Patent ID: 6908752

Claim:
A process for the preparation of a compound of the formula I, wherein the two R 1 residues are identical and are hydrogen, fluorine, (C 1 -C 4 )-alkyl, (C 2 -C 4 )-alkenyl, (C 2 -C 4 )-alkynyl or (C 3 -C 4 )-cycloalkyl, where alkyl, alkenyl, alkynyl and cycloalkyl can be substituted by 1, 2 or 3 identical or different substituents selected from the group consisting of fluorine, chlorine, bromine and methoxy, or the two R 1 residues together are tetramethylene —(CH 2 ) 4 — or pentamethylene —(CH 2 ) 5 —; R 2 is fluorine, chlorine, bromine, nitro, cyano, hydroxyl, methoxy, acetylamino, tert-butyloxycarbonylamino, benzyloxycarbonylamino, methylmercapto, tert-butylmercapto, (C 1 -C 10 )-alkyl, aryl, aryl-(C 1 -C 10 )-alkyl-, heteroaryl, heteroaryl-(C 1 -C 10 )-alkyl-, (C 2 -C 10 )-alkenyl, (C 2 -C 10 )-alkynyl, (C 3 -C 7 )-cycloalkyl or (C 3 -C 7 )-cycloalkyl-(C 1 -C 10 )-alkyl-, where alkyl, aryl, heteroaryl, alkenyl, alkynyl and cycloalkyl can be substituted by 1, 2 or 3 identical or different substituents selected from the group consisting of fluorine, chlorine, bromine, iodine, trifluoromethyl, methyl, nitro, cyano, acetylamino, 9-fluoroenylmethyloxycarbonylamino, tert-butyloxycarbonylamino, benzyloxycarbonylamino, mercapto, methylmercapto, tert-butylmercapto, hydroxyl, methoxy, ethoxy and COOR 4 ; R 3 is hydrogen, (C 1 -C 10 )-alkyl, aryl-(C 1 -C 10 )-alkyl-, (C 2 -C 10 )-alkenyl, (C 3 -C 10 )-alkynyl, (C 3 -C 7 )-cycloalkyl or (C 3 -C 7 )-cycloalkyl-(C 1 -C 10 )-alkyl-, where alkyl, aryl, alkenyl, alkynyl and cycloalkyl can be substituted by 1, 2 or 3 identical or different substituents selected from the group consisting of fluorine, chlorine, bromine, iodine, trifluoromethyl, methyl, cyano, nitro, acetylamino, tert-butyloxycarbonylamino, benzyloxycarbonylamino, hydroxyl, methoxy, ethoxy and COOR 5 ; R 4 and R 5 , which can be identical or different, are hydrogen, (C 1 -C 4 )-alkyl, (C 2 -C 4 )-alkenyl or aryl-(C 1 -C 10 )-alkyl-; or of a salt thereof, in substantially enantiomerically pure form, which process comprises subjecting a mixture of the enantiomeric forms of a compound of the formula I wherein R 3 has a meaning other than hydrogen, to an enzymatic hydrolysis with the aid of an esterase and separating the converted and unconverted compounds from each other.