Abstract:
Lactams of 1-amino-3-carboxylic acid cyclic compounds are produced in enantiomeric form, together with an enantiomer of the corresponding ring-opened amino-acid or ester, by reaction of the racemic lactam with a novel lactamase. The products are useful in the synthesis of chiral carbocyclic nucleotides. The enantiomer is preferrably 2-azabicyclo(2.2.1)hept-5-en-3-one. It is desirable to isolate the enantiomer comprising predominantly the (+) enantiomer and a residual amount of the (-) enantiomer, wherein the (+) enantiomer is present in an enantiomeric excess of at least about 88% over the (-) enantiomer or the enantiomer comprising predominantly the (-) enantiomer and a residual amount of the (+) enantiomer, wherein the (-) enantiomer is present in an enantiomeric excess of at least about 98% over the (+) enantiomer.

Description:
This application is a continuation of application Ser. No. 08/035,236, filed Mar. 22, 1993 now abandoned, which is a division of Ser. No. 07/596,306, filed Oct. 15, 1990, now U.S. Pat. No. 5,284,769. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to chiral compounds having utility as intermediates in the synthesis of anti-viral agents, and to a process for their preparation. 
     BACKGROUND OF THE INVENTION 
     Various 9-substituted purines are known as anti-viral and anti-neoplastic agents. One such compound, known as AZT, has been used for the treatment of AIDS. More recent examples are disclosed in EP-A-4268672, U.S. Pat. No. 4,742,064, and also in GB-A-2217320 which, by way of specific illustration, describes a compound known as Carbovir (carbocyclic 2&#39;,3&#39;-didehydro-2&#39;,3&#39;-dideoxyguanosine). Not surprisingly, the enantiomers of such chiral compounds have different activities. 
     Carbovir and known analogues are prepared from the known γ-lactam, 2-azabicyclo[2.2.1]hept-5-en-3-one, i.e. the compound of formula ##STR1## in which X is --CH 2  --, --Y--Z-- is --CH═CH--, and R n  is absent. The prior art indicates that the final product, or any intermediate or starting material, may be resolved by known methods, and that a racemic mixture of the product may be enzymatically converted to chirally pure compounds. The γ-lactam can be prepared by reacting cyclopentadiene with tosyl cyanide. 
     R. D. Allan et al, Eur. J. Pharmacol. 122 (1986) 339-348, disclose a series of GABA analogues, resolved and unresolved, including compounds of formula II ##STR2## in which --X-- is --CH 2  --, --Y--Z-- is --CH═CH--, and R 1  and R 2  are H. In particular, (+)-1R,4R-4-aminocyclopent-2-ene-1-carboxylic acid is disclosed, although its activity is much lower than that of the structurally-isomeric 1-ene. 
     SUMMARY OF THE INVENTION 
     The present invention is based on the surprising discovery of lactamases that will react with a γ-lactam of formula I to give a single enantiomer of the lactam and the corresponding ring-opened compound of formula II in an enantiomeric form. The enantiomers are novel compounds, and are excellent synthons for a desired enantiomer of Carbovir or a pharmacologically-active analogue. 
     In compounds of the invention, X is --CH 2  --, --(CH 2 ) 2  --, --Q--, --CH 2  --Q-- or --Q--CH 2  -- and Q is a heteroatom (including NH); either Y or Z are independently selected from --CH 2  -- and a heteroatom (including NH), or --Y--Z-- is --CH═CH--, --CH═N-- or --N═CH--; and R n  is absent or represents one or more independently-selected substituents at any available position(s) on the X,Y,Z-containing ring. R 1  is H or alkyl; R 2  is H or a blocking group. 
     While (+)-4-aminocyclopent-2-ene-1-carboxylic acid is known, the present invention provides a new utility, i.e. its conversion to another compound of formula II, e.g. as a synthon. 
     DESCRIPTION OF THE INVENTION 
     Unsaturated lactams of formula I (in which --Y--Z-- means --CH═CH--, --CH═N-- or --N═CH--) can be prepared, by generally known means, by a Dieis-Alder reaction which comprises reacting a corresponding compound of formula III ##STR3## with tosyl cyanide or chlorosulphonyl isocyanate. Depending on the nature of X, the compound of formula III may variously be, for example, a cyclopentadiene, 1,3-cyclohexadiene, furan, pyrrole or 1,2-dihydropyridine. 
     Unsaturated lactams of formula I may be reduced to give saturated lactams of formula I, if necessary with concomitant protection of susceptible groups, by known means. The unsaturated lactams may also be utilised in the preparation of compounds of the invention bearing one or more substituents R. 
     R n  may be any substituent or substituents which do not interfere with the lactamase reaction; examples of R (if present) are methyl, ethyl, n-butyl, OH, Cl, Br, F, CF 3  and azido. For example, F or another halogen may be a substituent of X, e.g. as --CHHal--. The total number of carbon atoms in the group or groups R will not usually exceed 8. n may be, for example, 1 or 2, but R n  is preferably absent. 
     The compound of formula III is preferably a 1,3-cyclohexadiene or, most preferably, a (symmetrical) 5-membered compound. X is preferably --CH 2  --. Specific examples of compounds of formula I are the following: ##STR4## Others are given in PCT/GB90/00574. 
     It is surprising that material of biological origin will react selectively with a γ-lactam of formula I, to give the desired products in good yield. The material is described herein, for convenience, as a lactamase. 
     Suitable activities are those present in certain novel wild-type isolates of the genera Pseudomonas, Alcaligenes, Arthrobacter, Brevibacterium, Nocardia, Rhodococcus and Corynebacterium, whilst not being limited to isolates of these genera. Selection for these activities may be conducted in the presence of compounds containing one or more N-acyl substituents. If necessary, elevated levels of activity may be produced by growth of cells in the presence of such compounds. Particular examples of suitable activity are those produced maximally active in cells of the unique strains ENZA-20 and Rhodococcus sp ENZA-1, the latter when cultivated in a suitable medium in the presence of N-acetyl-L-phenylalanine or N-acetyl-D,L-phenylalanine. 
     Rhodococcus sp ENZA-1 was isolated from soil samples by enrichment culture in mineral salts medium containing N-acetyl-L-phenylalanine as the sole source of carbon and energy. The isolate has been deposited at the NCIMB in Aberdeen, on 17.10.89. The accession number is NCIMB 40213. 
     ENZA-20 was obtained from a sewage sample, and deposited at the NCIMB on 16.01.90. The accession number is NCIMB 40249. 
     Novel microorganisms have the ability to cause stereoselective formation of enantiomers of, say, 2-azabicyclo[2.2.1]hept-5-en-3-one and (-)-4-aminocyclopent-2-ene-1-carboxylic acid. The depos may, if desired, be subjected to conventional mutagenic/selection procedures, with a view to attaining improved effects. 
     The reaction of the lactam with the material having lactamase activity gives a compound of formula II which can be separated, as necessary or desired, from admixture with the unreacted lactam. If desired, the product (R 2  =H) can be reacted with an acylating agent such as acetic anhydride, to give the corresponding compound (II: R 2  =acyl). Other conventional N-blocking groups can be introduced as desired, examples being CH 2  Ar, COOalk, CONHalk, SO 2  alk, Si(alk) 3 , CHO and COalk (alk meaning alkyl in its broadest sense). 
     If the lactamase reaction is conducted in the presence of water, R 1  is H. Alternatively, a nucleophile may be used to introduce an alkyl or other group R 1  directly. For example, the nucleophile is methanol. 
     (-)-4-Aminocyclopent-2-ene-1-carboxylicacid and other cis compounds of formula II are often the product of the process. If desired, racemic cis amino-acid may be converted to racemic cis/trans amino-acid which is then exterified, allowing ready separation. The, say, (+)-trans ester may be isolated. 
     One alternative synthesis of the amino-acids (II) is by cleavage of the racemic lactam (I) with a chiral nucleophile (e.g. an amine or alcohol). The cleavage may show some entantioselectivity, and the resultant diastereoisomers can be separated by fractional crystallisation. Acid hydrolysis (if needed) liberates the free amino-acid. 
     A further alternative comprises enantioselective reaction on the double bond. Potential examples are 
     (i) asymmetric hydroboration (as pioneered by H. C. Brown) 
     (ii) catalytic asymmetric dihydroxylation (B. Sharples) 
     (iii) binaphthyl rhodium/ruthenium-catalysed double bond displacement (Noyori; such a process is used in tonne scale menthol manufacture). 
     In each case, the unreacted lactam could be collected or the functional product be used to develop a route to a chiral drug. 
     Yet another alternative route to chiral products of the invention comprises conducting the Dieis-Alder addition with a chiral Lewis acid catalyst or by using a chiral dienophile in place of tosyl cyanide. Economically, the use of a chiral catalyst is preferred. The resultant optically-enriched lactam may be subjected to the novel biotransformation process, in order to obtain a higher yield of pure (-)-lactam than may be possible from the racemate. 
     The optical isomers of the lactam (I) have a higher melting point than the racemate and therefore a higher lattice energy. Enantiomers may therefore be obtained by preferential crystallisation (entrainment), i.e. by seeding a supersaturated solution of racemate in a suitable solvent with pure enantiomer, such that more enantiomer is recovered than put in. 
    
    
     The following Examples illustrate the invention. 
     EXAMPLE 1 
     Preparation of Cells 
     Rhodococcus sp ENZA-1 was grown in a medium containing KH 2  PO 4  (7 gl -1 ), Na 2  HPO 4  (2 gl -1 ), MgSO 4  (0.4 gl -1 ), NaCl (2 gl -1 ) CaCl 2 .6H 2  O (0.01 gl -1 ), FeCl 3 .7H 2  O (0.08 gl -1 ), ZnSO 4 .7H 2  O (0.0001 gl -1 ), (NH 4 ) 2  SO 4  (2 gl -1 ) yeast extract (1 gl -1 ) and glucose (10 gl -1 ). The medium was adjusted to pH 7.0 with 5M NaOH and sterilised by autoclaving at 121° C. for 20 min. N-acetyl-L-phenyl-alanine at pH 7 was filter-sterilised and added to the cooled medium to give a final concentration of 10 gl -1 . The medium was distributed in 1 liter volumes within 5 liter shake flasks and  100 ml volumes within 500 ml shake flasks. A loopful from a slant of ENZA-1 was inoculated into 100 ml of the above medium and grown at 30° C. with shaking at 200 rpm, for 24 hours. A 5 liter shake flask was then inoculated with 50 ml of the seed culture and grown under the same conditions. The whole culture was then harvested after 48 hours growth, by centrifugation, and the cell paste was stored at -20° C. until use. 
     Preparation of Catalyst 
     Following thawing, Rhodococcus sp ENZA-1 cell paste (1.50 g) was suspended in phosphate buffer solution at pH 7 (0.1 M, 3.5 ml). The suspension of cells was then disrupted by sonication, to yield an essentially cell-free extract. 
     Preparation of (+)-lactam and (-)-amino-acid 
     The racemic lactam 2-azabicyclo[2.2.1]hept-5-en-3-one (218 mg, 2 mmol) was dissolved in phosphate buffer solution at pH 7 (0.1 M, 5.0 ml) and 0.5 ml of the cell-free extract was added. The resulting mixture was then inoculated at 30° C. with shaking for 14 days. 
     Isolation Of the (+)-lactam 
     The reaction mixture prepared above was extracted with dichloromethane (3×25 ml), and the organic layers were dried with anhydrous MgSO 4 . Following filtration, the organic layers were concentrated by rotary evaporation at 30° C. at reduced pressure, to yield a white solid (110 mg) which was fractionated by chromatography on silica (5 g) in the presence of diethyl ether as the mobile phase, to give the (+)-lactam (97 mg, 0.9 mmol; 88% e.e.). 
     Derivation of the (-)-amino-acid and Isolation of the (-)-ester/amide 
     The aqueous layer resulting from dichloromethane extraction of the reaction mixture was acidified to pH 1 with dilute HCl (1M) and then concentrated to near dryness by rotary evaporation at 35° C. and reduced pressure. The resultant oil was then refluxed with benzene (25 ml) for 1 hour in a Dean-Stark apparatus to remove water. The resulting mixture was then concentrated by rotary evaporation at 35° C. and reduced pressure, to yield a brown solid which was refluxed with dry methanol (25 ml) for 5 hours. The resulting solution was then filtered and evaporated to dryness at 35° C. and reduced pressure. Dry pyridine (10 ml) was added, and the solution was cooled in an ice-bath. At this point, acetic anhydride (5 ml) was added dropwise with stirring, and the mixture was allowed to warm to room temperature. Following a further 2 hours stirring, the solution was concentrated by evaporation at 35° C. and reduced pressure. The resulting oil was then taken up in dichloromethane (150 ml) and washed consecutively with water (30 ml), sat. NaHCO 3  solution (2×30 ml), dilute HCl (0.1 M, 2×30 ml) and brine (30 ml), at which point the solution was dried over anhydrous MgSO 4 . Subsequently, the solution was concentrated by evaporation under reduced pressure at 35° C. and fractionated by chromatography on silica (10 g) in the presence of a mobile phase having the composition: CH 2  Cl 2  /(CH 3 ) 2  CO 80:20, to yield the (-)-ester/amide (II: X=CH 2  ; Y-Z=--CH═CH--; R 1  =CH 3  ; R 2  =Ac) (128 mg, 0.7 mmol; 81% e.e.). 
     EXAMPLE 2 
     The procedure of Example i was repeated, except that isolate ENZA-20 was used. In cell preparation, growth in the presence of N-acetyl-L-phenylalanine was not necessary. The products of reaction with the racemic lactam were the (-)-lactam and the (+)-amino-acid. The (-)-lactam had&gt;98% e.e. The (+)-ester/amide was isolated.