Description

Field of the Invention

[0001] The present invention relates to novel compounds of general formula I

X wherein X , Y, and R ¹ - R ⁵ and n have the meanings given in the description and claims, process for preparing these compounds and use thereof

Background Art

[0002] Fluorescent probes are powerful tools that are widely used in various research fields such as biology, biochemistry, chemical biology and medical diagnostics. Among their multiple usages, they are used to identify protein location and their conformational changes and to monitor in vivo processes.

[0003] Several families of fluorescent dyes have been developed and commercialized in the past few years covering the whole color spectra. They are designed to be easily cross-linked to the molecules or proteins of interest commonly via amide coupling or click chemistry.

[0004] Imidazo-pyridines have also been reported to be fluorescence active. The imidazo-pyridine core is found in many drugs and biologically active molecules (Kazmierczak, A. et al., J. Med. Chem. 2017, 60, 8781; Silva, D. G. et al., ACS Med. Chem. Lett. 2017, 8, 766; Sayeed, I. B. et al., Med. Chem. Commun. 2017, 8, 1000; Enguehard-Gueiffier, C. and Gueiffier, A. Mini-Rev. Med. Chem. 2007, 7, 888). As the neutral heterocycles, the corresponding salts do not only display biological activity but also proved to be fluorescence active. In the last decade, several research groups described the synthesis of imidazo-pyridines with interesting photochemical properties. They can be divided into two subcategories, the one embedding an imidazo[l,5-a] pyridine core and the one embedding an im!dazo[l,2- a]pyridine core. [0005] A novel route to Mused thiazole compounds starting from a phenacylpyridinium salt and KSCN is described by Babaev E. V. et al. (Russian chemical Bulletin International Edition Seriya Khimicheskaya, vol 171 (1), 2005, pp 231-237). The various activities of aminothiazolopyridinium salts were tested and described by Babaev E. V. et al. (Tetrahedron Letters, vol 40(42), 1999, pp 7553- 7556). Zhang Tongxin et al. (Dyes and Pigments, vol 171, 2019) have disclosed fluorescence-enhanced chemosensors for imaging living cells in extreme acidity. A new family of azonia aromatic heterocycles comprising imidazopyridinium cations and their use as DNA intercalators is described by Bosch P. et al. (Dyes and Pigments, vol 138, 2016, pp 135-146).

[0006] However, most of the so far developed fluorescent dyes show some limitations. They are synthetically challenging to make, poorly modular and are relatively expensive for the final customer. Thus, there is still a need for new and modular fluorescent dyes and approaches for synthesizing them. It is an aim of the present invention to provide a new pyridinium core with photophysical properties tunable over the whole color spectrum by post functionalization of the core. Summary of invention

[0007] It has surprisingly been found, that compounds of general formula I, wherein X , Y, and R ¹ - R ⁵ and n have the meanings below, exhibit photophysical properties and show fluorescent activity. Their photophysical properties can be easily tuned to cover the whole color spectrum by post functionalization of the pyridinium core and they can be easily functionalized to be later cross-linked to the molecules or proteins of interest. Described is also a modular and combinatorial synthesis of fluorescent poly-substituted imidazo[l,2-a] pyridinium salts and their analogs.

[0008] Various objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of embodiments of the invention.

Brief description of drawings

[0009] Fig 1: Solvatochromic effect of compound 4aj.

[0010] Fig 2: pH effect of compound 4aj. Description of embodiments

[0011] The present invention relates to compounds of general formula (I), wherein

X denotes an anion,

Y denotes N-R ¹ , 0, or S,

R ¹ , R ² , R ³ , R ⁴ , and R ⁵ denote R ^a or R ^b , or

R ² and R ³ together with the nitrogen atom form a 4- to 10-membered heterocyclic group, which optionally may be substituted by one or more, identical or different R ^a and/or R ^b n denotes 0, 1, 2, 3, or 4, and each R ^a independently of one another denotes hydrogen or a group, optionally substituted by one or more, identical or different R ^b and/or R ^c , selected from among C^ _! oalkyl, C _2-10 alkenyl, C _2-10 a I ky ny I , C _3„10 cycloalkyl, C _4„16 cycloalkylalkyl,

C _6-14 aryl, C _7„16 a ry la I ky 1 , 3-18 membered heterocyclyl, 4-22 membered heterocyclyla I ky 1 , 4-22 membered heterocyclylalkenyl, 5-12 membered heteroaryl and 6-18 membered heteroary lal ky I ; each R ^b is a suitable substituent and is selected in each case independently of one another from among =0, -OR ^c , C^haloalkyloxy, -OCF ₃ , =S, -SR ^C , =NR ^C , =NOR ^c ,

= NNR ^C R ^C , =NN(R ^g )C(0) NR ^c R ^c , -NR ^C R ^C , -ONR ^c R ^c , -N(OR ^c )R ^c , -N(R ^g ) NR ^c R ^c , halogen, -

CF ₃ , -ON, -NO, -OCN, -SON, -NO, -NO ₂ , =N ₂ , -N ₃ , -S(0)R ^c ,

-S(0)OR ^c , -S(0) ₂ R ^c , -S(0) ₂ OR ^c , -S(0) N R ^C R ^c , -S(0) ₂ N R ^C R ^c , -OS(0)R ^c ,

-OS(0) ₂ R ^c , -OS(0) ₂ OR ^c , -OS(0) N R ^C R ^c , -OS(0) ₂ N R ^C R ^c , -C(0)R ^c , -C(0)OR ^c ,

-C(0)SR ^c , -C(0) N R ^C R ^c , -C(0)NR ^c [(CH ₂ ) _n 0] _m R ^c , -C(0) N(R ^g )NR ^c R ^c , -C(0)N(R ^g )OR ^c , -C(NR ^g ) NR ^c R ^c , -C(NOH) R ^c , -C(NOH) N R ^C R ^c , -OC(0) R ^c , -OC(0)OR ^c , -OC(0)SR ^c , -OC(0) N R ^C R ^c , -OC(NR ^g )NR ^c R ^c , -SC(0) R ^c , -SC(0)OR ^c , -SC(0)NR ^c R ^c ,

-SC(NR ^g ) N R ^C R ^C , -N(R ^g )C(0) R ^c , -N [C(O) R ^c ] ₂ , -N (OR ^g )C(0) R ^c , -N(R ^g )C(NR ^g ) R ^c , -N(R ^g ) N(R ^g )C(0)R ^c , -N[C(0) R ^c ] NR ^c R ^c , -N(R ^g )C(S) R ^c , -N(R ^g )S(0) R ^c ,

-N(R ^g )S(0)OR ^c , -N(R ^g )S(0) ₂ R ^c , -N[S(0) ₂ R ^c ] ₂ , -N(R ^g )S(0) ₂ OR ^c , -N(R ^g )S(0) ₂ NR ^c R ^c , -N(R ^g )[S(0) ₂ ] ₂ R ^c , -N(R ^g )C(0)0R ^c , -N(R ^g )C(0)SR ^c , -N(R ^g )C(0)NR ^c R ^c , -N(R ^g )C(0)NR ^g NR ^c R ^c , -N(R ^g )N(R ^g )C(0)NR ^c R ^c ,

-N(R ^g )C(S)NR ^c R ^c , -[N(R ^g )C(0)] ₂ R ^c , -N(R ^g ) [C(0)] ₂ R ^c , -N{[C(0)] ₂ R ^c } ₂ ,

-N(R ^g ) [C(0)] ₂ 0R ^c , -N(R ^g ) [C(0)] ₂ NR ^C R ^c , -N{[C(0)] ₂ 0R ^c } ₂ , -N{[C(0)] ₂ N R ^C R ^c } ₂ , -[N(R ^g )C(0)] ₂ 0R ^c , -N(R ^g )C(NR ^g )0R ^c , -N(R ^g )C(N0H)R ^c , -N(R ^g )C(NR ^g )SR ^c and ~N(R ^g )C(NR ^g )NR ^c R ^c ; each R ^c independently of one another denotes hydrogen or a group, optionally substituted by one or more, identical or different R ^d and/or R ^e , selected from among C^alkyl, C _2„6 alkenyl, C _2-6 a I ky ny I , C _3„10 cycloalkyl, C _4-16 cycloa I ky la I ky I ,

C _6-10 aryl, C _7-16 a ry la I ky 1 , 3-18 membered heterocyclyl, 4-22 membered heterocyclyla I ky 1 , 4-22 membered heterocyclylalkenyl, 5-12 membered heteroaryl and 6-18 membered heteroary lal ky I ; each R ^d denotes a suitable substituent and is selected in each case independently of one another from among =0, -OR ^e , C _1.3 haloalkyloxy,-OCF ₃ , =S, -SR ^e , =NR ^e ,

= NOR ^e , =NNR ^e R ^e , =NN(R ^g )C(0)NR ^e R ^e , -NR ^e R ^e , -ONR ^e R ^e , -N(OR ^e )R ^e ,

-N(R ^g ) NR ^e R ^e , halogen, -CF ₃ , -CN, -NC, -OCN, -SCN, -NO, -N0 ₂ , =N ₂ ,

-N ₃ , -S(0) R ^e , -S(0)OR ^e , -S(0) ₂ R ^e , -S(0) ₂ OR ^e , -S(0)NR ^e R ^e , -S(0) ₂ NR ^e R ^e ,

-OS(0)R ^e , -OS(0) ₂ R ^e , -OS(0) ₂ OR ^e , -OS(0)NR ^e R ^e , -OS(0) ₂ NR ^e R ^e , -C(0)R ^e ,

-C(0)OR ^e , -C(0)SR ^e , -C(0)NR ^e R ^e , -C(0)N(R ^g )NR ^e R ^e , -C(0)N(R ^g )OR ^e ,

-C(NR ^g ) NR ^e R ^e , -C(NOH)R ^e , -C(NOH)NR ^e R ^e , -OC(0) R ^e , -OC(0)OR ^e ,

-OC(0)SR ^e , -OC(0)NR ^e R ^e , -OC(NR ^g ) NR ^e R ^e , -SC(0) R ^e , -SC(0)OR ^e ,

-SC(0)NR ^e R ^e , -SC(NR ^g )NR ^e R ^e , -N(R ^g )C(0)R ^e , -N [C(0)R ^e ] ₂ , -N(OR ^g )C(0)R ^e , -N(R ^g )C(NR ^g )R ^e , -N(R ^g )N(R ^g )C(0) R ^e , -N[C(0)R ^e ]NR ^e R ^e , -N(R ^g )C(S)R ^e ,

-N(R ^g )S(0)R ^e , -N(R ^g )S(0)OR ^e , -N(R ^g )S(0) ₂ R ^e , -N[S(0) ₂ R ^e ] ₂ , -N(R ^g )S(0) ₂ OR ^e , -N(R ^g )S(0) ₂ NR ^e R ^e , -N(R ^g )[S(0) ₂ ] ₂ R ^e , -N(R ^g )C(0)OR ^e , -N(R ^g )C(0)SR ^e , -N(R ^g )C(0)NR ^e R ^e , -N(R ^g )C(0)NR ^g NR ^e R ^e , -N(R ^g )N(R ^g )C(0)NR ^e R ^e ,

-N(R ^g )C(S)NR ^e R ^e , -[N(R ^g )C(0)] ₂ R ^e , -N(R ^g )[C(0)] ₂ R ^e , -N{[C(0)] ₂ R ^e } ₂ ,

-N(R ^g ) [C(0)] ₂ OR ^e , -N(R ^g ) [C(0)] ₂ NR ^e R ^e , -N{[C(0)] ₂ 0R ^e } ₂ , -N{[C(0)] ₂ NR ^e R ^e } ₂ , ^~ [N(R ^g )C(0)] ₂ OR ^e , -N(R ^g )C(NR ^g )OR ^e , -N(R ^g )C(NOH)R ^e , -N(R ^g )C(NR ^g )SR ^e and -N(R ^g )C(NR ^g )NR ^e R ^e ; and each R ^e independently of one another, optionally substituted by one or more, identical or different R ^f , denotes hydrogen or a group selected from among C^alkyl, C _2„6 alkenyl, C _2„6 a I ky ny I , C _3.8 cycloalkyl, C _6-10 aryl, 3-8 membered heterocyclyl, and 5- 12 membered heteroaryl; and each R ^f denotes a suitable substituent and is selected in each case independently of one another from among =0, -OR ^g , ~C(0)R ^g , -NR ^g R ^g , each R ^g independently of one another denotes hydrogen, C _h alky I, C _2-6 alkenyl,

C _2-6 a I ky nyl , C _3„8 cycloalkyl, C _6-10 aryl, 3-8 membered heterocyclyl, or 5-12 membered heteroaryl; and each R ^h denotes a suitable substituent and is selected in each case independently of one another from among halogen, =0, -OH, -C(0)H, -C(0)CH ₃ , -NH ₂ ,

”N(CH ₃ ) ₂ , and wherein at least one of R ² and R ³ does not denote H, and optionally in the form of the tautomers, the racemates, the enantiomers, the diastereomers, hydrates, isotopes, and mixtures thereof.

[0012] A further embodiment of the invention relates to compounds as described herein, wherein _2-10 alkenyl,

C _4-16 cycloalkylal optionally substituted by one or more, identical or different R ^b and

[0013] A further embodiment of the invention relates to compounds as described herein, wherein R ² and R ³ form a 4- to 10-membered heterocyclyl group, which is optionally substituted by one or more, identical or different R ^a and/or R ^b .

[0014] A further embodiment of the invention relates to compounds as described herein, wherein R ² and R ³ independently from each other denote C^^alkyl, C _2-10 alkenyl, C _3-8 cycloalkyl, C _4-16 cycloal ky lal kyl, C _6-10 aryl, C _7-16 arylalkyl, optionally substituted by one or more, identical or different R ^b and/or R ^c .

[0015] A further embodiment of the invention relates to compounds as described herein, wherein R ⁴ denotes

C _6-14 aryl, 5-12 membered heteroaryl, optionally substituted by one or more, identical or different R ^b and/or R ^c .

[0016] A further embodiment of the invention relates to compounds as described herein, wherein R ⁵ denotes halogen, C _T-10 alkyl, C _2„10 alkenyl, C _6-10 aryl, 5-12 membered heteroaryl, 3-12 membered heterocyclyl, or two R ⁵ form a 6-membered cyclic ring, optionally substituted by one or more, identical or different R ^b and/or R ^c .

[0017] A further embodiment of the invention relates to compounds as described herein, wherein Y is N-R ¹ . [0018] A further embodiment of the invention relates to compounds as described herein, wherein X denotes a chloride, bromide, iodide, fluoride, acetate, formiate, perchlorate, fluorinated alkyl or aryl sulfonate, trifluorosulfonate, trifluoromethanesulfonate (triflate), nonafluorobutanesulfonate (nonaflate), halogenated carboxylate, trifluoroacetate, trichloroacetate, dichloroacetate, monochloroacetate, tetrafluoroborate, hexafluorophosphate, hexafluoroantimonate,.

[0019] One embodiment of the invention relates to compounds selected from

Table 1.

[0020] One embodiment of the invention relates to compounds as described herein, wherein the compounds exhibit photophysical properties, preferably fluorescence activity.

[0021] One embodiment of the invention relates to a process for manufacturing a compound of formula II, wherein compounds 1, 2, and 3 are reacted in the presence of a solvent and an electrophile to obtain a compound of formula (II) wherein

R _y is a leaving group,

R ² , R ³ , R ⁴ , and R ⁵ are as defined above.

[0022] A process for manufacturing a compound of formula III, wherein compounds 1 and 3 are reacted in the presence of a solvent and electrophile to obtain a compound of formula (III) wherein R _y is selected from the group consisting of a halogen,

R ² , R ³ , R ⁴ , and R ⁵ are as defined above.

[0023] A process for manufacturing a compound of formula IV, wherein a compound of formula (III) is reacted in the presence of a thiation agent and a solvent to obtain a compound of formula (IV)

(III) (IV) wherein

R ² , R ³ , and R ⁵ are as defined above, and X is an anion.

[0024] The invention provides use of the compounds for the use as a fluorescent probe.

[0025] Further, the invention provides use of the compounds as a fluorescent probe, wherein the fluorescent probe is used in the field of biology, biochemistry, chemical biology, or medical diagnostics.

Definitions

[0026] As used herein, the following definitions apply, unless stated otherwise: [0027] Unless specified otherwise, the term “alkyl”, when used alone or in combination with other groups or atoms, refers to a saturated straight or branched chain consisting solely of 1 to 6 hydrogen -substituted carbon atoms, and includes methyl, ethyl, propyl, isopropyl, n-butyl, 1-methylpropyl, isobutyl, t-butyl, 2,2- dimethylbutyl, 2,2-dimethyl propyl, n-pentyl, 2-methylpentyl, 3-methylpentyl, 4- methylpentyl, n-hexyl and the like.

[0028] Unless specified otherwise, the term “alkenyl” refers to a partially unsaturated straight or branched chain consisting solely of 2 to 6 hydrogen- substituted carbon atoms that contains at least one double bond, and includes vinyl, allyl, 2-methylprop-l-enyl, but-l-enyl, but-2-enyl, but-3-enyl, buta-l,3-dienyl, penta-l,3-dienyl, penta-2,4-dienyl, 2-methylbut-l-enyl, 2-methylpent-l-enyl, 4- methylpent-l-enyl, 4-methylpent-2-enyl, 2-methylpent-2-enyl, 4-methylpenta-l,3- dienyl, hexen-l-yl and the like.

[0029] Unless specified otherwise, the term “alkynyl” refers to a partially unsaturated straight or branched chain consisting solely of 2 to 6 hydrogen- substituted carbon atoms that contains at least one triple bond, and includes ethynyl, 1-propynyl, 2-propynyl, 2-methylprop-l-ynyl, 1-butynyl, 2-butynyl, 3- butynyl, 1,3-butadiynyl, 3-methylbut-l-ynyl, 4-methylbut-ynyl, 4-methylbut-2-ynyl, 2-methylbut-l-ynyl, 1-pentynyl, 2-pentynyl, 3-pentynyl, 4-pentynyl, 1,3-pentadiynyl, 1,4-pentadiynyl, 3-methylpent-l-ynyl, 4-methylpent-2-ynyl, 4-methylpent-2-ynyl, 1-hexynyl, and the like.

[0030] Unless specified otherwise, the term “cycloalkyl”, when used alone or in combination with other groups or atoms, refers to monocyclic hydrocarbon rings, bicyclic hydrocarbon rings or spirohydrocarbon rings, which each may be either saturated or unsaturated (cycloalkenyl). The term unsaturated means that in the ring system in question there is at least one double bond, but no aromatic system is formed. In bicyclic hydrocarbon rings, two rings are linked such that they have at least two carbon atoms in common. In spirohydrocarbon rings, one carbon atom (spiroatom) is shared by two rings. If a cycloalkyl is substituted, the substitution may be mono- or polysubstitution in each case, at all the hydrogen-carrying carbon atoms, independently of one another. Cycloalkyl itself may be linked to the molecule as substituent via any suitable position of the ring system.

[0031] Typical examples of individual sub-groups are listed below.

Monocyclic saturated hydrocarbon rings: cyclopropyl; cyclobutyl; cyclopentyl; cyclohexyl; cycloheptyl, etc.

Monocyclic unsaturated hydrocarbon rings: cycloprop-l-enyl; cycloprop-2-enyl; cyclobut-l-enyl; cyclobut-2-enyl; cyclopent-l-enyl; cyclopent-2-enyl; cyclopent-3- enyl; cyclohex-l-enyl; cyclohex-2-enyl; cyclohex-3-enyl; cyclohept-l-enyl; cyclohept-2-enyl; cyclohept-3-enyl; cyclohept-4-enyl; cyclobuta-1,3- dienyl; cyclopenta-l,4-dienyl; cyclopenta-l,3-dienyl; cyclopenta-2,4-dienyl; cyclohexa-1,3- dienyl; cyclohexa-l,5-dienyl; cyclohexa-2,4-dienyl; cyclohexa-1 ,4-dienyl; cyclohexa-2,5- dienyl, etc.

Saturated and unsaturated bicyclic hydrocarbon rings: bicyclo [2.2.0] hexyl ; bicyclo [3.2.0] hepty I ; bicyclo [3.2.1] octyl ; bicyclo[2.2.2]octyl; bicyclo[4.3.0]nonyl (octahydroindenyl); bicyclo[4.4.0]decyl (deca hydronaphthalene); bicyclo [2,2,1] hepty I (norbornyl); (bicyclo[2.2.1]hepta-2,5-dienyl (norborna-2,5- dienyl); bicyclo [2,2,1] hept-2-eny I (norbornenyl); bicyclo [4.1.0] heptyl (norcaranyl); bicyclo- [3.1.1] heptyl (pinanyl), etc. Saturated and unsaturated spirohydrocarbon rings: spiro [2.5] octyl, spi ro [3.3] hepty I , spiro[4.5]dec-2-ene, etc.

[0032] “Cycloalkylalkyl” denotes the combination of the above-defined groups alkyl, alkenyl, alkynyl, and cycloalkyl, in each case in their broadest sense. The alkyl group as substituent is directly linked to the molecule and is in turn substituted by a cycloalkyl group. The alkyl and cycloalkyl may be linked in both groups via any carbon atoms suitable for this purpose. The respective sub-groups of alkyl and cycloalkyl are also included in the combination of the two groups.

[0033] Unless specified otherwise, the term “aryl” refers to an aromatic mono- or bicyclic group containing from 6 to 14 carbon atoms, preferably 6 to 10 carbon atoms, that may be optionally fused with a fully or partially saturated or unsaturated carbocyclic ring and may optionally be substituted with one or more, identical or different substituents, suitably one to three substituents. Examples of aryl groups include phenyl, naphthyl, indanyl, and the like.

[0034] “Arylalkyl” denotes the combination of an alkyl, alkenyl, or alkynyl group and aryl as hereinbefore defined, in each case in their broadest sense. The alkyl group as substituent is directly linked to the molecule and is in turn substituted by an aryl group. The alkyl and aryl may be linked in both groups via any carbon atoms suitable for this purpose. Typical examples include benzyl, 1-phenylethyl, 2- pheny lethy I , phenylvinyl, phenylallyl, etc.

[0035] Unless specified otherwise, the term “heteroaryl” refers to an aromatic mono- or bicyclic group containing from 5 to 14 carbon atoms, preferably 5 to 12 carbon atoms, of which one to five is replaced with a heteroatom selected from N,

S and O, that may optionally be reduced to a non-aromatic heterocycle and may optionally be substituted with one or more, identical or different substituents. Examples of heteroaryl groups include pyrrolyl, dihydropyrrolyl, pyrrolidinyl, oxopyrrolidinyl, indolyl, isoindolyl, indolizinyl, imidazolyl, pyrazolyl, benzimidazolyl, imidazo(l,2-a) pyridinyl, indazolyl, purinyl, py rrolo(2,3-c) pyridinyl, pyrrolo(3,2- c) pyridinyl, py rrolo (2, 3- b) pyridinyl, pyrazolo(l,5-a) pyridinyl, 1,2,3-triazolyl, 1,2,4- triazolyl, tetrazolyl, oxazolyl, 1,2 oxazolyl, isoxazolyl, 1,3,4-oxadiazolyl, 1,2,5- oxadiazolyl, 1,2,4-oxadiazolyl, 1,2,3-oxadiazolyl, thiazolyl, isothiazolyl, 1,3,4- thiadiazolyl, 1,2,5-thiadiazolyl, 1 ,2,4-th iad iazoly 1 , 1,2,3-thiadiazolyl, furanyl, dihydrofuranyl, tetrahydrofuranyl, benzofuranyl, isobenzofuranyl, thiophenyl, dihydrothiophenyl, tetrahydrothiophenyl, benzothiophenyl, benzoisothiophenyl, pyridyl, piperidinyl, quinolinyl, isoquinolinyl, tetrahydroisoqinolinyl, quinolizinyl, pyrazinyl, pyridazinyl, pyrimidinyl, pyranyl, tetrahydropyranyl, 1,2,3-triazinyl, 1,2,4- triazinyl, 1,3,5-triazinyl, chromenyl, morpholinyl, diazepinyl, benzodiazepinyl, and the like.

[0036] “Heteroarylalkyl” denotes the combination of the alkyl, alkenyl, alkynyl, and heteroaryl groups defined hereinbefore, in each case in their broadest sense. The alkyl group as substituent is directly linked to the molecule and is in turn substituted by a heteroaryl group. The linking of the alkyl and heteroaryl may be achieved on the alkyl side via any carbon atoms suitable for this purpose and on the heteroaryl side by any carbon or nitrogen atoms suitable for this purpose.

[0037] By the term “heterocyclyl” are meant groups which are derived from cycloalkyl as hereinbefore defined if in the hydrocarbon rings one or more of the groups -CH ₂ - are replaced independently of one another by the groups -0-, -S- or - NH- or one or more of the groups =CH- are replaced by the group =N-, while not more than five heteroatoms may be present in total, there must be at least one carbon atom between two oxygen atoms and between two sulphur atoms or between one oxygen and one sulphur atom and the group as a whole must be chemically stable. Heteroatoms may simultaneously be present in all the possible oxidation stages (sulphur -> sulphoxide -SO-, sulphone -S0 ₂ -; nitrogen -> N- oxide). It is immediately apparent from the indirect definition/derivation from cycloalkyl that heterocyclyl is made up of the sub-groups monocyclic hetero-rings, bicyclic hetero-rings and spirohetero-rings, while each sub-group can also be further subdivided into saturated and partially unsaturated heterocyclyl groups.

The term partially unsaturated means that in the ring system in question there is at least one double bond, but no aromatic system is formed. In bicyclic hetero-rings two rings are linked such that they have at least two atoms in common. In spirohetero-rings one carbon atom (spiroatom) is shared by two rings. If a heterocyclyl is substituted, the substitution may be mono- or polysubstitution in each case, at all the hydrogen-carrying carbon and/or nitrogen atoms, independently of one another. Heterocyclyl itself as substituent may be linked to the molecule via any suitable position of the ring system.

[0038] The term “heterocyclyl group” as used herein also refers to a heterocyclyl group as defined above, which optionally may be fused to an aromatic aryl or heteroaryl group. [0039] Typical examples of individual sub-groups are listed below:

Monocyclic heterorings (saturated and unsaturated): oxolane, pyrrolidinyl, pyrrolinyl, imidazolidinyl, thiazolidinyl, imidazolinyl, pyrazolidinyl, pyrazolinyl, piperidinyl, piperazinyl, oxiranyl, aziridinyl, azetidinyl, 1,4-dioxanyl, azepanyl, diazepanyl, morpholinyl, thiomorpholinyl, homomorpholinyl, homopiperidinyl, homopiperazinyl, homothiomorpholinyl, thiomorpholinyl-S-oxide, thiomorpholinyl- S,S-dioxide, 1,3-dioxolanyl, oxane, tetrahydrothiopyranyl, 1,4-oxazepanyl, tetrahydrothienyl, homothiomorpholinyl-S,S-dioxide, oxazolidinonyl, dihydropyrazolyl, dihydropyrrolyl, dihydropyrazinyl, dihydropyridyl, dihydro- pyrimidinyl, dihydrofuryl, dihydropyranyl, tetrahydrothienyl-S-oxide, tetrahydrothienyl-S,S-dioxide, homothiomorpholinyl-S-oxide, 2,3-dihydroazet, 2 H- pyrrolyl, 4/7-pyranyl, 1,4- dihydropyridinyl, etc;

Bicyclic heterorings or polycyclic heterorings (saturated and unsaturated): 8- azabicyclo[3.2.1]octyl, 8-azabicyclo[5.1.0]octyl, 2-oxa-5-azabicyclo[2.2.1]heptyl, 8- oxa- 3-aza-bicyclo[3.2.1]octyl, 3,8-diaza-bicyclo [3.2.1] octyl, 2,5-diaza-bicyclo- [2.2.1] heptyl, l-aza-bicyclo[2.2.2]octyl, 3,8-diaza-bicyclo [3.2.1] octyl, 3,9-diaza- bicyclo [4.2.1] nonyl, 2,6-diaza-bicyclo[3.2.2]nonyl, hexahydro-furo[3,2-b]furyl, 1H- thieno[3,4-r/]imidazol, hexahydro-lH-thieno[3,4-c]imidazol, saturated or unsaturated cyclopentaphenanthrene, etc;

Spiro-heterorings (saturated and unsaturated): l,4-dioxa-spiro[4.5]decyl; 1-oxa- 3,8-diaza-spiro[4.5]decyl; 2,6-diaza-spiro[3.3]heptyl; 2,7-diaza-spiro[4.4]nonyl; 2,6- diaza-spiro[3.4]octyl; 3,9-diaza-spiro[5.5]undecyl; 2,8-diaza- spiro [4.5] decyl, etc. [0040] “Heterocyclylalkyl” denotes the combination of the alkyl and heterocyclyl groups defined hereinbefore, in each case in their broadest sense. The alkyl group as substituent is directly linked to the molecule and is in turn substituted by a heterocyclyl group. The linking of the alkyl and heterocyclyl may be achieved on the alkyl side via any carbon atoms suitable for this purpose and on the heterocyclyl side by any carbon or nitrogen atoms suitable for this purpose.

“Heterocyclylalkenyl” denotes the combination of the alkenyl group and heterocyclyl groups defined hereinbefore, in each case in their broadest sense. The alkenyl group as substituent is directly linked to the molecule and is in turn substituted by a heterocyclyl group. The linking of the alkenyl and heterocyclyl may be achieved on the alkenyl side via any carbon atoms suitable for this purpose and on the heterocyclyl side by any carbon or nitrogen atoms suitable for this purpose. “Heterocyclylalkynyl” denotes the combination of the alkynyl group and heterocyclyl groups defined hereinbefore, in each case in their broadest sense. The alkynyl group as substituent is directly linked to the molecule and is in turn substituted by a heterocyclyl group. The linking of the alkynyl and heterocyclyl may be achieved on the alkenyl side via any carbon atoms suitable for this purpose and on the heterocyclyl side by any carbon or nitrogen atoms suitable for this purpose. [0041] By the term "suitable substituent" is meant a substituent that on the one hand is fitting on account of its valency and on the other hand leads to a system with chemical stability.

[0042] It is intended that the definition of any substituent or variable at a particular location in a molecule be independent of its definitions elsewhere in that molecule. It is understood that substituents and substitution patterns on the compounds of this invention can be selected by one of ordinary skill in the art to provide compounds that are chemically stable and that can be readily synthesized by techniques known in the art as well as those methods set forth herein.

[0043] The term "tautomers" refer to compounds that are interchangeable forms of a particular compound structure, and that vary in the displacement of hydrogen atoms and electrons. Thus, two structures may be in equilibrium through the movement of p electrons and an atom (usually H). For example, enols and ketones are tautomers because they are rapidly interconverted by treatment with either acid or base.

[0044] It is also to be understood that compounds (e.g., dihydro bases described herein) that have the same molecular formula but differ in the nature or sequence of bonding of their atoms or the arrangement of their atoms in space are termed "isomers". Isomers that differ in the arrangement of their atoms in space are termed "stereoisomers".

[0045] Stereoisomers that are not mirror images of one another are termed "diastereomers" and those that are non-superimposable mirror images of each other are termed "enantiomers". When a compound has an asymmetric center, for example, it is bonded to four different groups, a pair of enantiomers is possible. An enantiomer can be characterized by the absolute configuration of its asymmetric center and is described by the R- and S-sequencing rules of Cahn and Prelog, or by the manner in which the molecule rotates the plane of polarized light and designated as dextrorotatory or levorotatory (i.e. , as (+) or (-)-isomers respectively).

[0046] A 50:50 mixture of enantiomers is referred to as a racemic mixture or a racemate, which may occur where there has been no stereoselection or stereospecificity in a chemical reaction or process. The terms "racemic mixture" and "racemate" refer to an equimolar mixture of two enantiomeric species, devoid of optical activity.

[0047] The term "hydrate" refers to a compound which is associated with water. Typically, the number of the water molecules contained in a hydrate of a compound is in a definite ratio to the number of the compound molecules in the hydrate.

[0048] Any formula or structure given herein, including Formula I compounds, is also intended to represent unlabeled forms as well as isotopically-labeled forms of the compounds. Isotopically-labeled compounds have structures depicted by the formulas given herein except that one or more atoms are replaced by an atom having a selected atomic mass or mass number. Examples of isotopes that can be incorporated into compounds of the invention include isotopes of hydrogen, carbon, nitrogen, oxygen, phosphorous, fluorine, and chlorine, such as, but not limited to 2H (deuterium, D), 3H (tritium), 11C, 13C, 14C, 15N, 18F, 31P, 32P, 35S, 36CI, and 125J.

[0049] The term “leaving group” has the meaning conventionally associated with synthetic organic chemistry, i.e., an atom or a group capable of being displaced by a nucleophile and includes halo (such as chloro, bromo, and iodo), alkanesulfonyloxy, arenesulfonyloxy, alkylcarbonyloxy (e.g., acetoxy), arylcarbonyloxy, mesyloxy, tosyloxy, trifluoromethanesulfonyloxy, aryloxy (e.g., 2,4- dinitrophenoxy), methoxy, N , O - dimethyl hydroxy la mi no, and the like. Specific examples of leaving groups include, but are not limited to, Cl, Br, I and sulfonate esters such as OMs (mesylate), OTs (tosylate), ONs (nosylate), OTf (triflate) and any other sulfonate esters.

[0050] The term “electrophile”, “electrophile molecule”, or a reactant comprising an electrophilic moiety is a compound which is at least partially formed of C- and Fl-Atoms and comprises an electron-poor site which may react with another molecule comprising a site with an increased electron density. The "electrophile" typically carries a positive charge or a positive partial charge. In principle, various types of electrophiles or reactants comprising an electrophilic moiety can be used. In particular, as an electrophile, a Lewis-acidic organic or inorganic reactant with an electron poor site can be used. Exemplary electrophiles are molecules selected from the group consisting of triflic anhydride, PCI ₃ , and (COCI) ₂ .

[0051] The term “thiation agent” describes for example, any agent known in the art for the conversion of an oxo group to a thioxo group such as, for example, 2,4-bis- (4- methoxyphenyl)-l, 3-d ithia-2,4-diphosphetane-2, 4-disulphide (Lawesson's Reagent) or P ₂ S ₅ (phosphorus pentasulphide). The thiation reaction is generally carried out with the required stoichiometric amount of thiation reagent in order to reduce the risk of damage to other functional groups. In general, the reaction is carried out in a suitable solvent or diluent such as toluene, xylene or tetrahydrofuran and at a temperature, for example, at or near the reflux temperature of the solvent or diluent, that is in the range 65 to 150° C.

[0052] The term “anion” means an ionic compound preferably inorganic comprising one or several negative charges. Anion denotes, for example, an organic or inorganic anion, such as halogen, preferably chloride and fluoride, sulfate, hydrogen sulfate, phosphate, boron tetrafluoride, carbonate, bicarbonate, oxalate or alkyl sulfate, especially methyl sulfate or ethyl sulfate; anion also denotes lactate, formate, acetate, propionate, perchlorate, fluorinated alkyl sulfonate or fluorinated aryl sulfonate, trifluorosulfonate, trifluoromethanesulfonate (triflate), nonafluorobutanesulfonate (nonaflate), halogenated carboxylate, trifluoroacetate, trichloroacetate, dichloroacetate, monochloroacetate, tetrafluoroborate, hexafluorophosphate, and hexafluoroantimonate.

Examples

[0053] The Examples which follow are set forth to aid in the understanding of the invention but are not intended to, and should not be construed to limit the scope of the invention in any way. The Examples do not include detailed descriptions of conventional methods, such methods are well-known to those of ordinary skill in the art.

GENERAL EXPERIMENTAL DETAILS

GENERAL SCHEME 1: Synthesis of imidazo[l,2-a]pyridinium salts GENERAL PROCEDURE 1 (SCHEME 1): Synthesis of imidazo[l,2-a]pyridinium salts

[0054] Based on the electrophilic activation of amides, a metal-free three- component synthesis of imidazo[l,2-a]pyridinium triflates using simple amides, azides and halopyridines as starting materials is described (SCHEME 1).

GENERAL SCHEME 2: Synthesis of imidazo[l,2-a]pyridinium salt 4a

GENERAL PROCEDURE 2 (SCHEME 2): Synthesis of imidazo[l,2-a]pyridinium salt 4a

[0055] The reactivity of activated amides with nucleophilic azides for nucleophilic a -amination was investigated. Surprisingly, the use of benzyl amides did not process with the formation of the expected azirinium intermediate II, but allowed the addition of a pyridine molecule to the enaminium intermediate I, resulting in the formation of imidazo[l,2-a]pyridinium fluorescent salt 4a. A similar umpolung of reactivity was already observed in the case of enolonium species generated from the addition of pyridine N-oxides to the keteniminium ion.

GENERAL SCHEME 3: Optimization of reaction conditions for the synthesis of imidazo[l,2-a]pyridinium salt 4a

GENERAL PROCEDURE 3 (SCHEME 3): Optimization of reaction conditions for the synthesis of imidazo[l _» 2-a]pyridinium salt 4a [0056] Interested by the formation of these imidazo[l,2-a]pyridinium salts, the reaction conditions were optimized (Table 2). By increasing the equivalents of pyridine (entries 3-5) and switching from a 2-fluoro to a 2-chloro substituted pyridine, the NMR yield was improved from 30% to 73% (entry 6).

Table 2: Optimization of the reaction conditions of imidazo[l,2-a]pyridinium salts

GENERAL SCHEME 4: Synthesis of imidazo[l,2-a]pyridinium salts - amides

[0057] The scope of the reaction was further investigated. Both electron withdrawing and electron donating groups were tolerated on the aromatic group (4b, d, e, f, g, h, db, do) as well as a naphthyl group (4c). The presence of a substituent in ortho position allowed the formation of the atropisomeric product (4i). Piperidine (4j), diethyl (4k) and azetidine amides (4I) worked smoothly to afford the desired imidazo[l,2-a]pyridinium.

[0058] Amides with an aromatic substituent on the nitrogen lead only to traces of compounds 4o-q. [0059] Compound 4da having a trifluoroacetate as a counterion could be isolated in a high yield after reverse phase column chromatography.

[0060] The photophysical properties of this series of fluorophores were evaluated (Table 3). Spectra were measured using methanol as solvent. All substrates 4a-l present blue fluorescence, with a maximum of emission l _GT1 between 412 nm and 448 nm and are excited around 300 nm. Electron-donating substituents on the aryl moiety induce a bathochromic shift in fluorescence emission (4d). They also display large Stoke shift (difference between X _em and l^ ₃ ), over 100 nm, which helps to minimize cross-talk between the excitation source and the fluorescent emission, and they show quite useful quantum yields, above 50%.

GENERAL SCHEME 5: Synthesis of imidazo[l,2-a]pyridinium salts - azides

[0061] Then the scope of tolerated azides in this reaction was investigated. As expected, the reaction was compatible with alkyl azides bearing no functional group (4r,t). Interestingly, a terminal alkene as well as an iodoalkyl chain were tolerated (4u, v). Some aromatic variations of azide were tolerated (4y, z).

[0062] Compound 4dd containing an azide could be obtained using 10 equivalents of the corresponding bis-azide reactant. Notably, the compound contains a reactive azide that could be used as a handle for further functionalization.

[0063] The photophysical properties of these new fluorophores were studied (Table 4). Spectra were measured using methanol as solvent. The substitution brought by the azide partner did not have any influence on the optical properties of the compounds which all showed the same excitation and emission wavelengths as well as comparable quantum yields. This allows for introduction of several functional groups in FT position for late-stage functionalization without repercussion on the photo physical properties of the fluorophore.

GENERAL SCHEME 6: Synthesis of imidazo[l,2-a]pyridinium salts - pyridines GENERAL PROCEDURE 6 (SCHEME 6): Synthesis of imidazo[l,2-a]pyridinium salts - pyridines

[0064] Further, the pyridine partners in the formation of irmidazo[l,2-a]pyridiniums via amide activation were investigated. Modification of the pyridine allowed for the introduction of various aromatic rings with various electronic properties (4ab-df). GENERAL SCHEME 7: Synthesis of imidazo[l,2-a]pyridinium salts - variation of azide and pyridine partners salts - variation of pyridine and azide partners

[0065] 2-Chloro-quinoline and 2-chloro-isoquinoline could also be used to afford the tricyclic products 4al-ao with acceptable yields.

[0066] 2,4-dich loro-8- (/V-di methyl) quinoline only produced compound 4dg in a low yield. The presence of halide substituents in compounds 4dg, 4dh, 4di, 4dj makes them valuable as potential substrates for coupling reactions. The presence of halide substituents would potentially allow late-stage functionalization of the fluorescent probes.

[0067] The usage of 2-chloro-4-iodo pyridine with the corresponding azides gave compounds 4dh-dj.

GENERAL PROCEDURE 8 (SCHEME 6 and SCHEME 7): Photophysical properties of pyridines and variations of pyridines and azides

[0068] This series extending the p-system showed more variability in the photophysical properties (Table 5). All biaryls systems 4ab-ai showed high quantum yields as well as a bathochromic effect for fluorescence emission compared to compound 4a. Tricyclic fluorophores 4al and 4ao also showed a fluorescence emission shifted to the red compared to 4a. Spectra were measured using methanol as solvent.

Table 5: Photophysical data of selected imidazo[l,2-a]pyridinium salts - pyridines

GENERAL SCHEME 8: Synthesis of imidazo[l,2-a]pyridinium salts - variation of amide and azide partners

GENERAL PROCEDURE 9 (SCHEME 8): Synthesis of imidazo[l,2-a]pyridinium salts - variation of amide and azide partners

[0069] Then the scope of tolerated variations of amides and azides in the reaction was investigated. As expected, the reaction was compatible with a variation of amides and azides (4s). GENERAL SCHEME 9: Synthesis of imidazo[l,2-a]pyridinium salts - aryl groups

GENERAL PROCEDURE 10 (SCHE E 9): Synthesis of imidazo[l,2-a]pyridinium salts - aryl groups

[0070] 4do was obtained after reduction of the nitro group in 4h. Sandmeyer reaction followed by NaN ₃ addition afforded compound 4dp which could react with an alkyne under copper catalysis to get 4dq. The compounds contain a free amine (4do), an azide (4dp) and a triazole heterocycle (4dq), functionalities that are ubiquitous in medicinally relevant and bioactive molecules.

GENERAL SCHEME 10: Synthesis of imidazo[l,2-a]pyridinium salts - intermediates

GENERAL PROCEDURE 11 (SCHEME 10): Synthesis of imidazo[l,2- ajpyridinium salts - intermediates

[0071] Compounds 4dr, 4eg and 4ej were obtained after Suzuki coupling from 4dh or 4dj. After hydrolysis and amide coupling, conjugates 4ds,4dt, 4eb, 4ec, 4ek and 4eu were obtained. 4eg and 4ej were hydrolyzed to 4eh and 4ex and then coupled with the corresponding amines to give 4ei and 4em. 4ed and 4ee were obtained after hydrolysis and amide coupling or esterification of 4dh. The chloride counterparts of 4ed and 4ee were obtained via S _N Ar with chloride anion nucleophile.

GENERAL SCHEME 11: Synthesis of imidazo[l,2-a]pyridinium salts - others GENERAL PROCEDURE 12 (SCHEME 11): Synthesis of imidazo[l,2- ajpyridinium salts - others

[0072] 4du was obtained from 4ag via S _N Ar with piperidine. Staudinger reduction of 4dk afforded 4dv in good yield. Compounds 4dw-dz were obtained by hydrolysis of 4dl and 4dm.

[0073] 4eo, 4ep, 4eq and 4es were obtained via palladium-catalyzed phosphorylation and sulphonylation followed by deprotection.

[0074] 4ea and 4en were obtained according to the standard procedure for the formation of imidazo[1.2a]pyridinium salts. 4en was then employed in two consecutive palladium-catalyzed cross-couplings to afford 4ew.

GENERAL SCHEME 12: Pd-catalyzed cross couplings for the postfunctionalization of imidazo[l,2-a]pyridinium salts

GENERAL PROCEDURE 13 (SCHEME 12): Suzuki-Miyaura cross coupling for the post-functionalization of imidazo[l,2-a] pyridinium salts

[0075] Currently performed synthesis of modified pyridine required several steps and huge amount of starting materials due to the use of 5 equivalents to form the imidazo[l,2-a]pyridinium salts. Therefore, a more economical and convergent strategy to access different substitutions on the pyridinium moiety was designed. Commercially available 2-chloro-4-iodopyridine allowed an easy access to grams of 4ag, which can be further functionalized via Suzuki Miyaura cross coupling (Scheme 8-a). The commercial or synthetic availability of aryl boronic acid or esters allowed to screen easily several pyridyl substituents in a high throughput manner. Biaryls fluorophores 4a j, 4aq, 4a r and 4dm were isolated and fully characterized. The usage of 4di and 4dj as starting materials in the Suzuki coupling afforded compounds 4dk, 4dl and 4dn in good to excellent yields. Styryl substitution was also evaluated. For this purpose vinylic boronic esters, synthesized using the boron Wittig methodology developed by Morken and coworkers, were used in Suzuki coupling affording compounds 4ay, 4ba-bg. It was also proven, that when using a di-iodoaryl derivative, the Suzuki coupling could be selectively achieved on the pyridinium moiety (4ef, 4er).

GENERAL SCHEME 13: Pd-catalyzed cross couplings for the postfunctionalization of imidazo[l,2-a] pyridinium salts - coumarin-acid

GENERAL PROCEDURE 14 (SCHEME 13): Decarboxylative cross-coupling with coumarin-acid

[0076] The fluorophore 4bh is obtained by a decarboxylative coupling with a coumarin-acid.

GENERAL PROCEDURE 15 (SCHEME 12 and SCHEME 13): Photophysical Data of post-functionalized imidazo[l,2-a]pyridinium salts

[0077] Concerning the optical properties, quite interesting wavelengths following two strategies were obtained (Table 6). Spectra were measured using methanol as a solvent. Increasing the push-pull effect with electron donating substituent allowed to get a bathochromic effect (4a j, 4aq), with a particularly excellent quantum yield for compound 4a j. On the other hand, expending the p system via conjugated alkenes gave even larger bathochromic shifts. The highly conjugated systems present a turquoise to red fluorescence, from 498 nm for to 685 nm. Quantum yields were quite lower compared to the biaryl systems but remain useful giving their high wavelengths emissions.

Table 6: Photophysical Data of selected imidazo[l,2-a]pyridinium salts - boronic acids or esters or coumarin 4bf 470 685 215 31,2 0,18 5,5

4bg 397 532 135 41,6 0,37 15,5

4bh 443 522 79 30,7 0,84 25,8

GENERAL PROCEDURE 16: Solvatochromism 4aj

[0078] The solvatochromatic effect of 4aj was determined (Table 7). As a cationic species whose fluorescence emission is based on a push-pull effect, imidazo[l,2- a]pyridinium salts are sensitive to solvent polarity. Typically, 4aj displayed strong sensitivity to solvent polarity. In polar solvent, it presents a larger Stoke shift, especially in water, which adds potential value for fluorescence imaging application. Spectra are shown in Figure 1.

Table 7: Solvatochromic effect on 4a j.

Stoke Shift

Solvent Xabs (nm) lbhi (nm) (nm)

Toluene 411 497 86

CH CI 422 483 61

MeCN 407 497 90

DMF 404 494 90

MeOH 405 493 88

H2O 394 497 103

Toluene 411 497 86 ain MeOH at [l*E-4]; ^b in MeOH at [l*E-6],

GENERAL PROCEDURE 17: pH effect on spectral properties of 4aj

[0079] 4aj contains a diethyl aniline substituent, which under acidic condition (pH inferior to 5) undergoes a hypsochromic shift of A _abs and A _em , from 395 to 356 nm and 497 to 480 nm respectively. Thus, it could be used to identify specific cell organelles or cells itself which are acidic, such as certain type of cancer cells. Spectra are shown in Figure 2.

GENERAL SCHEME 14: Synthesis of oxazo[l,2-a]pyridinium salts GENERAL PROCEDURE 18 (SCHEME 14): Synthesis of oxazo[l,2-a]pyridinium salts

[0080] It was also hypothesized that other heteroatoms could be installed in the bicyclic core of the new dyes and fluorescent properties may also be tuned.

GENERAL SCHEME 15: Optimization of reaction conditions of oxazo[l,2- ajpyridinium salts action conditions of oxazo[l,2-a]pyridinium salts

[0081] In the case of the imidazo-pyridinium, the nitrogen was brought by the azide partner. Pyridine oxide was considered to be an appropriate oxygen donor for the formation of oxazo-pyridiniums. Indeed, substituting the azide partner by 2,6- I utidine- AAoxide (LNO) in the imidazole formation reaction permitted to obtain the desired scaffold with 35% yield (Table 8, Entry 1). A short optimization showed that the number of equivalents of LNO could be reduced to 1.5 (Table 8, Entries 2, 4, 5), whereas 2 equivalents of triflic anhydride were required to completely consume the starting amide (Table 8, entries 2,6). The treatment of the reaction was of major importance: LNO by-products were found to be difficult to separate from the oxazo-pyridinium salts. They could be washed away via an acidic extraction but this also resulted in a loss of yield (Table 8, entry 3). T rituration in Et ₂ 0 was an efficient way to separate these impurities from the desired product (Table 8, entries 1,2). This process was more efficient when performed after a first column chromatography (Table 8, entries 5, 7). Finally other pyridine-oxides were tested (Table 8, Entries 8,9). 2,6-Dichloropyridine-/V-oxide afforded 7a in compatible yield, however, its low accessibility will limit its use.

GENERAL SCHEME 16: Synthesis of oxazo[l,2-a]pyridinium salts - amides

GENERAL PROCEDURE 20 (SCHEME 16): Synthesis of oxazo[l,2-a]pyridinium salts - amides

[0082] Next the scope of this transformation (Scheme 16) with different amides as well as different pyridines was explored, choosing the substrates according to the best imidazo-pyrazoles in terms of yields and fluorescence properties. The best nitrogen substitution on the amide part proved to be the more electron-donating pyrrolidine and azetidine. Diethyl substituent permitted to afford product 7c with only a moderate yield and dial lyl or dimethyl substituents provided 7ac and 7ab in poor yields. The aryl part of the starting amide could also be tuned. Para-\o6o and nitro substituents showed yields comparable to the standard substrate 7a. An ortho substituent was well-tolerated (7aa). However, 7af and 7ag bearing respectively a para- trifluoromethyl thio and para-ester, meta-m ethoxy substituents were only obtained in low yields. A thiophene could be utilized which yielded product 7f.

Other bases could also be used, such as the 2-ch loro-4- iodopy rid ine affording product 7g with 44% yield on gram-scale. Finally, product 7h could be obtain in good yield using 2-chloro-isoquinoline as a base.

GENERAL SCHEME 17: Synthesis of oxazo[l,2-a]pyridinium salts - pyridines Photophysical properties of oxazo[l,2-a]pyridinium salts

[0083] Then photophysical properties of some substrates were evaluated (Table 9). Spectra were measured using methanol as solvent. A bathochromic shift was observed compared to the associated imidazo-pyridinium. Consequently, compounds 7a,b,h exhibited a maximal absorption around 350 nm (compared to 295 nm for imidazo-pyridiniums) and an emission comprised between 450 and 469 nm.

GENERAL SCHEME 18: Pd-catalyzed cross couplings for the postfunctionalization of oxazo[l,2-a]pyridinium salts

GENERAL PROCEDURE 22 (SCHEME 18): Pd-catalyzed cross couplings for the post-functionalization of oxazo[l,2-a]pyridinium salts

[0084] Starting from the iodo compound 7g Suzuki couplings was performed with several boronic acid and esters, affording compounds (7i-7n) and (7ah-7ak) with moderate to excellent yields.

GENERAL PROCEDURE 23 (SCHEME 18): Photophysical properties of post- functionalized oxazo[l,2-a]pyridinium salts

[0085] The optical properties of the synthetized fluorophores were characterized (Table 10). Spectra were measured using methanol as a solvent. No standard for quantum yield calculation matched excitation and emission wavelength. They showed the same general trend as the imidazo-pyridiniums with a bathochromic shift. A very large range of emission wavelengths from 508 nm to 720 nm was achieved. The result for 7n is of particular interest since the quantum yield of 10% is quite good for a red emission. Compound 7i, an analogue of 4a j, displayed a green emission at 510 nm with a quantum yield of 66% which is quite remarkable.

GENERAL SCHEME 19: Synthesis of thiazo-pyridiniums

GENERAL PROCEDURE 24 (SCHE E 19): Synthesis of thiazo-pyridiniums

[0086] Intrigued by the impact of the nature of the heteroatom on the photophysical properties, thiazo-pyridiniums were synthesized. These new structures were easily obtained by reacting oxazo-pyridiniums 7 with Lawesson’s reagent at 80 C in dioxane. Non-substituted thiazo-pyridinium 8a was obtained in 81% yield and iodinated derivative 8b was obtained in 68% yield. GENERAL PROCEDURE 25 (SCHEME 19): Photophysical properties of thiazo- pyridiniums

[0087] In order to explore the photophysical properties of those compounds and compare them to oxazo-pyridinium and imidazo-pyridiniums, compound 8b was further reacted with two different boronic esters to obtain compounds 8c and 8d. [0088] Photophysical properties of those new compounds were finally evaluated (Table 11). Spectra were measured using methanol as a solvent. No standard for quantum yield calculation matched excitation and emission wavelength. As anticipated, a bathochromic shift both in absorption and emission was observed. Interestingly, the quantum yields for 8a and 8b were lower than their nitrogenated and oxygenated counterparts whereas 8d had a greater quantum yield than 71 and

4ay.