Abstract: Compounds of formula (I) wherein R.sub.1, R.sub.2, R.sub.3, R.sub.4, X are as defined in the specification and are prepared by elimination of one of the groups X.sub.1 and X.sub.2 from a compound of formula (II) wherein X.sub.1 and X.sub.2 each independently represent a carboxylic acid group or salt thereof, or a protected carboxylic acid group, the non-eliminated group X.sub.1 or X.sub.2 corresponding to the group X in the desired compound of formula (I).

Abstract: compounds of formula I ##STR1## wherein R.sup.1 represents a C.sub.1-8 alkyl, C.sub.2-8 alkenyl, C.sub.2-8 alkynyl, C.sub.3-8 cycloalkyl(C.sub.1-8)alkyl, C.sub.1-8 hydroxyalkyl, C.sub.1-8 alkylthio, phenyl or substituted phenyl;R.sup.3 represents a C.sub.1-8 alkyl, C.sub.2-8 alkenyl, C.sub.2-8 alkynyl, CO2(C.sub.1-8)alkyl, CO.sub.2 (C.sub.2-8) alkenyl, C.sub.1-8 alkylthio, (C.sub.1-2)alkyl CO.sub.2 (C.sub.1-8) alkyl or C.sub.1-8 aldehydroalkyl where the aldehyde function is protected by a suitable protecting group (for example, an acetal such as a dimethyl acetal);R.sup.4 represents a hydrogen atom, C.sub.1-8 alkyl or C.sub.2-8 alkenyl;Z represents a group (CH.sub.2).sub.n or a branched alkyl chain;n is 1 to 8;and each of a, b and c is independently a single or a double bond;can be prepared relatively easily and in good xxx at room temperature by reacting a compound of general formula II ##STR2## wherein R.sup.2, R.sup.3, R.sup.