The invention relates to an improved process for the preparation of compounds of formula ##STR1## or salts thereof, starting from compounds of formulae ##STR2## via compounds of formulae ##STR3## in which compounds R.sub.1 is hydrogen or lower alkyl, R.sub.2 is an organic radical which may carry a functional group which may be protected by a customary protective group R.sub.2.sup.o, R.sub.3 is hydrogen or a customary carboxyl protective group R.sub.3.sup.o, W is a group which can be replaced by a thiocarboxylic acid radical of formula III, and Z is oxygen or sulfur.
该发明涉及一种改进的制备公式##STR1##或其盐的化合物的方法,其起始化合物为公式##STR2##的化合物,经由公式##STR3##的化合物制备而成。其中,化合物R.sub.1为氢或低碳基,化合物R.sub.2为有机基团,可以携带一个功能基团,该功能基团可以由惯用的保护基团R.sub.2.sup.o保护;化合物R.sub.3为氢或惯用的
羧酸保护基团R.sub.3.sup.o;化合物W为一个可以被公式III的
硫代
羧酸基团取代的基团;化合物Z为氧或
硫。