Compounds of formula I ##STR1## wherein R is C.sub.1 -C.sub.4 alkyl or benzyl, are prepared by reacting a butanal of formula II ##STR2## wherein X and Y are each independently of the other chloro, bromo or iodo, in an inert solvent, with a compound of formula III R--S--Me (III) wherein R is as defined above and Me is an alkali metal cation, and converting the resultant compound of formula IV ##STR3## wherein X and R are as defined above, in an inert solvent and in the presence of a base, into a compound of formula I.