| 摘要 |
A process for preparing a compound of formula IIa or IIb: <IMAGE> IIa <IMAGE> IIb wherein R2, R3, R4 and R5 independently represent hydrogen or C1-C4 alkyl; and A is an optionally substituted aromatic or heteroaromatic ring system; the process comprising the steps of: (a) reacting a racemic mixture of a compound of formula II: <IMAGE> II wherein R2, R3, R4, R5 and A are as defined for formulae IIa and IIb; with a sterically hindered chiral esterifying agent to form enantiomers of formulae IIIa and IIIb: <IMAGE> IIIa <IMAGE> IIIb wherein R2, R3, R4, R5 and A are as defined for formulae IIa and IIb and R15 is a chiral sterically hindered residue; b) separating the diastereoisomers of formulae IIIa and IIIb; and (c) converting the diastereoisomers of formulae IIIa and IIIb separately to compounds of formulae IIa and IIb respectively by acid or base hydrolysis. If required, the unwanted enantiomer of formula IIa or IIb may be inverted to give the preferred isomer.
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