THE PREPARATION OF CHIRAL cis-5-AMINO-2-CYCLOPENTEN-1-OL DERIVATIVES

Abstract: A process for the preparation of a cyclic carbamate of formula (1), comprises treatment of a cyclopentene of formula (2) wherein R<1> is H or an organic group of up to 20 C atoms and R<2> is C1-10 alkyl, aralkyl or aryl, with a source of halogen and a base.

Description: THE PREPARATION OF CHIRAL .y-5-AMINO-2-C YCLOPENTEN- 1 -PL DERIV ATIVES Field of the Invention This invention relates to a process for the production of chiral cw-5-amino-2- cyclopenten-1-ol as its cyclic carbamate. Background to the Invention cw-5-Amino-2-cyclopenten-l-ol is an extremely useful synthon, especially in the form of derivatives such as the cyclic carbamate 1 (3,3a,4,6a-tetrahydro- cyclopentaoxazol-2-one). For example, conversion of 1 to N-Boc derivative gives an intermediate which is a suitable substrate for Pd°-catalysed nucleophilic allylic displacement (Muxworthy et al , Tetrahedron Letters, 1995, 36, 7539-7540), as shown in the following Scheme. H BOC α N v BOC 2 0, DMAP /^r-* N v Pd°- Nucleophile Nt /\- NHBOC 0^° CD" 0 - ' (1) This reaction was demonstrated for 2 model nucleophiles. Products of such reactions establish 1 ,3-chiral centres on the cyclopentane ring. This structural motif is an important synthetic target, particularly for elaboration into carbocyclic nucleosides and analogues of nucleoside derivatives, reviewed for example by Agrofoglio et al. (Tetrahedron, 1994, 50, 10611-10670). As further demonstration of the synthetic utility of the N-Boc derivative of compound 1, Anderson et al. (J. Org. Chem., 1998, 63, 7594-7595) report diastereoselective allylic amination reactions. Synthesis of the cyclic carbamate 1 represents a challenge, especially when it is required in the form of a single enantiomer. Two syntheses of the latter have been reported. In the earlier of these, Muxworthy et al. (as above) used a mandelic acid derivative to introduce chirality. Thus an acyl nitroso intermediate, generated in situ from (R)-α-hydroxyphenylacetohydroxamic acid, undergoes a Diels- Alder reaction with cyclopentadiene, and the resulting adduct is treated with dilute HC1 to effect rearrangement to give a salt derivative of cw-5-amino-2-cyclopenten-l-ol. Further treatment with either EtaN/TsCl or t-BuOK/PhNTf 2 yields 1 in modest yield (30-35%) and requires chromatographic purification. Overall, this route appears impractical to operate at scale, because of the extremely low temperature required in the initial cycloaddition. In addition, a stoichiometric amount of the expensive (R)-mandelic acid is required as chiral auxiliary, recovery and reuse of which is precluded by its mode of removal. In the second route, Mulvihill et al. (J. Org. Chem. , 1998, 63, 3357-3363) employed a four-step sequence from cyclopentadiene, comprising acyl-nitroso cycloaddition, N-O bond scission, bioresolution and rearrangement of functional groups. The requirement for excess Mo(CO) 6 in the second step renders the overall approach unscaleable. (.single enantiomer) The carbocyclic nucleoside abacavir, a potent reverse transcriptase inhibitor, is synthesised using enantiomerically pure 2-azabicyclo[2.2. l]hept-5-en-3-one as a chiral building block. This drug has been produced at multi-tonne scale, and an economical bioresolution of 2-azabicyclo[2.2. l]hept-5-en-3-one has been developed to provide the chiral building block as a single enantiomer (WO-A-98/ 10075). This uses a cloned lactamase at high volume efficiency: Recombinant lactamase Racemic 500g/l >98% ee (+) >98% ee (-) Both the residual lactam and the product amino acid can be converted into compounds of the following structures (2 & 3) using standard chemical methods. HOzC / ^ HBOC HO_>C ''//'NNV NHBOC (2) (3) Maier et al (Synlett, Aug. 1998, 891-3) disclose the synthesis of cyclohexenylamines by ring-closing metathesis. One product of such reaction is further reacted as follows B Summary of the Invention This invention is based on the discovery of an economical and convenient process to the cyclic carbamate 1. This process can be operated at scale, to access single enantiomer forms of 1 from starting materials 2 and 3 which are readily available in quantity via bioresolution of 2-azabicyclo[2.2.1]hept-5-en-3-one, and of which the simplest embodiment may be represented as follows: Halolactonisation of unsaturated carboxylic acids or their salts with iodine to form lactones has been known for several decades, and is a well-understood process. It was surprising that, when the cyclopentene carboxylic acid 2 was treated with iodine in a biphasic system of diethyl ether and saturated sodium bicarbonate, the expected γ- lactone 4 was not formed. Instead, the NHBoc function cyclises onto the adjacent carbon, to generate a putative intermediate 5 which undergoes decarboxylative elimination under the mildly basic conditions to give compound 1. The latter is obtained directly in high chemical purity. This is a scaleable reaction, and represents a superior route to the routes described above. Instead of iodine, another halogen may be used. If required, a different carbamate protecting group can be used instead of N-Boc in the process of the present invention. The process may also be applied to N-substituted derivatives containing Ν-R 1 instead of Ν-H, R 1 being an organic group (alkyl, aryl, aralkyl etc, optionally substituted) of up to 20 C atoms. Also, the CO 2 H group may instead be CO 2 R 2 , R 2 being H, C 10 alkyl, aralkyl or aryl, e.g. of up to 10 C atoms. Detailed Description of the Invention In a preferred embodiment, the base is an aqueous alkaline base, preferably aqueous sodium carbonate, and the reaction is conducted in a biphasic system. Preferably, enantiomerically enriched carbamate 1 of at least 80% ee, and more preferably at least 95% ee, is prepared. If not already protected, the product may be converted, by known means, to a protected, e.g. the N-Boc, derivative. This is then ready for processing to a desired final product. The following Examples illustrate the invention. Example 1 N-Boc-(---)-a-s-3-amino-4-cyclopentene-l-carboxylic acid Methanol (300ml) and 60g (0.465mol) (+)-cw-3-amino-4- cyclopentenecarboxylic acid (obtained from the bioresolution of 2- azabicyclo[2.2.1]hept-5-en-3-one) were stirred together, then 194ml (1.39 mol) triethylamine slowly added. The reaction was cooled to 8°C with ice, then 99g (0.45 mol) di-t-butyl dicarbonate dissolved in 400ml methanol was slowly added. After stirring for 48 hrs at ambient temperature the reaction was concentrated under reduced pressure to 100ml and acidified to pH 3 with 1M KHSO 4 . The solution was extracted twice with an equal volume of methyl t-butyl ether, the organic layer dried with MgSO 4 , then evaporated under reduced pressure to give an off-white solid. This was recrystallised from a mixture of methyl t-butyl ether and heptane to give 90g of N-Boc (+)-amino acid whose structure was verified by proton NMR. Example 2 3,3a,4,6a-Tetrahydrocyclopentaoxazol-2-one Into a 100ml round bottom flask were placed 1.5g (6.6mmol) of N-Boc (+)- amino acid, 7ml of diethyl ether and 30ml of saturated sodium bicarbonate solution. The resulting solution was cooled to 0°C with an ice bath and a solution of 5.03g (20mmol) of iodine in 20ml of THF was slowly added. After the addition was complete, the flask was protected from light with tin foil and left stirring at 0°C overnight. The reaction was quenched with sodium sulfite solution and diluted with 10ml saturated sodium bicarbonate solution. The solution was extracted with 3 x 30ml dichloromethane and the combined organic extracts dried over magnesium sulfate. Solvent was removed by rotary evaporator to yield 208mg of an off-white solid. Analysis by GC-MS showed a single component with a retention time of 14.82 minutes: m/z: 125 (M + ), 96, 80. ΝMR (CDC1 3 ): 6.5 (1H, s, ΝH), 6.15-6.00 (1H, m), 5.95-5.75 (1H, m), 5.56 (1H, d, J=7.1), 4.45 (1H, t, J=6.6), 2.8-2.2 (2H, m).

Claims: CLAIMS 1. A process for the preparation of a cyclic carbamate of formula (1), which comprises treatment of a cyclopentene of formula (2) (1) (2) wherein R 1 is H or an organic group of up to 20 C atoms and R 2 is C,., 0 alkyl, aralkyl or aryl, with a source of halogen and a base. 2. A process according to claim 1, wherein R 1 is H. 3. A process according to claim 1 or claim 2, wherein R 2 is tert-butyl. 4. A process according to any preceding claim, wherein the source of halogen is iodine. 5. A process according to any preceding claim, wherein the base is an aqueous alkaline base, and the reaction is conducted in a biphasic system. 6. A process according to claim 5, wherein the base is sodium carbonate. 7. A process according to any preceding claim, wherein enantiomerically enriched carbamate 1 of at least 80% ee is prepared. 8. A process according to claim 7, wherein enantiomerically enriched carbamate 1 of at least 95% ee is prepared. 9. A process according to any preceding claim, which further comprises conversion of compound 1 to the N-Boc derivative.

Inventors: LLOYD MICHAEL, LLOYD, MICHAEL
Applicants: CHIROTECH TECHNOLOGY LTD, LLOYD MICHAEL, CHIROTECH TECHNOLOGY LIMITED, LLOYD, MICHAEL
Application year: 2000
Main classification: C07D263
IPC codes: C07D263