the invention is concerned with a new process for the preparation of compounds of formula ## str1 ## wherein r 1 is trityl , acetyl , tetrahydropyranyl or cyclopentyl ; r 2 is hydrogen , hydroxy , lower alkyl , cycloalkyl , lower alkoxy , lower alkenyl , lower alkynyl , aryl , aryloxy , aryl - lower alkyl , aryl - lower alkoxy or heterocyclyl or heterocyclyl - lower alkyl ; the lower alkyl , cycloalkyl , lower alkoxy , lower alkenyl , cycloalkenyl , lower alkynyl , aryl - lower alkyl , aryl , aryloxy , aryl - lower alkoxy , the heterocyclyl moieties being unsubstituted or substituted with at least one group selected from carboxy , amino , nitro , cyano , lower alkyl , lower alkoxy , hydroxy , halogen , -- conr 21 r 22 , -- ncoor 23 , r 22 co --, r 22 oco -- or r 22 coo --, wherein r 21 is hydrogen , lower alkyl , or cycloalkyl ; r 22 is hydrogen or lower alkyl ; r 23 is lower alkyl , lower alkenyl or a carboxylic acid protecting group ; y is -- s -- and z is -- ch 2 -- or y is -- ch 2 -- and z is -- o --, by acylation of a compound of formula ## str2 ## with an activated carboxylic acid derivative of formula ## str3 ## wherein r 3 is lower alkyl , and r 1 , r 2 , x , y , z have the significance given above ; and it is further concerned with compounds of formula iii .