Compounds of the formula (I) ##STR1## (in which X represents HS-- or a group R.sub.1 R.sub.2 N-- wherein R.sub.1 and R.sub.2 which may be the same or different represent hydrogen or a straight or branched chain alkyl group containing from 1 to 4 carbon atoms or a benzyl group and R.sub.3 represents a C.sub.3-7 alkyl group or a C.sub.3-7 cycloalkyl group) are prepared by reacting an .alpha.-ketoester of the formula (II) ##STR2## (in which Alk represents an alkyl group) or a precursor thereof with a compound of the formula (III) ##STR3## The precursor of the compound of formula (II) may be a mixture of isomeric enol esters ##STR4## The compounds of formula (I) may be converted into compounds of formula VIII ##STR5## in which R.sub.1 and R.sub.2 which may be the same or different represent hydrogen or a straight or branched chain alkyl group containing from 1 to 4 carbon atoms, and R.sub.3 is as defined above.
式(I)的化合物 ##STR1## (其中X代表HS--或基团R.sub.1 R.sub.2 N--,其中R.sub.1和R.sub.2可能相同或不同,代表氢或含有1至4个碳原子的直链或支链烷基或苄基,而R.sub.3代表C.sub.3-7烷基或C.sub.3-7环烷基)可通过将式(II)的α-
酮酯或其前体与式(III)的化合物反应制备而成。 ##STR2## (其中Alk代表烷基)。化合物(II)的前体可以是异构烯醇酯的混合物 ##STR4## 式(I)的化合物可以转化为式(VIII)的化合物 ##STR5## 其中R.sub.1和R.sub.2可能相同或不同,代表氢或含有1至4个碳原子的直链或支链烷基,而R.sub.3如上所定义。