Chiral azabicyloheptanone and a process for their preparation

Abstract: Lactamase of 1-amino-3-carboxylic acid cyclic compounds are provided 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.

Description: 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, US-A-4742064, and also in GB-A-2217320 which, by way of specific illustration, describes a compound known as Carbovir (carbocyclic 2',3'-didehydro-2',3'-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 See, for example, Daluge et al , J. Org. Chem. 43 (12): 2311-20 (1978). 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 the compound of formula II in which R₁ is 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. Caamano et al , Heterocycles 27 (12):2839-41 (1988), describe an enantioselective synthesis of 2-azabicyclo[2.2.1]hept-5-en-3-one, by hydrolysis of the adduct formed by Diels-Alder addition of cyclopentadiene to the chiral dienophile (+)-10-camphorsulphonyl cyanide. The enantiomeric excess was 13%. Sicsic et al , Tetrahedron Letters 28 :1887-8 (1987), discloses that pig liver esterase is an enantioselective hydrolytic catalyst of (±)-methyl-4-cis-acetamidocyclopent-2-ene-1-carboxylate. The (+) and (-) enantiomers were obtained in 87% ee and 97% ee respectively. Ikbal et al , Fur. J. Med. Chem. 24 :415-420 (1989), discloses the chemical synthesis of racemic methyl 4-acetamido-cis-cyclopent-2-ene-1-carboxylic acid from 2-azabicyclo[2.2.1]hept-5-en-3-one, and its enzymatic resolution using pig liver esterase. Summary of the Invention The present invention is based on the surprising discovery of lactamases that will react with the 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 substantially pure enantiomers of formula I wherein the enantiomeric excess is not less than or equal to 13% are novel compounds, and are excellent synthons for a desired enantiomer of Carbovir. Description of the Invention It is surprising that material of biological origin will react selectively with the γ-lactam of formula I, to give the desired product 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 deposited microorganisms 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 the lactamase reaction is conducted in the presence of water, R₁ is H. Alternatively, a nucleophile may be used to introduce an alkyl group R₁ directly. For example, the nucleophile is methanol. (-)-4-Aminocyclopent-2-ene-1-carboxylic acid of formula II is 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. The following Examples illustrate the invention. Example 1 Preparation of Cells Rhodococcus sp ENZA-1 was grown in a medium containing KH₂PO₄ (7 gl⁻¹), Na₂HPO₄ (2 gl⁻¹), MgSO₄ (0.4 gl⁻¹), NaCl (2 gl⁻¹), CaCl₂.6H₂O (0.01 gl⁻¹), FeCl₃.7H₂O (0.08 gl⁻¹), ZnSO₄.7H₂O (0.0001 gl⁻¹), (NH₄)₂SO₄ (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 litre volumes within 5 litre 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 litre 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 x 25 ml), and the organic layers were dried with anhydrous MgSO₄. 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. NaHCO₃ solution (2 x 30 ml), dilute HCl (0.1 M, 2 x 30 ml) and brine (30 ml), at which point the solution was dried over anhydrous MgSO₄. 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: CH₂Cl₂/(CH₃)₂CO 80:20, to yield the (-)-ester/amide (II: X = CH₂; Y-Z = -CH=CH-; R₁ = CH₃; R₂ = Ac) (128 mg, 0.7 mmol; 81% e.e.). Example 2 The procedure of Example 1 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 >98% e.e. The (+)-ester/amide was isolated.

Claims: Claims for the following Contracting States : AT, BE, CH, DE, DK, FR, GB, GR, IT, LU, NL, SE An enzyme having lactamase activity capable of stereospecific reaction with one of the enantiomers of 2-azabicyclo[2.2.1]hept-5-en-3-one. An enzyme according to claim 1, capable of stereoselective reaction with racemic 2-azabicyclo[2.2.1]-hept-5-en-3-one, to form a mixture of an enantiomer of 2-azabicyclo[2.2.1]hept-5-en-3-one and the opposite enantiomer of 4-aminocyclopent-2-ene-1-carboxylic acid or an alkyl ester thereof. A microorganism having lactamase activity as defined in claim 1 or claim 2. A microorganism according to claim 3, having the characteristics of that deposited as NCIMB 40213, or a mutant thereof. A microorganism according to claim 4, having the characteristics of that deposited as NCIMB 40249, or a mutant thereof. A process for preparing an enantiomer of 2-azabicyclo[2.2.1]hept-5-en-3-one, which comprises reacting a mixture of enantiomers of 2-azabicyclo[2.2.1]hept-5-en-3-one with an enzyme according to claim 1 or claim 2 or a microorganism according to any of claims 3 to 5. A process according to claim 6, for preparing a mixture as defined in claim 2, which is conducted in the presence of a nucleophile, whereby the alkyl ester is formed. A mixture of an enantiomer of 2-azabicyclo[2.2.1]hept-5-en-3-one, at least substantially free of the other enantiomer, wherein the enantiomeric excess is not less than or equal to 13%, and the opposite enantiomer of 4-aminocyclopent-2-ene-1-carboxylic acid or an alkyl ester thereof. An enantiomer of 2-azabicyclo[2.2.1]hept-5-en-3-one, at least substantially free of the other enantiomer, wherein the enantiomeric excess is not less than or equal to 13%, obtainable by the process of claim 6 or by the separation of a mixture according to claim 8. An enantiomer according to claim 9, which is (+)-2-azabicyclo[2.2.1]hept-5-en-3-one, in at least 88% enantiomeric excess, obtainable by the process of claim 6 or by the separation of a mixture according to claim 8. An enantiomer according to claim 9, which is (-)-2-azabicyclo[2.2.1]hept-5-en-3-one, in more than 98% enantiomeric excess, obtainable by the process of claim 6 or by the separation of a mixture according to claim 8. Claims for the following Contracting State : ES A process for preparing an enantiomer of 2-azabicyclo[2.2.1]hept-5-en-3-one, at least substantially free of the other enantiomer, which comprises reacting a mixture of the enantiomers with an enzyme having lactamase activity capable of stereospecific reaction with one of the enantiomers. A process according to claim 1, wherein the enantiomer produced is (+)-2-azabicyclo[2.2.1]hept-5-en-3-one, in at least 88% enantiomeric excess. A process according to claim 1, wherein the enantiomer produced is (-)-2-azabicyclo[2.2.1]hept-5-en-3-one, in more than 98% enantiomeric excess. A process according to any preceding claim, wherein the enantiomer produced is in admixture with the opposite enantiomer of 4-aminocyclopent-2-ene-1-carboxylic acid or an alkyl ester thereof. A process according to claim 4, which is conducted in the presence of a nucleophile, whereby the alkyl ester is formed. A process according to any of claims 1 to 5, wherein the enzyme is as found in a microorganism having the characteristics of that deposited as NCIMB 40123, or a mutant thereof. A process according to any of claims 1 to 5, wherein the enzyme is as found in a microorganism having the characteristics of that deposited as NCIMB 40239, or a mutant thereof.

Inventors: EVANS CHRISTOPHER THOMAS, ROBERTS STANLEY MICHAEL, EVANS, CHRISTOPHER THOMAS, ROBERTS, STANLEY MICHAEL
Applicants: CHIROSCIENCE LTD, CHIROSCIENCE LIMITED
Application year: 1990
Main classification: C07D209
IPC codes: C07D498, C12R1, C12P41, C12N1, C12N9, C07D491, C07D209, C12P13, C12P17