| 摘要 |
The invention relates to an improved process for the preparation of compounds of formula <IMAGE> (I) or salts thereof, starting from compounds of formulae <IMAGE> (II) <IMAGE> (III) via compounds of formulae <IMAGE> (IV) <IMAGE> (VII) <IMAGE> (VIII) in which compounds R1 is hydrogen or lower alkyl, R2 is an organic radical which may carry a functional group which may be protected by a customary protective group R2o, R3 is hydrogen or a customary carboxyl protective group R3o, W is a group which can be replaced by a thiocarboxylic acid radical of formula III, and Z is oxygen or sulfur.
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