“Process for the purification of methyl-2, 2-dithienylglycolate”

FIELD OF THE INVENTION

The present invention relates to a purification process for obtaining methyl-2, 2-dithienylglycolate with a reduced content of methyl-2-

(thiophene-2-yl)-2-(thiophene-3-yl) glycolate for use in the pharmaceutical industry.

BACKGROUND OF THE INVENTION

Methyl-2, 2-dithienylglycolate, methyl 2-hydroxy-2,2-dithiophen-2-yl acetate (MDTG, Formula (I)), is an important reaction intermediate in the synthesis of various anticholinergic agents with specificity for muscarinic receptors, useful in the treatment of chronic obstructive pulmonary disease (COPD), a chronic pulmonary disease caused by inflammation of the airways and lung parenchyma.

In particular, MDTG is commonly used, reacting with the appropriate polycyclic compounds, in the synthesis processes of tiotropium bromide, (1 a,2 ,4 ,5a,7 )-7-[(hydroxydi-2-thienylacetyl)oxy]-9,9- dimethyl-3-oxa-9-azoniatricyclo [3.3.1.0] nonane bromide (Formula (II)), and aclidinium bromide, [(8R)-1 -(3-phenoxypropyl)-1 -azoniabicyclo [2.2.2] octane-8-yl] 2-hydroxy-2,2-dithiophen-2-yl-acetate bromide (Formula (III)), and derivatives thereof.

Synthesis processes of tiotropium bromide and derivatives thereof, which use MDTG as a reaction intermediate, are for example described in US 5,610,163, US 6,486,321 , US 6,506,900, US 7,491 ,824, US 8,008,495, and EP 2018379.

Synthesis processes of aclidinium bromide and derivatives thereof, which use MDTG as a reaction intermediate, are for example described in in EP 1200431 , US 6,750,226, US 7,109,210, and WO 2016/162878.

Commonly, MDTG is synthesized by Grignard reaction of the dimethyloxalate with 2-thienylmagnesium bromide, according to the following scheme (a).

The reaction described above allows to obtain MDTG with good yields, leading however to the formation of a regioisomeric impurity represented by the methyl-2-(thiophene-2-yl)-2-(thiophene-3-yl) glycolate (2,3-MDTG, Formula (lb)). When the so obtained MDTG is used in the synthesis processes above, the 2,3-MDTG regioisomer is able to react with the polycyclic compounds leading in turn to the formation of the regioisomer impurities of formula (Mb) and (lllb), shown below, having chemical-physical characteristics very similar to the desired products and therefore difficult to separate.

It is therefore necessary to have a purification method allowing to obtain MDTG with a high degree of purity, in particular with a low content of 2, 3-MDTG regioisomeric impurity.

Methods of purification of MDTG are known in the art.

WO 2016/162878 discloses a purification process of MDTG, by adding toluene and activated carbon. The solid obtained by filtration of this mixture is subsequently dissolved in hot cyclohexane and cold- precipitated. The final product is obtained with an 85% purification yield. EP 2018379 discloses the synthesis of MDTG according to scheme

(a) above, and its purification by crystallization from ethanol (96%)/n- heptane, absolute ethanol/n-heptane, isopropanol/n-heptane, and toluene/n-heptane mixtures. The final product is obtained with a purification yield of 70-80%.

Indian patent application no. 1605/MUM/2014 discloses a method of purification of MDTG by cold precipitation from a 1 ,4-dioxane/water mixture, with a purification yield of 40%.

All the methods mentioned above, however, do not address the problem of the purification of MTDG from its 2, 3-MDTG regioisomer, or when they face it, as in the Indian application no. 1605/MUM/2014, they propose purification methods with extremely low yields, not compatible with an industrial production.

SUMMARY OF THE INVENTION

The Applicant has noted that there is a need to obtain methyl-2, 2- dithienylglycolate with high yields and purity, in particular with low content of the regioisomehc impurity methyl-2-(thiophene-2-yl)-2- (thiophene-3-yl) glycolate.

The Applicant has further noted that none of the methyl 2,2- dithienylglycolate purification methods known in the art allows to obtain high yields of methyl 2,2-dithienylglycolate comprising a 2,3-MDTG content sufficiently low to make it suitable for the pharmaceutical industry.

Accordingly, the Applicant has faced the problem of obtaining methyl 2,2-dithieneylglycolate with high yields and reduced content of 2,3- MDTG.

After extensive experimentation, the Applicant has surprisingly found a purification process that allows to obtain methyl-2, 2-dithienylglycolate with reduced 2,3-MDTG content and high yields, in particular with a content of 2,3- MDTG lower than or equal to 0.10% and yields higher than or equal to 70%.

Accordingly, object of the present invention is a process for the purification of methyl 2,2-dithienylglycolate from a mixture comprising methyl-2-(thiophene-2-yl)-2-(thiophene-3-yl) glycolate, comprising the following steps:

a) adding to said mixture an organic solvent selected from the group consisting of: linear alkyl ethers, linear alkenyl ethers, isobutyl acetate, toluene, and their mixtures with at least one co-solvent; b) bringing the mixture with the organic solvent to a first temperature selected in the range of from 30°C to 70°C;

c) maintaining the mixture with the organic solvent at said first temperature for at least 1 hour;

d) bringing the mixture with the organic solvent to a second temperature selected in the range of from 0°C to 25°C;

e) maintaining the mixture with the organic solvent at said second temperature until a crystalline solid is obtained; and f) separating the obtained crystalline solid from the mixture with the organic solvent,

wherein said crystalline solid is methyl-2, 2-d ith ienylg lycolate with a reduced content of methyl-2-(thiophene-2-yl)-2-(thiophene-3-yl) glycolate.

A further object of the present invention is crystalline methyl-2, 2- dithienylglycolate obtained from the purification process according to the present invention, characterized in that it comprises a percentage of methyl-2-(thiophene-2-yl)-2-(thiophene-3-yl) glycolate lower than or equal to 0.10%.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood by reading the following examples, intended as a guide and not as a limitation, to be read with the enclosed diagrams, wherein:

- Figure 1 shows the chromatogram of an MDTG sample obtained as described in example 1 ;

- Figure 2 shows the chromatogram of an MDTG sample obtained as described in example 6;

- Figure 3 shows the XRPD (X-Ray Powder Diffraction) spectrum of an MDTG sample obtained as described in example 6;

- Figure 4 shows the TGA (Thermal gravimetric Analysis) profile of an MDTG sample obtained as described in example 6;

- Figure 5 shows the DSC (Differential Scanning Calorimetry) profile of an MDTG sample obtained as described in example 6.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a process for the purification of methyl-2, 2-dithienylglycolate allowing to obtain methyl-2, 2- dithienylglycolate with a reduced content of methyl-2-(thiophene-2-yl)-2- (thiophene-3-yl) glycolate and high yields. Advantageously, the purification process according to the present invention allows to obtain, with a yield equal to or higher than 70%, methyl-2, 2-dithienylglycolate comprising a 2,3-MDTG percentage lower than or equal to 0.10% as measured via high performance liquid chromatography (HPLC).

In particular, the present invention relates to a process for the purification of methyl 2, 2-dithienylglycolate, from a mixture comprising methyl-2-(thiophene-2-yl)-2-(thiophene-3-yl) glycolate, comprising the following steps:

a) adding to said mixture an organic solvent selected from the group consisting of: linear alkyl ethers, linear alkenyl ethers, isobutyl acetate, toluene, and their mixtures with at least one co-solvent; b) bringing the mixture with the organic solvent to a first temperature selected in the range of from 30°C to 70°C;

c) maintaining the mixture with the organic solvent at said first temperature for at least 1 hour;

d) bringing the mixture with the organic solvent to a second temperature selected in the range of from 0°C to 25°C;

e) maintaining the mixture with the organic solvent at said second temperature until a crystalline solid is obtained; and

f) separating the obtained crystalline solid from the mixture with the organic solvent,

wherein said crystalline solid is methyl-2, 2-dithienylglycolate with a reduced content of methyl-2-(thiophene-2-yl)-2-(thiophene-3-yl) glycolate.

Said reduced content of the regioisomer methyl-2-(thiophene-2-yl)-2- (thiophene-3-yl) glycolate corresponds to 0.10%, measured as the percentage ratio of the area of the peaks obtained by HPLC, as better described in the Experimental Section. The purification process according to the present invention is carried out on a mixture comprising methyl-2, 2-d ith ienylg lycolate and methyl-2- (thiophene-2-yl)-2-(thiophene-3-yl) glycolate, i.e. on a dried or substantially solid methyl 2,2-dithienylglycolate crude or residue comprising methyl-2-(thiophene-2-yl)-2-(thiophene-3-yl) glycolate, and optionally variable amounts of other impurities.

Advantageously, the methyl-2, 2-dithienylglycolate used in the purification process according to the present invention can be commercially obtained, or it can be synthesized starting from 2- thienylmagnesium bromide, according to the following scheme (1 ).

Briefly, 2-thienylmagnesium bromide, prepared from 2- bromotiophene and magnesium shavings, is dissolved in tetrahydrofuran and this solution is added to a dimethyloxalate solution in tetrahydrofuran. The mixture is subsequently brought to the reflux temperature, and an aqueous NH ₄ CI solution is slowly added during stirring of the mixture. The product is extracted from the reaction mixture with chloroform and subsequently treated with ethanol, to give methyl-2, 2-dithienylglycolate. This preparation typically includes a percentage of methyl-2-(thiophene-2-yl)-2-(thiophene-3-yl) glycolate of 0.18-0.20%, which however is not suitable for subsequent uses in the pharmaceutical field.

The process according to the present invention allows to obtain methyl-2, 2-dithienylglycolate with a purification yield equal to or higher than 70%, preferably equal to or higher than 75%, even more preferably equal to or higher than 80%.

The crystalline solid obtained from the purification process according to the present invention is methyl-2, 2-dithienylglycolate comprising a percentage of methyl-2-(thiophene-2-yl)-2-(thiophene-3-yl) glycolate lower than or equal to 0.10%, thus suitable as synthetic intermediate for products intended for the pharmaceutical industry.

Preferably, said crystalline solid obtained from the purification process according to the present invention is methyl-2,2- dithienylglycolate comprising a percentage of methyl-2-(thiophene-2-yl)- 2-(thiophene-3-yl) glycolate of from 0.08% to 0.10%.

According to a particularly preferred aspect, said crystalline solid obtained from the purification process according to the present invention is methyl-2, 2-dithienylglycolate comprising a percentage of methyl-2-(thiophene-2-yl)-2-(thiophene-3-yl) glycolate equal to or lower than 0.08%.

In a crystallization purification process, such as that according to the present invention, the choice of the solvent is of fundamental importance.

After extensive experimentation, the Applicant has found that methyl-

2, 2-dithienylglycolate is completely soluble, already at room temperature, in solvents such as acetone, acetonitrile, dimethyl sulfoxide (DMSO), dimethylformamide (DMF), N-methyl pyrrol idone and tetrahydrofuran (THF), and only partially soluble in methanol, pyridine, and 1 -methyl imidazole.

Furthermore, methyl-2, 2-dithienylglycolate is completely insoluble in solvents such as water, hexane and heptane, even after heating the mixture.

These solvents are therefore considered unsuitable, if used alone, for the crystallization process. For the same reasons, water, hexane and heptane, are also unsuitable as co-solvents for the crystallization process according to the present invention. Indeed, the term“co-solvent” means a solvent which, when used in admixture with a different solvent, effectively aids the solubilisation of a solute.

Also considered to be unsuitable for the purification process according to the present invention are all those solvents leading to methyl-2, 2-dithienylglycolate comprising a percentage of methyl-2- (thiophene-2-yl)-2-(thiophene-3-yl) glycolate higher than 0.10%.

The preferred solvents, among those leading to methyl-2, 2- dithieneylglycolate comprising a percentage of methyl-2-(thiophene-2- yl)-2-(thiophene-3-yl) glycolate lower than or equal to 0.10%, are those solvents able to guarantee higher purification yields, that are less toxic and allow simpler processing conditions, for example being characterized by a not too high boiling point, e.g. not higher than 120°C, preferably not higher than 100°C.

Therefore, according to step a) of the process, an organic solvent selected in the group consisting of linear alkyl ethers, linear alkenyl ethers, isobutyl acetate, toluene, and their mixtures with a co-solvent is added to the methyl-2, 2-dithienylglycolate. Preferably, said mixture of solvents is pre-prepared before addition to the methyl-2, 2- dithienylglycolate.

According to a preferred aspect, said solvent or mixture of solvents is added at room temperature to the methyl-2, 2-dithienylglycolate, without achieving complete dissolution. According to this preferred aspect, said complete dissolution is achieved in step b) of the process, by raising the temperature.

According to a preferred aspect, said organic solvent is selected from methyl-t-butyl ether (MTBE), diisopropyl ether, isopropenylmethyl ether, isobutyl acetate, toluene, and their mixtures with an organic co-solvent. According to a more preferred aspect, when the selected organic solvent is toluene, preferably no co-solvent is used.

Preferably, said organic solvent is selected from MTBE and its mixtures with an organic co-solvent.

Even more preferably, said organic solvent is MTBE.

Preferably, the co-solvent is a solvent able to effectively solubilize methyl-2, 2-dithienylglycolate, when used in admixture with MTBE.

According to a preferred aspect, the co-solvent is an organic solvent selected from butanol, dimethyl acetamide, and xylene.

Advantageously, said co-solvent is used from 1 % to 90% with respect to the volume of said mixture, preferably from 5% to 50% with respect to the volume of said mixture, even more preferably from 10% to 30% with respect to the volume of said mixture.

In order to obtain working conditions compatible with laboratory practice, as well as with industrial scale practice, it is necessary that the starting mixture of the process according to the present invention has a concentration sufficiently high to minimize product losses due to processing, and at the same time to guarantee the good handling of the mixture.

Therefore, according to a preferred aspect, in step a) of the process the organic solvent is used in an amount of from 2 to 5 volumes with respect to the weight of methyl-2, 2-dithienylglycolate, preferably from 2.5 to 4 volumes with respect to the weight of the methyl-2, 2- dithienylglycolate, even more preferably from 3 to 3.5 volumes with respect to the weight of methyl-2, 2-dithienylglycolate.

Thereafter, the mixture is brought to a first temperature preferably selected according to the solubility of methyl-2, 2-dithienylglycolate in the solvent or mixture of solvent used, as well as to the boiling temperature of the selected solvent, or solvent mixture. Preferably, said temperature must be equal to or lower than the boiling point of the solvent, or mixture of solvents. The selected temperature must ensure complete solubilisation of methyl-2, 2-dithienylglycolate in step b) of the process.

Therefore, according to step b) of the process, the mixture is brought to a first temperature selected in the range of from 30°C to 70°C, preferably of from 40°C to 60°C, even more preferably of from 50°C to 55°C.

According to a preferred embodiment, the crystallization of the purified methyl-2, 2-dithienylglycolate is induced by seeding with pure methyl-2, 2-dithiene glycolate crystals. The crystals used for seeding may be obtained by subsequent repetitions of the purification process of the present invention, and preferably comprise a percentage of methyl- 2-(thiophene-2-yl)-2-(thiophene-3-yl) glycolate lower than 0.07%, more preferably lower than or equal to 0.05%.

According to step c) of the process, the mixture is left at said first temperature for at least 1 hour, preferably between 1 and 2 hours, even more preferably for more than 2 hours.

Thereafter, the mixture is cooled to a second temperature selected according to the solubility at room temperature of methyl-2,2- dithienylglycolate in the medium used. In particular, said second temperature must guarantee complete precipitation of methyl-2, 2- dithienylglycolate from the mixture.

According to step d) of the process, the mixture is brought to a second temperature selected in the range of from 0°C to 25°C.

Preferably, said second temperature is selected in the range of from

15°C to 25°C when the solvents used are: methyl-t-butyl ether, diisopropyl ether, isopropenylmethyl ether, isobutyl acetate, and their mixtures with an organic co-solvent.

Alternatively, said second temperature is selected in the range of from 0°C to 10°C, for example when the solvent is MTBE, toluene. Advantageously, the cooling from the first temperature according to step c) to the second temperature according to step d) takes place in a time interval of at least 1 hour, preferably between 1 and 2 hours, even more preferably of more than 2 hours.

According to step e) of the process, the mixture is kept at said second temperature until a crystalline solid is obtained.

Advantageously, the mixture is kept at said second temperature for at least 1 hour, preferably between 1 and 2 hours, even more preferably for more than 2 hours.

According to step f) of the process, the crystalline solid obtained is then separated from the mixture, preferably by filtration.

According to this preferred embodiment, the mixture is preferably filtered on sintered glass, cold-washed several times with the organic solvent used for the process, and the crystalline solid is dried under reduced pressure.

The crystalline solid obtained from the purification process according to the present invention, consisting of methyl-2, 2-dithienylglycolate with a methyl-2-(thiophene-2-yl)-2-(thiophene-3-yl) glycolate content lower than or equal to 0.10%, can be characterized by High Performance Liquid Chromatography (HPLC), Electrospray Ionization Mass Spectrometry (ESI-MS), proton and carbon Nuclear Magnetic Resonance ( ¹ H-NMR, ¹³ C-NMR), Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), and X-ray Powder Diffraction (XRPD).

The crystalline methyl-2, 2-dithienylglycolate obtained from the purification process according to the present invention is characterized in that it comprises an amount of methyl-2-(thiophene-2-yl)-2- (thiophene-3-yl) glycolate lower than or equal to 0.10%, preferably of from 0.08% to 0.10%, even more preferably lower than or equal to 0.08% as measured by HPLC. Moreover, the crystalline methyl-2, 2- dithienylglycolate obtained from the purification process according to the present invention is characterized by an HPLC profile substantially as shown in Figure 2.

Furthermore, the crystalline methyl-2, 2-dithienylglycolate obtained from the purification process according to the present invention is characterized by an XRPD spectrum having distinguishable peaks at 7.94°, 13.34°, 15.91 °, 21.86°, 22.65°, 22.94° (2-Theta, ± 0.1 ). Furthermore, the crystalline methyl-2, 2-dithienylglycolate obtained from the purification process according to the present invention is characterized by an XRPD spectrum substantially as shown in Figure 3.

Alternatively, the crystalline methyl-2, 2-dithienylglycolate obtained from the purification process according to the present invention is characterized by a DSC profile having an endothermic peak at 93.25°C ± 1 °C with onset at 92.78°C ± 1 °C. Furthermore, the crystalline methyl- 2, 2-dithienylglycolate obtained from the purification process according to the present invention is characterized by a DSC profile substantially as shown in Figure 5.

The Applicant, to the best of its knowledge, is not aware of a characterization of the crystalline form of methyl-2, 2-dithienylglycolate such as that carried out in the present invention and shown in Figures 3-5. Moreover, the FIPLC analytical method used for the detection of methyl-2-(thiophene-2-yl)-2-(thiophene-3-yl) glycolate, herein reported in Example 2 and whose results are shown in Figures 1 -2, was not known.

EXPERIMENTAL SECTION

EXAMPLE 1

Synthesis of methyl-2,2-dithienylalvcolate

A tetrahydrofuran solution (TFIF, 2.5 L) of 2-thienylmagnesium bromide, prepared from 2-bromotiophene (330 g, 195 mL, 2 mol) and magnesium shavings (50 g, 2 mol), was added dropwise to a dimethyloxalate solution (120 g, 1 mol) in THF (0.5 L) under nitrogen atmosphere. After 1 hour at room temperature and 2 hours at the reflux temperature, the reaction mixture was cooled using an ice bath, and an aqueous solution of NH ₄ CI (1.5 L) was slowly added during stirring of the mixture, which was subsequently extracted with chloroform. The organic phases were combined and washed with an aqueous solution of NaHC0 ₃ and a saturated aqueous NaCI solution, dried over Na ₂ S0 ₄ and then concentrated.

The residue was then treated with ethanol to give 1 15 g of methyl- 2,2-dithienylglycolate and analysed by HPLC (MDTG, yield 45%, methyl-2-(thiophene-2-yl)-2-(thiophene-3-yl) glycolate (2,3-MDTG): 0.18-0.20%, as measured by HPLC).

EXAMPLE 2

HPLC evaluation of the 2,3-MDTG percentage comprised in a MDTG sample

An HPLC analytical method for the analysis of the impurity percentage due to 2,3-MDTG in an MDTG sample was optimized.

HPLC analyses were carried out using a Shimadzu instrument equipped with binary pump (LC-20AT), UV detector (SPD-20A), autosampler (SIL-20AC), degasser (DGU-20A5) and column thermostat

(CTO-20AC).

The chromatogram of a methyl-2, 2-dithienylglycolate sample obtained as described in Example 1 is shown in Figure 1 , where a clear separation between the two peaks is observed.

The conditions chosen were as follows:

Column: Zorbax Rx-C8 or equivalent

150x4,6 mm, 5pm

Mobile phase A: 10 ml of triethylamine (TEA) in 1000 ml

ultrapure water, brought to pH 3.0 with perchloric acid Mobile phase B: acetonitrile

Gradient:

Mobile phase B

Minutes

(v/v %)

25

25

51

25

25

25

Flow: 2 ml/min

Wavelength: 254 nm

Column temperature: 25°C

Injection volume: 5 pi

Acquisition time: 45 minutes

EXAMPLE 3

Crystallization purification of MDTG: solvent screening

Crystallization purification experiments have been carried out using a wide range of solvents considering the different solubility of the two isomers.

Briefly, a suitable amount of solvent was added to a methyl-2, 2- dithienylglycolate sample prepared according to Example 1 and the suspension was brought to a temperature T1. The mixture was then left under stirring at this temperature for one hour, cooled to a temperature T2 and maintained at this temperature for 30 minutes.

In all the experiments, the least amount of solvent needed to completely dissolve the solid sample was used. The T1 temperature was chosen taking into account the boiling temperature of the solvent used. The T2 temperature was chosen according to the solubility at room temperature of the MDTG in the given solvent.

The sample was then filtered on sintered glass and analysed by HPLC to measure the amount of 2,3-MDTG present.

Methyl-2, 2-dithienylglycolate was completely soluble already at room temperature in acetone, acetonitrile, DMSO, DMF, N-methylpyrrolidone and THF, and partially soluble in methanol, pyridine, and 1 -methyl imidazole. Furthermore, MDTG was completely insoluble in water, hexane and heptane even after heating the mixture. These solvents were therefore unsuitable, if used alone, for the crystallization process.

The solvents, temperatures and MDTG/solvent ratios used, together with the percentage of 2,3-MDTG measured at the end of the procedure, are shown in the following Table 1.

TABLE 1

a p/V ratio

MTBE: methyl-t-butyl ether

As evidenced by the results reported in Table 1 , the solvents: isobutyl acetate, toluene, diisopropyl ether, isopropenylmethyl ether and MTBE allow to obtain MDTG with a percentage of 2,3-MDTG equal to or lower than 0.10%.

EXAMPLE 4

Crystallization purification of MDTG: co-solvents

In order to further improve the results obtained in Example 3, crystallization tests were carried out using MTBE and a number of co- solvents.

The specific solvents, amounts and temperatures used, together with the percentage of 2,3-MDTG detected, are summarized in the following Table 2. The results obtained in Example 3 using pure MTBE are reported in the table for comparison (tests A(i) and B(i) of the invention).

TABLE 2

a percentage amount in MTBE/co-solvent mixture

b p N ratio

As evidenced by the results summarized in Table 2 (tests 4, 5 and 8), crystallizations carried out using MTBE and a co-solvent such as butanol, dimethyl acetamide or xylene, yielded MDTG with acceptable purity (2, 3-MDTG < 0.10%).

A more in-depth study was therefore conducted on the use of MTBE/butanol, MTBE/dimethyl acetamide and MTBE/xylene binary mixtures. Table 3 below shows the ratios and temperatures used, together with the yield obtained and the percentage of 2, 3-MDTG detected, for crystallization experiments carried out with MTBE/butanol mixtures. Table 3 also shows the results obtained in Example 3 using pure MTBE at T1 and T2 temperatures comparable to those used with butanol (test B(i) of the invention).

TABLE 3

a percentage amount in MTBE/co-solvent mixture

b p N ratio

Table 3 results show that all the tested MTBE/butanol binary mixtures allow to obtain MDTG with satisfactory yields and purity. Test 6 also confirms the results described in Example 3, namely that the use of butanol alone does not allow to obtain sufficiently pure MDTG (2,3- MDTG > 0.10%).

The experiments carried out on binary mixtures MTBE/dimethyl acetamide and MTBE/xylene gave results comparable to those obtained on MTBE/butanol mixtures.

EXAMPLE 5

Crystallization purification of MDTG according to the invention: procedure

3 volumes of MTBE were added to a methyl 2,2-dithienylglycolate sample prepared according to example 1 , and the suspension was brought to the reflux temperature (53-54°C) until the product was completely dissolved. The solution was then brought to 45°C and crystallization induced by seeding with methyl-2, 2-dithienylglycolate crystals comprising a percentage of 2,3-MDTG of about 0.05%, obtained by one or more successive repetitions of the purification process according to test B(i) of the invention described in Example 4. The mixture was then left stirring at 45°C for one hour, then cooled to 20-25°C in 2 hours and maintained at this temperature for 30 minutes.

The sample was filtered on sintered glass, washed with cold MTBE and analysed by HPLC. The crystalline white solid thus obtained (80% purification yield, purity >99.9%, 2.3-MDTG 0.08%) was dried under reduced pressure.

The residual solution was concentrated and placed at 4°C for about 2 hours. The sample was then filtered on sintered glass, washed with cold MTBE and analysed by HPLC. This second crystalline sample was re- crystallized twice to obtain a sample with the same purity as the first one.

EXAMPLE 6

Crystallization purification of MDTG according to the invention: scale- UJD

3 volumes of MTBE (1.5 L) were added to a sample of methyl-2, 2- dithienylglycolate (510 g, 2.0 mol) prepared according to example 1 , and the suspension was brought to the reflux temperature (53-54°C) until the product was completely dissolved. The solution was then brought to 45°C and crystallization induced by seeding with methyl-2, 2- dithienylglycolate crystals comprising a percentage of 2,3-MDTG of about 0.05%, obtained by one or more subsequent repetitions of the purification process according to test B(i) of the invention described in Example 4.

The mixture was then left stirring at 45°C for one hour, then slowly cooled to 20-25°C over 2 hours and kept at this temperature for 1 hour.

The sample was filtered on sintered glass, washed with cold MTBE and analysed by HPLC (Figure 2). The thus obtained crystalline white solid (370 g, 1.45 mol, crystallization yield >75%, purity >99%, 2,3- MDTG: 0.08%) was dried under reduced pressure and subjected to mass spectrometry analysis ESI-MS, ¹ H-NMR, ¹³ C-NMR, DSC, and XRPD. The significant peaks of the spectra obtained and the conditions of analysis are shown below.

Mass spectrometry analyses were performed using a Waters QTof Micro with positive ESI.

The significant peaks of the mass spectrum recorded for the thus obtained methyl-2, 2-d ith ienylg lycolate sample are shown in the following Table 4. TABLE 4

Proton and carbon nuclear magnetic resonance analyses were recorded in deuterated chloroform (CDCI ₃ ) on a Varian Mercury 400 spectrometer, equipped with a 5 mm ATH ¹ H/ ¹⁹ F/X probe. The spectrum was obtained by dissolving 5 mg of the test sample in 0.6 mL of deuterated solvent.

The peaks of the ¹ H-NMR and ¹³ C-NMR spectra recorded for the methyl-2, 2-dithienylglycolate sample obtained as described above are shown in the following Table 5 with the respective structure assignments.

TABLE 5

Diffraction measurements were carried out under room conditions on a PANalytical X'Pert PRO Q-Q diffractometer of 240 mm radius in reflection geometry, equipped with Cu Ka 1 radiation and an X'Celerator detector, operated at 40 kV and at 40 mA. The sample was mounted on a silicon sample holder with a null background and was rotated at 1 rpm during data collection. The angular interval was 3.0-40.0° (20) with an interval of 0.017°. The scanning speed was 0.082 s (40.8 s/interval).

The obtained methyl-2, 2-d ith ienylg lycolate sample can be characterized by an XRPD spectrum substantially as shown in Figure 3, which significant peaks are shown in the following Table 6.

TABLE 6

TGA analyses were performed using a Mettler Toledo TGA/DSC1 instrument, equipped with an XP5 scale and coupled with a Thermo Nicolet iS10 IR-spectrometer. The measurements were performed on approximately 100 pL of sample, in the temperature range of from 25°C to 320°C, with a temperature increase of 10°C/min.

The TGA profile recorded for the obtained methyl-2, 2- dithienylglycolate sample (Figure 4) shows only one phenomenon of weight decrease of the analysed sample, which starts around 200°C, indicating the thermal decomposition of the compound.

Finally, the DSC analyses were carried out on a Mettler-Toledo DSC- 1 equipped with version 13.00 of the STARe software. The test sample was heated at 10°C/min from 20 to 350°C under a nitrogen flow of 50 mL/min.

The obtained methyl-2, 2-d ith ienylg lycolate sample can be characterized by a DSC profile substantially as shown in Figure 5, and having an endothermic peak at 93.25°C ± 1 °C with onset at 92.78°C ± 1 °C.

COMPARATIVE EXAMPLE 1

Purification of MDTG according to procedures known in the art

Further purification tests were performed using procedures described in the art. The solvents, amounts and temperatures used, together with the yield obtained and the percentage of 2,3-MDTG detected, are shown in the following Table 7.

In particular, the results shown were obtained in MDTG purification by crystallization from absolute heptane/ethanol and heptane/toluene mixtures (tests 1 (c) and 2 (c)), and a MDTG purification by cold- precipitation from a 1 ,4-dioxane/water mixture (test 3 (c)).

The results obtained in Examples 3 and 4, using pure MTBE or 90:10 MTBE/butanol mixture as solvent and co-solvent (tests B(i) - C(i) of the invention, respectively), are also shown in Table 7.

TABLE 7

a percentage amount in solvent mixture

b p/V ratio

As evidenced by the results shown in Table 7, crystallization from heptane containing mixtures (tests 1 (c) and 2(c)) allowed to obtain good purification yields but an unsatisfactory amount of 2,3-MDTG (>0.15%) and in any case much higher than those obtained with the procedures of the invention.

Furthermore, the purification by cold-filtration from a 1 ,4- dioxane/water mixture (test 3(c)) gave absolutely unacceptable results both in terms of the purification yield of 30% and the amount of 2,3- MDTG detected (0.19%).

In all three comparative tests, were obtained impurity levels (2,3- MDTG >0,15%) much higher than that obtainable with any of the procedures according to the invention hereinbefore described (2,3- MDTG £0.10%).

COMPARATIVE EXAMPLE 2

Purification of MDTG by extraction

Extraction purification experiments were performed in consideration of the different LogP value of the two isomers, indicative of a different affinity to water.

A methyl 2,2-dithienylglycolate sample prepared as described in Example 1 was dissolved in an organic solvent and the mixture was extracted 3 times with an aqueous solution. The organic phase was then dried over Na2S0 ₄ and the solvent removed under reduced pressure. The solid thus obtained was analysed by HPLC to measure the percentage of 2,3-MDTG impurity.

The solvents, the aqueous solutions and the amounts used, together with the percentage of 2,3-MDTG detected, are shown in the following Table 8.

TABLE 8

As evidenced by the results summarized in Table 8, none of the tested conditions allowed to obtain methyl-2, 2-dithienylglycolate containing a percentage of 2,3-MDTG lower than or equal to 0.10%. This purification technique therefore does not lead to a satisfactory purity degree.

COMPARATIVE EXAMPLE 3

Purification of MDTG by resuspension

Purification experiments by resuspension in different organic solvents have been carried out considering the different solubility of the two isomers.

A methyl 2, 2-dithienylglycolate sample prepared as described in Example 1 was resuspended in a suitable organic solvent and the mixture was stirred at room temperature for 1 hour. The suspension was then filtered and the filtrate dried under reduced pressure. The thus obtained solid was analysed by HPLC to measure the percentage of 2,3-MDTG impurity.

The solvents and the amounts used, together with the percentage of 2,3-MDTG detected, are shown in the following Table 9. TABLE 9

As evidenced by the results reported in Table 9, none of the tested conditions allowed to obtain methyl-2, 2-dithienylglycolate containing a percentage of 2,3-MDTG lower than or equal to 0.10%. This purification technique therefore does not lead to a satisfactory purity degree.