Patent Publication Number: US-2012028340-A1

Title: Kinetic resolution of (4s) -- 4- phenyl -- 3- [(5rs)-5-(4-flurophenyl)-5-
hydroxypentanoyl] --1,3-oxazolidin-2-one to the (5s) isomer via lipase
catalyzed enantioselective esterification of the (5r) isomer

Description:
FIELD OF INVENTION 
     The invention relates to novel method for synthesis of optically pure (4S)-4-phenyl-3-[(5S)-5-(4-fluorophenyl)-5-hydroxypentanoyl]-1,3 oxazolidin 2-one, of formula I, an intermediate used for the synthesis of ezetimibe (formula II) and (3R,4S)-4-(3,3′dihydroxybiphenyl-4-yl)3-[(3S)-3-(4-fluorophenyl)-3-hydroxypropyl]1 phenylazetidin-2-one (DEPA, formula III) through enzymatic kinetic resolution. 
     BACKGROUND OF THE INVENTION 
     This invention relates to the novel process for the chiral synthesis of ezetimibe and DEPA intermediate of Formula I, (4S)-4-phenyl-3-[(5S)-5-(4-fluorophenyl)-5-hydroxypentanoyl]-1,3 oxazolidin 2-one from the corresponding diastereoisomeric alcohols formula V. This is schematically represented in  FIG. 1 . 
     Ezetimibe (Formula II) (CAS. No. 163222-33-1), 1-(4-fluorophenyl)-3(R)-[3-(4-fluorophenyl)-3(S)-hydroxypropyl]-4(S)-(4-hydroxyphenyl)-2-azetidinone), a potent and selective cholesterol absorption inhibitor is disclosed in U.S. Pat. No. 5,767,115. 
     U.S. Pat. No. 7,320,972 describes DEPA (Formula III) (3R,4S)-4-(3,3′dihydroxybiphenyl 4yl)3-[(3S)-3-(4-fluorophenyl)-3-hydroxypropyl]1-phenylazetidin-2-one as a potent inhibitor of cholesterol absorption. 
     
       
         
         
             
             
         
       
     
     Compound of formula I can be converted to either Ezetimibe (II) or DEPA (III) through the process provided in U.S. Pat. No. 6,207,822 and WO2006122216 respectively. 
     Prior art reveals that compound I has been synthesized by following methods:
         1) Reduction of corresponding ketone (IV) by using borane as a reducing agent such as Borane-dimethyl sulfide (as in U.S. Pat. No. 6,207,822), Borane-tetrahydrofuran complex (as in Tetrahedron Letters, 44 (2003) 801-804), or Borane diethylaniline complex (as in Tetrahedron Letters, 48(2007) 2123-2125) in presence of chiral catalyst such as (R)-1-methyl-3,3-diphenyltetahydro-3H-pyrrolo[1,2-c][1,3,2]oxazaborole.   2) Compound ‘I’ has also been synthesized by microbial reduction of ketone (IV) as per the process provided in U.S. Pat. No. 5,618,707.       

     However, most of the reported methods suffer from the following disadvantages
         1) Boranes are highly reactive compounds and some are pyrophoric in nature. Most of these are highly poisonous and require special handling precautions, necessitating special operation procedure and higher capital cost.   2) Boranes are volatile and flammable in nature.   3) Some boranes are reported to be unstable on storage, such as borane tetrahydrofuran complex (THF ring opening) and also reported to be explosive on prolonged storage.   4) Borane-dimethyl sulfide has an unpleasant odor and a fact not liked by operation.   5) Enantiomeric purity is never exceed 95%, often requiring further purification.   6) Efficiency of microbial reduction process for desired compound I is low and required high dilution and hence not a practical process. Further, any microbial process is associated with fermentation, which needs special equipment such as sterile condition and environment. Use of autoclave to sterilize the fermentation media, bioreactor, etc.       

     Non-chiral reduction of corresponding ketone (IV) to racemic hydroxy compounds V (Tetrahedron Letters, 44 (2003)), which is diastereoisomer and in-principle could be separated by crystallization. However, such separation is not simple and easy for the present case due to lower melting points of the individual isomers (39.7° C. for compound of formula I). 
     Summarizing it is evident that there is a need for development of an eco-friendly, hazard free, “green”, cost effective process for the synthesis of compound I. This invention provides that. 
     OBJECTS OF THE INVENTION 
     Thus the object of the present invention is to provide enzymatic kinetic resolutions of 4S-phenyl-3-[(5RS)-5-(4-fluorophenyl)-5-hydroxypentanoyl]-1,3 oxazolidin 2-one to obtain optically pure compound I with enantiomeric purity of at least about 98%. 
     Other object of the present invention is to provide an eco-friendly and hazard free process for the preparation of compound of formula I. 
     Another object of the present invention is to provide a process for the preparation of compound of formula I with better efficiency and selectivity. 
     A further object of the present invention is to provide an improved industrial process for the preparation of compound of formula I that produces minimum by-products. 
     SUMMARY OF INVENTION 
     The present invention provides a process for synthesis of 4S-phenyl-3-[(5S)-5-(4-fluorophenyl)-5-hydroxypentanoyl]-1,3 oxazolidin 2-one comprising of resolution of 4S-phenyl-3-[(5RS)-5-(4-fluorophenyl)-5-hydroxypentanoyl]-1,3 oxazolidin 2-one by selective esterification of 4S-phenyl-3-[(5R)-5-(4-fluorophenyl)-5-hydroxypentanoyl]-1,3 oxazolidin 2-one using appropriate esterification reagent in an organic solvent in presence of Lipase enzyme at a temperature ranging from 0° to 100° C., and further isolation. 
     The esterification agent used in the said process is vinyl acetate 
     Lipase enzyme used in the process of invention is selected from the group of Lipase AS, Lipase PS, Novozym 435, Lipozyme TL IM, or Lipozyme RM IM; more preferably it is Lipozyme TL IM. 
     Organic solvent used for the esterification reaction is selected from Toluene, diisopropyl ether or a mixture thereof. 
     The process of invention is carried out more preferably at 40° C. 
     The isolation of desired compound I is carried out by column chromatography or crystallization. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed towards the method for preparation of enantiomerically pure 4S-phenyl-3-[(5S)-5-(4-fluorophenyl)-5-hydroxypentanoyl]-1,3 oxazolidin 2-one (I). The method of the present invention involves a kinetic resolution of 4S-phenyl-3-[(5SR)-5-(4-fluorophenyl)-5-hydroxypentanoyl]-1,3 oxazolidin 2-one (VI) by selective acetylation of one isomer in presence of lipase. 
     Interestingly, the present inventors found that 4S-phenyl-3-[(5R)-5-(4-fluorophenyl)-5-hydroxypentanoyl]-1,3 oxazolidin 2-one (the undesired enantiomer) specifically undergoes the acetylation reaction to give compound of formula VI and the desired compound (I) remains un-reacted. Compound of formula I is easily separated from compound of formula VI by column chromatography or through crystallization by derivatization of the desired alcohol moiety. 
       FIG. 1  shows the inventive process developed for synthesis of optically pure desired compound I. 
     The process consists of reaction of vinyl acetate with compound of formula V in organic solvent in presence of specified lipase to yield a mixture containing compound I with high yield and high % ee, and undesired isomer is acetylated to give VI. The resulting mixture of alcohol of formula I and acetate of formula VI after usual work-up is purified to individual compounds by column/flash chromatography. 
     Such enantioselective acetylation reaction of racemic hydroxy compound in presence of lipase is well documented (Hydrolases in organic synthesis, ed Bornscheuer and Kazlauskas, Wiley VCH verlag GmbH &amp; Co, 2006, pp 61-183). However, apriori, it is difficult to predict required enzyme, solvent, temperature and turnover number of catalyst i.e. lipase. 
     The resulting hydroxy compound (Formula I) which is enantiomerically enriched undergoes further reaction to yield the desired essentially enantiomerically pure Ezitimibe (II) and DEPA (III). Compound VI can be reused in enzymatic resolution after subsequent racemization via ester hydrolysis and oxidation to obtain compound IV through known chemical methods, thereby improving overall yield. 
     The enzyme (or Biocatalyst) may be any protein that will catalyze the enatioselective esterification of one enantiomer to yield the ester of hydroxy compound. Useful enzymes for enantioselectively esterification of hydroxy compound to ester of hydroxy compound may thus include hydrolases, including lipases. Such enzyme may be obtained from a variety of natural sources, including animal organs and microorganisms. 
     As described hereinafter useful enzymes for the enantioselective conversion of the hydroxy compound to ester of undesired hydroxy compound include lipases obtained from various biological sources (Table 1). Preferably such lipases include enzyme derived from the microorganism Termomyces lanuginosus, such as available from Novozyme A/S. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Lipases screening for enantioselective esterification 
               
            
           
           
               
               
               
            
               
                 Sr. 
                   
                   
               
               
                 No 
                 Lipase 
                 Enzyme Supplier 
               
               
                   
               
               
                 1 
                 Lipase AS “Amano” ( Aspergillus niger ) 
                 Amano Japan 
               
               
                 2 
                 Lipase PS “Amano” ( Burkholderia cepacia ) 
                 Amano, Japan 
               
               
                 3 
                 Novozym 435 ( Candida antarctica  lipase B) 
                 Novozyme A/S 
               
               
                 4 
                 Lipozyme TL IM ( Thermomyces lanuginosus ) 
                 Novozyme A/S 
               
               
                 5 
                 Lipozyme RM IM ( Mucor miehei ) 
                 Novozyme A/S 
               
               
                   
               
            
           
         
       
     
     The reaction mixture may comprise a single phase or may comprise multiple phases. For example, the enantioselective hydrolysis may be carried out in two phases system comprised of solid phase, which contains the enzyme, and an solvent, which contains the initially racemic substrate, the undesired optically active ester and the desired optically active hydroxy compound I. 
     The amounts of the racemic substrate (Formula V) and the biocatalyst used in the enantioselective hydrolysis will depend on, the properties of the recemic substrate and enzyme. Reaction may generally employ an enzyme loading of about 10% to about 100% and in many cases, may employ an enzyme loading of about 10 to 50% (W/V) 
     The enantioselective esterification may be carried out over wide range of temperature. For example, the reaction may be carried out at temperature of about 25° C. to a 50° C., but typically carried out at 40° C. Such temperatures generally permit substantially full conversion e.g, 95 to 99% of the one enantiomer in a reasonable period of time e.g. 72 to 120 h without deactivating the enzyme. Enzyme can be reused, and generally the turn-over of immobilized enzyme is high. 
     The enantioselective esterification may be carried out in different solvents. For example, the reaction may be carried in solvent such as toluene, diisopropyl ether (DIPE), cyclohexane, n-heptane, n-hexane and THF. Preferentially aromatic solvent such as toluene is preferred, 
     Activated ester used in enantioselective esterification may be consisting of vinyl acetate. 
     After the completion of reaction, desired hydroxy compound of formula I and ester of undesired enantiomer (VI) is separated out by column chromatography using silica gel as stationary phase and cyclohexane:ethyl acetate (1:1) as a mobile phase. 
     The present invention is illustrated in more detail by referring to the following Examples, which are not to be construed as limiting the scope of the invention. 
     Enzymatic screening reactions were performed in an HLC Termomixer. All enzymes used in the screening plate were obtained from commercial enzyme suppliers including Amano (Japan) and Novozyme (Denmark) 
     Example 1 
     Enzyme screening via enzymatic esterification of (R/S)4-phenyl-3-[(5S)-5-(4-fluorophenyl)-5-hydroxypentanoyl]-1,3 oxazolidin 2-one 
     Enzyme screening was carried out in HLC parallel thermomixer, which consist of 14 chambers to carry out individual reaction in 10 ml vial (called as individual reactors). Each individual reactor was charged with 3 ml toluene, 100 mg of substrate, and 300 mg of vinyl acetate and stirred at room temperature for 15 min. In each reactor different type of lipases (50% w/w of substrate) was added to initiate reaction. The resulting mixture was stirred at 40° C. for 120 h. reaction was monitor with chiral HPLC for enantioselectivity of Lipases. Retention time of compound I matched with standard sample prepared by known method as provided in U.S. Pat. No. 6,207,822. 
     Retention Time of Compounds on Chiral HPLC 
     
         
         
           
             1) Compound I—20.60 min 
             2) Compound V—17.44- and 20.60 min 
             3) Compound IV—16.07 min 
           
         
       
    
     Chiral HPLC Condition 
     
         
         
           
             1) Column: Chiralcel ODH (4.6×250 mm) 
             3) Mobile Phase: n-hexane and ethanol (70:30), flow rate: 0.7 ml/min.; Detection (UV): at 210 nm. 
           
         
       
    
     Compound VI:  1 HNMR (200 MH z , CDCl 3 ): 
     δ 7.15-7.42 (m, 7H), 7.00 (t, 2H), 5.7 (t, 1H), 5.4 (dd, 1H), 4.68 (t 1H), 4.27 (dd 1H), 2.93 (dt 2H), 2.1 (m, 3H), 1.58-1.80 (m, 4H) ppm 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Lipases screening for enantioselective esterification of Compound V 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 % Conversion of 
                   
               
               
                   
                   
                 one enantiomer 
               
               
                   
                 Lipase 
                 (HPLC) 
                 % ee P   
               
               
                   
                   
               
               
                   
                 Novozyme 435 
                 21 
                 51 
               
               
                   
                 Lipozyme TL 
                 — 
                 — 
               
               
                   
                 RM 
               
               
                   
                 Amano, AS 
                 — 
                 — 
               
               
                   
                 Amano, PS 
                 20 
                 93 
               
               
                   
                 Lipozyme TL IM 
                 99 
                 99 
               
               
                   
                   
               
            
           
         
       
     
     Example 2 
     Effect of enzyme loading on enzymatic esterification of (R/S)4-phenyl-3-[(5S)-5-(4-fluorophenyl)-5-hydroxypentanoyl]-1,3 oxazolidin 2-one 
     Effect of enzyme loading was carried out in HLC parallel thermomixer, which consist of 14 chambers to carry out individual reaction in 10 ml vial (called as individual reactors). Each individual reactor was charged with 3 ml toluene, 100 mg of substrate, and 300 mg of vinyl acetate and stirred at room temperature for 15 min. In each reactor different % w/w of Lipozyme TL IM lipase was added to initiate reaction. The resulting mixture was stirred at 40° C. for 120 h. reaction was monitor with chiral HPLC for enantioselectivity of Lipases.  FIG. 2  gives effect of catalysts loading on the rate of reaction. 
     Example 3 
     Effect of different solvent on enantioselective enzymatic esterification of (R/S)4-phenyl-3-[(5S)-5-(4-fluorophenyl)-5-hydroxypentanoyl]-1,3 oxazolidin 2-one 
     Effect of enzyme loading was carried out in HLC parallel thermomixer, which consist of 14 chambers to carry out individual reaction in 10 ml vial (called as individual reactors). Each individual reactor was charged with 3 ml of solvent, 100 mg of substrate, and 300 mg of vinyl acetate and stirred at room temperature for 15 min. In each reactor of Lipozyme TL IM lipase was added to initiate reaction. The resulting mixture was stirred at 40° C. for 120 h. reaction was monitor with chiral HPLC for enantioselectivity of Lipases. Table 3. Effect of different solvent on enantioselectivity 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Effect solvent on enantioselective esterification of Formula A 
               
            
           
           
               
               
               
               
            
               
                   
                 Lipase 
                 Solvent 
                 % ee p   
               
               
                   
                   
               
               
                   
                 Lipozyme 
                 DIPE 
                 96 
               
               
                   
                 TL IM 
               
               
                   
                 Lipozyme 
                 Toluene 
                 99 
               
               
                   
                 TL IM 
               
               
                   
                   
               
            
           
         
       
     
     Example 4 
     Lipozyme TL IM catalyzed esterification of 4S-phenyl-3-[(5RS)-5-(4-fluorophenyl)-5-hydroxypentanoyl]-1,3 oxazolidin 2-one 
     4S-phenyl-3-[(5RS)-5-(4-fluorophenyl)-5-hydroxypentanoyl]-1,3 oxazolidin 2-one (1 gm), 3 ml. toluene and 3 g of vinyl acetate were stirred at room temperature for 15 min at an ambient temperature in a HLC thermomixer. The reaction mixture was heated to 40° C. and 300 mg of Lipozyme TL IM (Thermomyces lanuginosus) were added to it. Progress of the reaction was monitored using chiral HPLC. After complete conversion of unwanted 4S-phenyl-3-[(5R)-5-(4-fluorophenyl)-5-hydroxypentanoyl]-1,3 oxazolidin 2-one to the Compound VI, reaction was filtered to remove the enzyme. Filtrate was concentrated under vacuum to remove toluene to give crude product, which on column chromatography over silica gel gives 0.37 gm (yield 74%, 99% ee) of compound I, and 0.34 gm (yield 68%, 99% ee) of compound of formula VI.