Process for chiral resolution of trans-2,2-dichloro-3-(3,5-dichlorophenyl)-cyclopropane-1-carboxylic acid by chiral chromatography

Abstract

The present invention provides a process for chiral resolution of trans-2,2-dichloro-3-(3,5- dichlorophenyl)-cyclopropane-1-carboxylic acid by chiral chromatography using an eluent comprising a mixture of an apolar solvent and a polar co-solvent selected from aliphatic alcohols, ethyl acetate and acetonitrile, and using a chiral stationary phase selected from the list consisting of amylose tris (S)-α-methylbenzyl carbamate, amylose tris (3-chloro-4-methylphenylcarbamate), amylose tris (3,5-dimethylphenylcarbamate) and amylose tris (3-chloro-5-methylphenylcarbamate).

Description

[0001] Process for chiral resolution of trans-2, 2-dichloro-3-(3,5-dichlorophenyl)- cyclopropane-1 -carboxylic acid by chiral chromatography

[0002] Technical field

[0003] The present invention relates to the field of enantiomer resolution and particularly to a process of chiral resolution of trans-2, 2-dichloro-3-(3,5-dichlorophenyl)cyclopropane-1-carboxylic acid by chiral chromatography.

[0004] Background

[0005] Trans-2, 2-dichloro-3-(3,5-dichlorophenyl)cyclopropane-1 -carboxylic acid has Formula I as shown below. It is one of the intermediates in the synthesis of compounds known from LIS2018 / 0098541 A1 and WO2016 / 168059 A1. Formula I

[0006] In the present application only the trans isomers, namely the (R,R) and (S,S) enantiomers are considered: (1F?,3F?)-2,2-dichloro-3-(3,5-dichlorophenyl)cyclopropane-1 -carboxylic acid (“the (R,R) enantiomer”) and (1 S,3S)-2,2-dichloro-3-(3,5-dichlorophenyl)cyclopropane-1-carboxylic acid (“the (S,S) enantiomer”).

[0007] The compound of Formula I is often obtained as a racemic mixture of the (R,R) and (S,S) enantiomers. For further use of one of the two enantiomers it is desired to perform chiral resolution of the racemate.

[0008] It is an object of the present invention to provide a robust, reproducible and scalable method for the chiral resolution of the racemic compound of Formula I. Preferably, the method leads to a high optical purity and a high yield of the desired enantiomer. Preferably, the process is suitable to be used in an industrial environment. Preferably, the process can be implemented as a continuous process.

[0009] Summary of invention

[0010] The present invention provides a process of chiral resolution of a racemic compound of Formula I Formula I by comprising the following steps: a) absorbing the racemic compound onto a chiral stationary phase; b) passing an eluent through the chiral stationary phase in an amount sufficient to elute the enantiomers contained in the racemic mixture from the chiral stationary phase; c) isolating the enantiomers partially or completely separately from each other, wherein the eluent comprises a mixture of an apolar solvent and a polar co-solvent selected from aliphatic alcohols, ethyl acetate and acetonitrile, and wherein the chiral stationary phase selected from the list consisting of amylose tris (S)-a- methylbenzyl carbamate, amylose tris (3-chloro-4-methylphenylcarbamate), amylose tris (3,5- dimethylphenylcarbamate) and amylose tris (3-chloro-5-methylphenylcarbamate).

[0011] Detailed description

[0012] Inventors have developed a new and advantageous process for chiral resolution of a racemic mixture of trans-2, 2-dichloro-3-(3,5-dichlorophenyl)cyclopropane-1-carboxylic acid by chiral chromatography.

[0013] “Racemate”, or “racemic compound”, or “racemic mixture” is a mixture that has equal amounts of left- and right-handed (R and S) enantiomers of a chiral molecule. “Chiral molecule” means that the molecule is non-superimposable on its mirror image.

[0014] “Enantiomer” is each of the two non-superimposable images. The Cahn-lngold-Prelog convention is one of the systems used to designate each enantiomer. The chiral centers of the molecule are assigned a designation of R or S depending upon the configuration of the groups attached to the chiral center. Each of the four groups attached to an asymmetric carbon (chiral center) is ranked based on its atomic number. When the molecule is oriented, so the lowest ranked group is facing away from the viewer, the remaining groups are counted in descending order. If the order proceeds clockwise, the chiral center is designated R. If the order proceeds counterclockwise, the chiral center is designated S.

[0015] Chiral chromatography is a method to separate enantiomers of chiral compounds in a liquid chromatography column. The process uses an eluent (mobile phase) and a chiral stationary phase.

[0016] In an embodiment, chiral chromatography method comprises the following steps: a) absorbing a racemic mixture that is desired to separate onto a chiral stationary phase; b) passing an eluent through the chiral stationary phase in an amount sufficient to elute the enantiomers contained in the racemic mixture from the chiral stationary phase; c) isolating the enantiomers partially or completely separately from each other.

[0017] Although it is known that high performance liquid chromatography (HPLC) can be used for chiral resolution, there are several challenges associated with the use of this method in practice.

[0018] First of all, there is a challenge to find a suitable chiral stationary phase with a suitably high selectivity and low retention time. Chiral stationary phases (CSP) are typically silica-based materials derivatized with polysaccharides that are modified with chiral selectors and which are designed to separate mixtures of enantiomeric compounds.

[0019] The inventors have tested numerous chiral stationary phases and only a few phases were found suitable for a successful chiral resolution of the racemic compound of Formula I. These are amylose tris (S)-a-methylbenzyl carbamate, amylose tris (3-chloro-4-methylphenylcarbamate), amylose tris (3,5-dimethylphenylcarbamate) and amylose tris (3-chloro-5-methylphenylcarbamate). Preferably, amylose tris (3,5-dimethylphenylcarbamate) or amylose tris (3-chloro-5-methylphenylcarbamate) is used. More preferably, amylose tris (3,5-dimethylphenylcarbamate) is used because it allowed to reach a higher optical purity at certain compound loadings.

[0020] The mentioned chiral stationary phases are available commercially. Amylose tris (S)-a-methylbenzyl carbamate is for example available as Chiralpak® AS-V (from Daicel). Amylose tris (3-chloro-4- methylphenylcarbamate) is for example available as Chiralpak® AZ (from Daicel). Amylose tris (3,5- dimethylphenylcarbamate) is available as Lux Amylose 1 (from Phenomenex), Chiralpak® AD and Chiralpak® IA (both from Daicel). Amylose tris (3-chloro-5-methylphenylcarbamate) is available as Chiralpak® IG (from Daicel).

[0021] Also, a suitable solvent system to be used as an eluent needs to be found. The solvent system should, on one hand, be able to sufficiently dissolve the racemic mixture, which can be a challenge for poorly soluble compounds. On the other hand, the solvent system should be suitable for column chromatography, meaning that it for example should not lead to degradation of the stationary phase or interact with the chiral selector of the CSP too much.

[0022] The present inventors have found that the compound used in the present invention (Formula I), unlike other aliphatic carboxylic acids substituted with an aromatic lipophilic moiety, is well soluble in some organic solvents commonly used in column chromatography, particularly in some polar solvents such as short chain aliphatic alcohols, ethyl acetate and acetonitrile. This allowed to envisage their separation by chiral HPLC with sufficient productivity.

[0023] The solvent (eluent) preferably comprises a mixture of an apolar solvent and a polar co-solvent. The apolar solvent is preferably selected from alkanes or aromatic hydrocarbons, such as heptane, n- hexane, iso-hexane, cyclohexane, isooctane, toluene, preferably from heptane and hexane. The polar co-solvent is preferably selected from aliphatic alcohols, ethyl acetate and acetonitrile, preferably an aliphatic alcohol. Preferred aliphatic alcohols are primary and secondary alcohols, more preferably a primary alcohol. In other embodiments, a secondary alcohol can be used.

[0024] It was found by the inventors that the best results are achieved when the eluent comprises a mixture of heptane or hexane as the apolar solvent, and an alcohol as the polar co-solvent. In a preferred embodiment, a mixture of heptane and an alcohol is used. Preferably, n-heptane is used. In other embodiments, hexane and an alcohol can be used. Hexane can be n-hexane or iso-hexane. Suitable iso-hexanes are 2-methylpentane, 3-methylpentane or 2,3-dimethylbutane, preferably 2- methylpentane.

[0025] The polar solvent is preferably a short chain (Ci-Ce or preferably C1-C3) aliphatic alcohol, more preferably selected from the list consisting of methanol, ethanol, 1 -propanol and 2-propanol (isopropanol). Most preferably, ethanol is used because of superior selectivity compared to other alcohols.

[0026] In some embodiments, the eluent comprises a mixture of n-heptane and an alcohol selected from methanol, ethanol, 1-propanol and 2-propanol, preferably a mixture of n-heptane and ethanol. In other embodiments, the eluent comprises a mixture of n-heptane and methanol. In yet other embodiments, the eluent comprises a mixture of n-heptane and 1-propanol. In yet other embodiments, the eluent comprises a mixture of n-heptane and 2-propanol.

[0027] In other embodiments, the eluent can comprise a mixture of n-hexane and an alcohol selected from methanol, ethanol, 1-propanol and 2-propanol, preferably a mixture of n-hexane and ethanol. In other embodiments, the eluent comprises a mixture of n-hexane and methanol. In yet other embodiments, the eluent comprises a mixture of n-hexane and 1-propanol. In yet other embodiments, the eluent comprises a mixture of n-hexane and 2-propanol.

[0028] In yet other embodiments, the eluent can comprise a mixture of iso-hexane (2-methylpentane) and an alcohol selected from methanol, ethanol, 1-propanol and 2-propanol, preferably a mixture of isohexane and ethanol. In other embodiments, the eluent comprises a mixture of iso-hexane and methanol. In yet other embodiments, the eluent comprises a mixture of iso-hexane and 1-propanol. In yet other embodiments, the eluent comprises a mixture of iso-hexane and 2-propanol. The alcohol is preferably present in the eluent in an amount of 0.1-15% (vol / vol), more preferably 1- 10% (vol / vol), yet more preferably 5-10% (vol / vol). It has been observed that the content of more than 10% (vol / vol) alcohol led to overlapping peaks and did not allow baseline separation. The content of 5-10% (vol / vol) has been found optimal to allow baseline separation.

[0029] Preferably, the eluent further comprises an acid, for example a carboxylic acid. Examples of suitable acids include acetic acid, trifluoroacetic acid (TFA) and formic acid. The advantage of this is that the acid lowers the pH of the mixture, which prevents the racemate of Formula I from converting into a deprotonated form. Without the use of an acid, it was observed that the peaks were overlapping and tailing, which makes the separation difficult or impractical. Preferably, TFA is used as it leads to higher resolution results than other acids.

[0030] The acid can be present in the eluent in an amount of 0.01-5% (vol / vol), preferably 0.05-1 % (vol / vol).

[0031] The process can be conducted at about room temperature, such as in the range from 20 °C to 35 °C, preferably from 20 °C to 25 °C.

[0032] The process according to the invention surprisingly allows to achieve both high selectivity and productivity, which make it well suitable for a large-scale preparative separation.

[0033] In a preferred embodiment, the process is implemented as a continuous process. A continuous process operates on the bases of continuous flow, as opposed to batch, intermittent or sequence operations. A skilled person is aware of continuous liquid chromatography processes. Continuous processes for chiral chromatography can be implemented such as SMB (simulated moving bed) process, which is a multi-column continuous purification process, which consists of columns connected in series, with inlet / outlet lines connected between the columns. The SMB technique is based on a quasi counter-current contact between the stationary and mobile phases mimicking a true moving bed. Skilled person is aware of SMB systems and is able to determine how the process described above can be implemented in an SMB system.

[0034] Implementation of the above-described process as a continuous process allows to increase the yield considerably (e.g. from 20 % to 95 %), while having a high productivity at the same time.

[0035] The advantage of the claimed process is the efficient separation of the enantiomers in both high productivity and high selectivity. A process for producing a single enantiomer will normally involve a chiral separation of a racemic compound or an enantiomerically selective reaction. For processes that entail a chiral separation, it is desirable to have this chiral separation as early in the process as possible. This will increase efficiency as the undesired enantiomer is not carried through the subsequent steps. To be truly efficient, the process must be highly selective in separating the two enantiomers. Furthermore, the process must also have a sufficiently high throughput or productivity to produce enough of the desired enantiomer in a reasonable amount of time at affordable costs. Coated polysaccharide-based chiral stationary phases are phases in which the polysaccharide is not covalently bonded to the underlying silica. Immobilized polysaccharide chiral stationary phases are those in which the polysaccharide is covalently linked to the underlying silica.

[0036] The efficiency of a chiral separation process can be judged by the degree of separation achieved between the enantiomers of a racemate and the productivity as measured in terms of the mass of racemate processed per the mass of the chiral stationary phase used per a period of time.

[0037] The productivity of the described process is preferably greater than 1.0 KKD of racemate (or crude material treated), more preferably greater than 1.5 KKD. The productivity of the process can for example be in the range of 1.0 to 6.0 KKD of racemate. Productivity (KKD) is defined as the kg amount of racemate (crude material treated) that can be separated per kg of chiral stationary phase (CSP) per day.

[0038] The selectivity (a) is the ratio of the retention factors of the two peaks of the two enantiomers as shown in Equation 1. It can be visualized as the distance between the apices of the two UV signals.

[0039] Equation 1 :

[0040] In which TE2 is the retention time of the second eluting enantiomer, TEI is the retention time of the first eluting enantiomer and To the time taken by the mobile phase to pass through the column.

[0041] Preferably, the selectivity of the process is higher than 1.0, more preferably higher than 1.2, such as for example in the range from 1.0 to 4.0, or from 1 .3 to 3.5.

[0042] Retention factor is the ratio of the retention time of an enantiomer on the column to the retention time of the mobile phase. Retention factor of the second enantiomer (k’2) is therefore the ratio of the retention time of the second eluting enantiomer to the retention time of the mobile phase as shown in Equation 2.

[0043] Equation 2:

[0044] In which TE2 is the retention time of the second eluting enantiomer and To the time taken by the mobile phase to pass through the column. Retention factor is preferably low, e.g. less than 12, preferably in the range 2-11.

[0045] The invention will now be further described by the following, non-limiting, examples. Examples

[0046] Example 1 : Solubility tests

[0047] The solubility of the compound with Formula I was determined in a set of organic solvents and water. Solubility was determined by visual evaluation at room temperature, by stepwise addition of the solvent on a measured racemate weight. The results are shown in Table 1.

[0048] TFA = trifluoroacetic acid

[0049] Table 1

[0050] It was observed that the compound with Formula I is surprisingly highly soluble in polar solvents such as specifically ethanol, methanol, 2-propanol, ethyl acetate and acetonitrile, which are also preferred solvents of choice for performing chromatographic separations. Addition of an acid such as TFA was also tested because it was observed that without the use of acids the peaks were often overlapping and tailing, which makes separation difficult or ineffective. However, it was observed that the addition of TFA negatively impact solubility in some solvents.

[0051] Example 2: Chiral stationary phases screening The ability of the following chiral stationary phases to separate a racemic mixture of Formula I was determined. Diluted racemate was injected on each stationary phase, and the product was eluted with binary mixtures of solvents (A for apolar solvent, B for polar co-solvent) as mobile phases. The chiral stationary phases are recited in Table 2, the conditions are recited in Table 3, the combination of CSPs and mobile phases screened are in Table 4.

[0052] Table 2

[0053] Table 3

[0054] Column 4.6 x 250 mm columns packed with CSPs (see Table 2)

[0055] Flow rate 1 mL / min

[0056] Isocratic elution of binary compositions, mixed in line for

[0057] Mobile Phase screening, and pre-mixed for overloaded injections.

[0058] UV 220 nm for screening study

[0059] Detection ELSD for use of dichloromethane and AcOEt

[0060] Room temp, for screening

[0061] Temperature 25 °C by default for overloaded injections

[0062] Feed solution Around 1 g racemate / L in ethanol for screening

[0063] Inj

[0064] Jected quantity 7 / volume , 10 pL for screening per runr a

[0065] Run time (min) 30 min for screening Table 4

[0066] See Table 2 for the abbreviations A separation is deemed interesting when the selectivity between the 2 enantiomers peaks to be separated is high. Short run time (max 30 min), as well as high solubility, are also favorable to a good productivity in industrial process. The best combinations of a stationary phase associated with a mobile phase were then selected based on a balance between run time (retention factor), solubility, and separation quality (selectivity). The conditions for which the selectivity was the highest (a > 1.25), together with a reasonable retention factor for the second enantiomer (2 < k’2 s 11), were selected as the best performing. Only eight of twenty chiral stationary phases investigated allowed for an acceptable degree of separation of a racemic mixture of Formula I, which are listed in Table 5.

[0067] Table 5

[0068] The first screening tests used neutral mobile phases without TFA. However, even if separations were observed, the peaks obtained were very broad and tailing (e.g. in #1 and #3). This would make separation of enantiomers very difficult or ineffective. In order to sharpen the peaks and keep a single protonated form of the compound of Formula I, an acid such as trifluoroacetic acid (TFA) was added to the mobile phases.

[0069] From the combinations from Table 5 using TFA in the eluent, combinations #2 and #4-8 used four CSPs which are preferred in this invention: amylose tris (S)-a-methyl benzyl carbamate, amylose tris (3-chloro-4-methylphenylcarbamate), amylose tris (3,5-dimethylphenylcarbamate) and amylose tris (3-chloro-5-methylphenylcarbamate). Two groups of chiral selectors were found particularly suitable for the separation of a racemic mixture of Formula I: first the amylose tris (3,5- dimethylphenylcarbamate), corresponding to Lux Amylose 1 I Chiralpak® AD and Chiralpak® IA stationary phases, and secondly the amylose tris (3-chloro-5-methylphenylcarbamate), corresponding to Chiralpak® IG stationary phase. Both combined with a mobile phase composed of 5 to 10 vol.% of ethanol in n-heptane with trifluoroacetic acid (TFA). These specific combinations allowed for the optimal balance of high selectivity and a low retention factor. Example 3: Productivity

[0070] A sample of a racemic mixture of Formula I was solubilized at 20 g / L in eluent consisting of a n-heptane / ethanol 95 / 5 (vol / vol) + 0.1 (vol%) TFA. This solution was injected on an amylose tris (3,5- dimethylphenylcarbamate) coated on silica gel column (Chiralpak® AD). The sample was eluted with the eluent at 25 °C. Both enantiomers were obtained using multi-column continuous process Varicol. The productivity (KKD) was estimated to be 2.5 kg of racemate / kg of chiral stationary phase / day on a continuous process translating into 1 .3 kg of desired enantiomer / kg of chiral stationary phase / day.

[0071] A sample of a racemic mixture of Formula I was solubilized at 20 g / L in eluent consisting of a n-heptane / ethanol 95 / 5 (vol / vol) + 0.1 (vol%) TFA. This solution was injected on an amylose tris (3,5- dimethylphenylcarbamate) coated on silica gel column (Chiralpak® AD). The sample was eluted with the eluent at 30 °C. Both enantiomers were obtained using multi-column continuous process Varicol. The productivity (KKD) was estimated to be 2.9 kg of racemate / kg of chiral stationary phase / day on a continuous process translating into 1.5 kg of desired enantiomer / kg of chiral stationary phase / day.

[0072] A sample of a racemic mixture of Formula I was solubilized at 20 g / L in eluent consisting of a n-heptane / ethanol 90 / 10 (vol / vol) + 0.1 (vol%) TFA. This solution was injected on an amylose tris (3,5-dimethylphenylcarbamate) coated on silica gel column (Chiralpak® AD). The sample was eluted with the eluent at 25 °C. Both enantiomers were obtained using multi-column continuous process Varicol. The productivity (KKD) was estimated to be 2.1 kg of racemate / kg of chiral stationary phase / day on a continuous process translating into 1.1 kg of desired enantiomer / kg of chiral stationary phase / day.

[0073] For the above processes the optical purity was >99 area% (UV-HPLC at 280 nm) and the recovery >95%, translating into 47.5% yield with maximum achievable 50%.

Claims

Claims1. A process of chiral resolution of a racemic compound of Formula IFormula I by chiral chromatography comprising the following steps: a) absorbing the racemic compound onto a chiral stationary phase; b) passing an eluent through the chiral stationary phase in an amount sufficient to elute the enantiomers contained in the racemic mixture from the chiral stationary phase; c) isolating the enantiomers partially or completely separately from each other, wherein the eluent comprises a mixture of an apolar solvent and a polar co-solvent selected from aliphatic alcohols, ethyl acetate and acetonitrile, and wherein the chiral stationary phase selected from the list consisting of amylose tris (S)-a- methylbenzyl carbamate, amylose tris (3-chloro-4-methylphenylcarbamate), amylose tris (3,5- dimethylphenylcarbamate) and amylose tris (3-chloro-5-methylphenylcarbamate).

2. The process of claim 1, wherein the chiral stationary phase is amylose tris (3,5- dimethylphenylcarbamate).

3. The process of claim 1, wherein the chiral stationary phase is amylose tris (3-chloro-5- methylphenylcarbamate).

4. The process of any one of claims 1-3, wherein the apolar solvent is selected from heptane and hexane.

5. The process of claim 4, wherein the apolar solvent is n-heptane.

6. The process of any of claims 1-5, wherein the polar co-solvent is an aliphatic alcohol.

7. The process of claim 6, wherein the alcohol is selected from methanol, ethanol, 1-propanol and 2-propanol.

8. The process of claim 7, wherein the alcohol is ethanol.

9. The process of any one of claims 6-8, wherein the alcohol is present in the eluent in an amount of 0.1-15% (vol / vol).

10. The process of any one of claims 1-9, wherein the eluent further comprises an acid.

11. The process according to claim 10, wherein the acid is selected from trifluoroacetic acid, formic acid and acetic acid.

12. The process according to claim 11 , wherein the acid is trifluoroacetic acid.

13. The process according to any one of claims 10-12, wherein the amount of the acid in the eluent is in the range 0.01-5% (vol / vol).

14. The process of any one of claims 1-13, wherein the process is conducted at a temperature in the range from 20 °C to 35 °C, preferably from 20 °C to 25 °C.

15. The process of any one of claims 1-14, which is a continuous process.

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