis of Cis-cyclooct-5-ene-1,2-diol i)NMO, OsO ₄ , THF, H ₂ O, acetone, 0° to rt, 12 h, 93% ii) AgNO ₃ , hV, rt, 8 h, 0 °C to RT, 12 h, 51%; Cis-Z-cyclooct-5-ene-1,2-diol (22) To a stirred mixture of 1,5-cyclooctadiene (1 1.22 mL, 12.9 mmol), 4- methylmorpholine N-oxide (1.87 g, 13.75 mmol) and THF: H2 O : acetone (1:1:1) (90 mL) at 0 °C was added osmium tetroxide (12.7 mg, 0.05 mmol). After 12 h at 25 °C the reaction mixture was poured into an aqueous saturated solution of NaHSO ₃ (60 mL), extracted with EtOAc (3 x 150 mL), washed with water (2 x 50 mL) and brine (50 mL). Drying (MgSO ₄ ) and concentration followed by flash chromatography (silica, 30% EtOAc in heptane) afforded the desired compound 1.70 g (93%). ¹ H NMR (400 MHz, CDCl ₃ ) δ 5.72 – 5.61 (m, 2H), 4.03 – 3.96 (m, 2H), 2.56 – 2.44 (m, 2H), 2.10 – 1.96 (m, 4H), 1.86 – 1.74 (m, 2H); ¹³ C NMR (101 MHz, CDCl ₃ ) δ 130.09, 75.18, 32.08, 23.11. Cis-E-cyclooct-5-ene-1,2-diol (23) A flash cartridge (220g, screw top, luer lock end fittings, Cat# FCSTLL-220-6) was packed with 8 cm silica (15-40µm) on the bottom and silver nitrate impregnated silica until the top. The column was flushed with 9:1 diethyl ether/n-heptane (500 mL) and the column was protected from light with aluminium foil. The cooling fence and UV lamps were turned on and after 10 minutes no detection of silver leakage was observed. Methyl benzoate (1 mL), cis-Z-cyclooct-5-ene-1,2-diol (1 g) and an additional 50 mL 9:1 diethyl ether/n-heptane solution were added to a round-bottom flask. The mixture was then added to the quartz flask. The pump was turned on (flowrate = 100 mL/min) and the photoreactor was turned on and photoisomerization was conducted for 8 hours. After 8 hours, the photoreactor was turned off and the column was dried by a stream of air. The silica was removed from the column and washed with 400 mL ammonia and 400 mL DCM. The mixture was stirred for 30 minutes, filtered and the organic layer was collected. The organic layer was washed with brine, dried with MgSO4, filtered and concentrated to give 0.51 g (51%) of the product as a colorless oil. Example 5 Ligation iv) between a symmetric tetrazine and cis,5,6-disubstitutedTCOs Scheme 8. (cis)-1,4-di(pyridin-2-yl)-5,6,7,8,9,10-hexahydrocycloocta[d ]pyridazine-7,8-diol (24) 23 (40 mg, 0.28 mmol) was dissolved in mixture of MeCN/H ₂ O (4/1 v/v, 5 mL), to which was added 3,6-di(pyridin-2-yl)-1,2,4,5-tetrazine (66 mg, 0.28 mmol). The reaction was stirred for 30 minutes at room temperature. p-Chloranil (117 mg, 0.47 mmol) was added, and the reaction was stirred for 10 min. After which, the reaction was diluted with H ₂ O (50 mL) and filtered. The solution was concentrated under reduced pressure and purified by flash chromatography to give 80 mg (82%) of the desired product. ¹ H NMR (400 MHz, CDCl ₃ ) δ 9.47 – 8.40 (m, 2H), 7.92 (d, J = 8.0 Hz, 2H), 7.85 (dd, J = 7.7, 1.8 Hz, 2H), 7.37 (ddd, J = 7.5, 4.8, 1.3 Hz, 2H), 3.40 – 3.25 (m, 2H), 3.01 (q, J = 5.3 Hz, 4H), 2.34 (ddd, J = 11.5, 8.2, 4.7 Hz, 2H), 1.86 (dt, J = 15.8, 8.1 Hz, 2H); ¹³ C NMR (101 MHz, CDCl ₃ ) δ 158.55, 156.40, 148.60, 139.90, 136.96, 125.01, 123.55, 72.91, 35.43, 23.50. Radiolabelling General methods: All reagents and solvents were purchased from ABX, Sigma Aldrich, Fluorochem and VWR and used as received, without further purification, unless stated otherwise. Dry THF and DCM were obtained from a SG Water solvent purification system and dry dimethyl sulfoxide (DMSO), MeCN, pyridine and methanol (MeOH) were purchased from commercial suppliers. Room temperature corresponds to a temperature interval from 18–21 ˚C. Reactions requiring anhydrous conditions were carried out under inert atmosphere (nitrogen) and using oven-dried glassware (152 ˚C). NMR ( ¹ H, ¹³ C) spectra were acquired on a 600 MHz Bruker Avance III HD, a 400 MHz Bruker Avance II or a Bruker AC200. Thin-layer chromatography (TLC) was run on silica plated aluminum sheets (Silica gel 60 F254) from Merck and the spots were visualized by ultraviolet light at 254 nm, by anisaldehyde and/or by potassium permanganate staining. Example 6 ¹⁸ F Radiolabeling of symmetrical tetrazines Scheme 9. Radiolabeling of symmetrical tetrazines. i) [ ¹⁸ F]Bu ₄ NF/Bu ₄ NOMs PO ₄ ^3- , t- BuOH/DMSO, 100 ºC, 5 min; ii) TFA, CH ₃ CN, 80 ºC, 10 min. Symmetrical ¹⁸ F-labeled tetrazine [ ¹⁸ F]I was prepared as follows: The aqueous [ ¹⁸ F]fluoride solution received from the cyclotron was passed through a preconditioned anion exchange resin (Sep-Pak Light QMA cartridge). The QMA was preconditioned by flushing it with 10 mL 0.5 M K ₃ PO ₄ and washing it with 10 mL H ₂ O afterwards. [ ¹⁸ F]F- was eluted from the QMA into a 4 mL v-shaped vial with 1 mL Bu ₄ NOMs dissolved in MeOH. The eluate was dried at 100 °C for 5 min under N ₂ - flow. Precursor XIII (9.3 µmol, 6 mg) was dissolved in 167 µL DMSO and then diluted with 833 µL tBuOH. The solution was added to the dried [18F]fluoride solution and allowed to react for 5 min at 100 °C. The reaction was cooled to 50 °C with air before addition of 3 mL H ₂ O. Radiochemical conversion (RCC) determined by radio-HPLC after the first step was 54%. The crude mixture was applied to a Sep-pak plus C18 solid phase extraction (SPE) cartridge that was preconditioned by flushing it with 10 mL EtOH followed by 10 mL of H ₂ O. The SPE was flushed with another 5 mL of H ₂ O and dried with N ₂ . The product was eluted from the SPE with 2 mL MeCN into a 7 mL v-shaped vial containing 600 µL TFA. This mixture was reacted for 10 min at 80 °C. The RCC of [ ¹⁸ F]I determined by radio-HPLC was 95% (Figure 4). Radio-HPLC was performed on a Luna 5 µm C18(2) column (150 × 4.6 mm) using a gradient of acetonitrile (CH ₃ CN) in water with 0.1% TFA. Gradient conditions: 0 min – 0% CH ₃ CN, 0-10 min – linear increase of CH ₃ CN content to 100%, 10-12 min – 100% CH ₃ CN, 12-13 min - linear decrease of CH ₃ CN content to 0%, 13-15 min – 0% CH ₃ CN, elution speed 2 mL/min. Figure 4. shows the Radio-HPLC of [ ¹⁸ F]1 at end of deprotection. Example 7 ¹⁸ F Radiolabeling of unsymmetrical tetrazines Unsymmetrical ¹⁸ F-labeled tetrazine [ ¹⁸ F]X was prepared from the nosyl precursor XXXVI via nucleophilic substitution as disclosed in Battisti, U.M.; Bratteby, K.; Jørgensen, J.T.; Hvass, L.; Shalgunov, V.; Mikula, H.; Kjær, A.; Herth, M.M. Development of the First Aliphatic 18F-Labeled Tetrazine Suitable for Pretargeted PET Imaging—Expanding the Bioorthogonal Tool Box. J. Med. Chem. 2021, 64, 15297–15312 Unsymmetrical ¹⁸ F-labeled tetrazine [ ¹⁸ F]XII was prepared from a trimethylstannyl precursor XXXVIII as disclosed in García-Vázquez, R.; Battisti, U.M.; Jørgensen, J.T.; Shalgunov, V.; Hvass, L.; Stares, D.L.; Petersen, I.N.; Crestey, F.C.; Löffler, A.; Svatunek, D.; et al. Direct Cu-mediated aromatic 18F-labeling of highly reactive tetrazines for pretargeted bioorthogonal PET imaging. Chem. Sci.2021, 12, 11668– 11675 Example 8 Screening of oxidants for the oxidation of the dihydropyridazines to pyridazines, yielding single end-products Figure 5 shows the reaction between a symmetrical tetrazine and an enantiopure TCO. The cycloaddition is completed within 5 minutes to give several isomers. The oxidants is then added to give the final single isomeric product. Each oxidant (5 equivalents) was added to the mixture and the reaction was analyzed by HPLC-MS after 10 minutes. The results are shown in Figure 5. These screening tests surprisingly showed that not all oxidants could be applied to provide a single isomeric form of the tetrazine-TCO pyridazine. Example 9 Provision of single isomeric tetrazine-TCO pyridazines The following oxidation conditions was used in this example: Tz (1 equiv) was dissolved in a mixture of EtOH to which was added a solution containing TCO-OH (1.5 equiv) in a mixture of EtOH. The reaction was stirred for 5 min, followed by the addition of Oxidant (5 equiv). This reaction was stirred for 10 min and subsequently analysed by analytical HPLC. Figure 6a-b. shows the result of the HPLC-MS analysis after oxidation confirming that only one single product was obtained. Compatibility of targeting vectors with oxidation conditions: In order to test whether the conditions leading to the oxidation of click product will not lead to the degradation of typical targeting vectors, we subjected a series of vectors relevant for theranostic radiopharmaceutical development to oxidation conditions previously shown to result in efficient conversion of dihydropyridazines to single- product pyridazines. Structures of tested vectors are shown in Figure 7. Vector oxidation test procedure: solution of targeting vector (70 µM) and oxidant (350 µM, 5 eq) in EtOH/water mixture (89-94% EtOH v/v) was stirred for 10 min at 25°C and subsequently analysed by analytical HPLC and LC/ESI-MS. The results are shown in Figure 8. None of the tested compounds showed oxidation and/or degradation meaning that they do not react with oxidants. The results showed that these vectors are compatible with the tested oxidants. Example 10 Measurement of second-order rate constants The second-order rate constant of all the click reactions made during the previous examples were measured by stopped-flow spectrometry in phosphate-buffered saline (PBS) at 25 °C in accordance with the method described in Battisti et al. J. Med. Chem. 2021, 64, 20, 15297–15312 (see page 15310 for experimental details and influencing factors). In short, stopped-flow measurements were performed using an SX20-LED stopped-flow spectrophotometer (Applied Photophysics) equipped with a 535 nm LED (optical pathlength 10 mm and full width half-maximum 34 nm) to monitor the characteristic tetrazine visible light absorbance (520−540 nm). The reagent syringes were loaded with a solution of axial-TCO-PEG ₄ , and the instrument was primed. The subsequent data were collected in triplicate for each tetrazine. Reactions were conducted at 25 °C in PBS and recorded automatically at the time of acquisition. The data sets were analyzed by fitting an exponential decay using Prism 6 (GraphPad) to calculate the observed pseudo-first-order rate constants that were converted to second-order rate constants by dividing with the concentration of the excess TCO compound. Only reactions that showed a minimum second-order rate constant of 500 M ^-1 s ^-1 in phosphate-buffered saline at 25 °C are considered suitable for providing the sufficient speed kinetics and therefore, reactions wherein the reaction kinetics was lower were disregarded for the purpose of the method according to the invention.