Abstract:
The present invention provides a process for preparation of optically active azabicyclo heptanone derivates using lactamases that will react with racemic lactam of formula (I) to give a single enanotiomer of lactam (III) and the corresponding ring opened compound of formula (IV) in an enantiomerically pure form.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a process for the preparation of an optically active azabicyclo heptanone derivative of general Formula III                           
     wherein R 1 =H, X=CH 2 , Y—Z=—CH═CH—, from a racemic mixture of the γ-lactams of Formula (I)                           
     wherein R 2  is H or COR 3 , (R 3  is C 1-4  alkyoxy, aryl or aryloxy), 
     X′ is O or CHR 4  (R 4  is F, OH, Br, or H). 
     Y′—Z′ is CH═CH,                           
     or —CH(R 6 )CH 2 — (R 6 =Br, OH, PhCH 2 O, or N 3 , 
     The present invention also relates to a process for the preparation of an optically active azabicyclo heptanone derivative of general Formula III wherein R 1 =H, X′=CH 2 , Y′—Z′=CH═CH 2  is useful as an intermediate in the synthesis of antiviral agents. 
     BACKGROUND OF THE INVENTION 
     Carbocyclic analogues of purines are known as antiviral and anti neoplastic agents. For example the compound of Formula II is described as having potent activity against human immunodeficiency virus (HIV) and Hepatitis B virus (HBV) (EP 0434450).                           
     wherein R 5 =Cyclopropylamino, or N-Cyclopropyl, N-Methylarnine. 
     Prior art discloses the preparation of 9-substituted-2-amino purines starting from a pyrimidine compound, coupling with enantiomerically pure sugar/carbocyclic analogues residue and cyclization to form the imidazole ring followed by introduction of suitable 6-substituent (PCT/GB95/00225). Carbovir and known analogues are prepared from the known γ-lactam (Vince lactam) 2-azabicyclo[2,2,1]hept-5-en-3-one (Formula I) wherein X′ is —CH 2 , —Y′—Z′— is —CH═CH— and R 2  is H. 
     Prior art indicates that the final product or any intermediate or starting material may be resolved by known methods or the racemic mixture of the product may be enzymatically converted to chirally pure compound. The γ-lactam can be prepared by reacting cyclopentadiene with tosylcyanide, (Vince J. Org. Chem. 1978, 43, 2311). 
     There are several synthetic pathways where chemical resolution into the enantiomer has been effected but the enzymatic resolution of γ-lactam will be the most economical commercial process. γ-lactamase methodology has been reported based on enantio-complementary biotransformation. Enzymatic resolution of bicyclic lactam using whole cell cultures ENZA1 and ENZA2 has been reported to give both the optical forms of lactam (S. V. Teylor, J. C. S. Chem. Comm., 1121, 1990, Tet. Assy., 4, 1117-1128). The detailed process has been described in patent (EP 0424064). The racemic lactam was treated with ENZA-½ cell free extract at 30° C. with shaking for 14 days. The crude (+)/(−) lactam was isolated by extraction with dichloromethane and purified by column chromatography on silica gel to provide the (+) lactam with 88% optical purity, ee, and the (−) lactam with 98% ee, optical purity. 
     Enzymatic resolution of N-Acyl bicyclic lactam using acylase has been described in patent (PCT/EP99/04814) in 31% yield with 98% ee. The conversion of the optically active N-Acetyl-lactam to (+)/(−) lactam is tedious. 
     The prior art methods to the cyclopentane moiety of carbocyclic nucleosides starting from non-carbohydrate synthons or readily available meso compounds generally involve a number of steps, which are often difficult to perform and provide poor yields, making the practical scale-up of these processes difficult and uneconomical. 
     OBJECTS OF THE INVENTION 
     The main object of the present invention is to provide a process for the preparation of an optically active azabicyclo heptanone derivative which obviates the drawbacks of the prior art processes and use cheaper and easily available microbial whole cell enzyme. 
     It is another object of the invention to provide a process for the preparation of (−) 2-Azabicyclo[2,2,1]-hept-5-ene-3-one Formula III, which is economical and efficient. 
     SUMMARY OF THE INVENTION 
     The present invention provides a process for the preparation of optically active azabicyclo heptanone derivatives using lactamases that will react with racemic γ-lactam of Formula I to give a single enantiomer of lactam (III) and the corresponding ring opened compound of formula (IV) in an enantiomerically pure form.                           
     Accordingly, the present invention provides a process for the preparation of (−)2-Azabicyclo[2,2,1]-hept-5-ene-3-one Formula III wherein R 1 =H, X=CH 2 , Y—Z=—CH═CH—,                           
     which comprises reacting a racemic mixture of a compound of Formula I:                           
     wherein R 2  is H or COR 3 , (R 3  is C 1-4  alkoxy, aryl or aryloxy), 
     X′ is O or CHR 4  (R 4  is F, OH, Br, or H), 
     Y′—Z′ is CH═CH,                           
     or —CH(R 6 )CH 2 — (R 6 =Br, OH, PhCH 2 O, or N 3 , 
     with an enzyme, a lactamase or the whole cells of a microorgnism in a buffer containing organic solvent at temperature ranging between 25-30° C. for a period ranging from 14 to 24 hr., extracting the mixture into an organic solvent, separating the organic layer and removing the organic solvent to obtain the product. 
     In one embodiment of the invention the microorganisms or enzymes used are selected from Bacillus, Klyuvera or Eschericha. 
     In another embodiment of the invention the whole cell is obtained from growing a culture of  Klyuvera Citrophila , ATCC No.21285 ( American Tissue and Type Culture, Box  1549, Manassas, Va. 20108), in a conventional culture medium. 
     In another embodiment of the invention, the cell extract or enzyme used comprises an enzyme or a cell extract from Klyuvera sp. (ATCC No.21285). 
     In another embodiment of the invention the buffer used is selected from the group consisting of phosphate buffer (0.05 M-0.1 M, 6-8 pH), citrate buffer (0.05 M-0.1 M 6-7.5 pH) and Trisbuffer (0.05M-0.2M, 7-8 pH). 
     In another embodiment of the invention the buffer used comprises phosphate buffer (0.2 M, 7.4 pH). 
     In another embodiment of the invention the organic solvent used for the reaction along with buffer is selected from the group consisting of alcohols, alkyl acetates, ketones and sulfoxides. 
     In a further embodiment of the invention, the organic solvent is selected from the group consisting of methanol, ethanol, butanol, ethyl acetate, acetone, dimethyl sulfoxide and dimethylformamide. 
     In a further embodiment of the invention, the organic solvent comprises acetone. 
     In another embodiment of the invention the percent of organic solvent used for the reaction along with buffer is in the range of 5% to 50% (v/v). 
     In another embodiment of the invention the percent of organic solvent used for the reaction along with buffer comprises is 10%(v/v). 
     In another embodiment of the invention the solvent used for extraction comprises a chlorinated solvent selected from the group consisting of chloroform, ethylene dichloride, methylene dichloride and an alkyl acetate. 
     In another embodiment of the invention, the alkyl acetate used as the solvent for extraction comprises ethyl acetate. 
     In another embodiment of the invention the solvent used for extraction comprises methylene chloride. 
     In a feature of the invention the chemical yield (−) 2-Azabicyclo[2,2,1]-hept-5-ene-3-one is 39.3% and the optical purity is 98%. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The process of the present invention is highly efficient with maximizes cost effectiveness by a fast resolution process to provide (−) lactam enantiomer, an important starting material for the production of the anti HIV agent (−) Carbovir and Abacavir. 
     The process of the present invention is described herein below with references to the following examples, which are illustrative only and should not be construed to limit the scope of the present invention in any manner. 
     EXAMPLE 1 
     This example describes general procedure for a biomass preparation using whole cell or an enzyme pre-inoculum (5-10 ml) prepared by growing a microorganism in a medium containing a yeast extract (0.5%), peptone (1%), sodium chloride (0.2%), sodium glutamate (0.5%) and phenyl acetic acid (15 mM) at pH 7.2-7.3 for 24 hr with shaking at 150 rpm. 300 ml of the above mentioned growth medium subsequently transferred to a 1 liter flask and incubated at 28-30° C. for 24 hours on a rotary shakers (150 rpm). The biomass of grown cells were separated by centrifuge and washed with phosphate buffer pH 6.8, The separated biomass or cell mass was used for the reaction described in Example 2. 
     EXAMPLE 2 
     The general procedure for the enantioselectsive hydrolysis of (±)-2-azabicyclo(2,2,1)hept-5-en-3-one, (Vince&#39;s lactam) (1) is as follows: 
     0.1 g (0.00092 mole parts) of (±)-2-azabicyclo[2,2,1]-hept-5-en-3-one (1) was suspended in phosphate buffer (5 parts) and 50 mg of the wet biomass of culture (ATCC No.21285) was added. The mixture was kept stirring for 72 hr. The cell mass was removed by filtering through celite and the filtrate was extracted with dichloromethane (5×10 parts). Removal of the solvent gave optically active Formula III in 31.8% chemical yield, and 58.2% ee. 
     EXAMPLE 3 
     The general procedure for the enantioselective hydrolysis of (±)-2-azabicyclo(2,2,1)hept-5-en-3-one, using the cell mass obtained from culture (ATCC No.21285) was followed. 0.1 g (0.00092 mole parts) of the (±) racemic mixture was suspended in phosphate buffer (5 parts) and an amount of the cell mass as indicated in Table 1 was added. The mixture was kept stirring for 24 hrs. The cell mass was then removed by filtering through celite and the filtrate was extracted with dichloromethane (5×10 parts). Removal of the solvent gave optically active Formula III. The results are summarized in Table 1. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Sr. No. 
                 Cells Wet/Wt. % 
                 Chemical Yield 
                 Ratio R:S 
                 ee % 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1. 
                 5 
                 39.1 
                 46.36:53.64 
                 7.29 
               
               
                 2. 
                 10 
                 33.2 
                 45.15:54.85 
                 9.71 
               
               
                 3. 
                 20 
                 31.4 
                 36.70:63.30 
                 26.61 
               
               
                 4. 
                 30 
                 33.0 
                 27.43:72.57 
                 45.14 
               
               
                 5. 
                 40 
                 34.2 
                 25.71:74.29 
                 48.58 
               
               
                 6. 
                 50 
                 30.5 
                 21.23:78.77 
                 57.55 
               
               
                   
               
             
          
         
       
     
     EXAMPLE 4 
     The general procedure for enantioselective hydrolysis of (±)2-azabicyclo(2,2,1)hept-5-en-3-one, using the cell mass from culture (ATCC No.21285) was followed. 0.2 g (0.00184 mole parts) of (±) was suspended in phosphate buffer and an organic solvent,(as indicated in Table 2) 10 parts. 0.1 gm of wet cell mass was added and kept stirring 24 hrs. The cell mass was removed by filtering through celite and the filtrate was extracted with dichloromethane (5×10 parts). Removal of the solvent gave optically active Formula III. The results are summarized in Table 2. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Sr. No. 
                 Organic Solvent 
                 Chemical Yield 
                 Ratio R:S 
                 ee % 
               
               
                   
               
             
             
               
                 1. 
                 Ethyl acetate 
                 27.1 
                 13.39:86.61 
                 73.22 
               
               
                 2. 
                 Methanol 
                 25.5 
                 14.90:85.10 
                 70.20 
               
               
                 3. 
                 Ethanol 
                 33.1 
                  9.27:90.73 
                 81.46 
               
               
                 4. 
                 Acetone 
                 44.2 
                  9.69:90.31 
                 80.62 
               
               
                 5. 
                 Dimethyl sulfoxide 
                 42.1 
                 36.10:63.90 
                 27.80 
               
               
                   
               
             
          
         
       
     
     The ratio of phosphate buffer (0.2M, pH 7.4) to organic solvent is (9:1). 
     EXAMPLE 5 
     The general procedure for enantioselective hydrolysis of (±)2-azabicyclo(2,2,1)hept-5-en-3-one, using the cell mass from culture (ATCC No.21285) was followed. 0.2 g (0.00184 mole parts) of (±) was suspended in phosphate buffer and acetone (as indicated in Table 2) 10 parts. 0.1 gm of wet cell mass was added and the mixture was kept stirring for 24 hrs. The cell mass was removed by filtering through celite and the filtrate was extracted with dichloromethane (5×10 parts). Removal of the solvent gave optically active Formula III. The results of different proportions of acetone are summarized in Table 3. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Sr. No. 
                 Buffer:Acetone (v/v) 
                 Chemical Yield 
                 Ratio R:S 
                 ee % 
               
               
                   
               
             
             
               
                 1. 
                 9.5:0.5 
                 40.8 
                  4.58:95.85 
                 90.85 
               
               
                 2. 
                 9.0:1.0 
                 41.2 
                  9.69:90.31 
                 80.62 
               
               
                 3. 
                 8.0:2.0 
                 41.3 
                 12.34:87.66 
                 75.33 
               
               
                 4. 
                 5.0:5.0 
                 Slow reaction 
                 — 
                 — 
               
               
                   
               
             
          
         
       
     
     EXAMPLE 6 
     The general procedure for the enantioselective hydrolysis of (±)2-azabicyclo(2,2,1)hept-5-en-3-one, using cell mass from culture (ATCC No.21285) was followed. 10.0 g (0.918 mole parts) of (±) was suspended in mixture of 475 parts of phosphate buffer and 25 parts of acetone in a 1 liter flask. Cell mass (wet. Weight˜5 parts) was added and the reaction mixture was stirred at room temperature (28±1° C. After completion of the reaction (monitored by chiral HPLC) the reaction mixture was centrifuged in order to remove the cell mass. The supernatent liquid was extracted using a continuous extractor by dichloromethane. On evaporation of the solvent under reduced pressure, (1S, 4R) azabicyclo(2,2,1)hept-5-en-3-one III (3.93 gm) was obtained. Crystallization with a dichloromethane:ether mixture gave a product of 98% optical purity.