The thiazolidine derivative may further be selected from compounds where X is methylene and R1 is of the formula IV ##STR8## wherein X is an oxygen or sulfur atom, R41 and R42 are each independently hydrogen or a hydrocarbon residue which may optionally be substituted and R41 and R42 may jointly, together with the oxazole or thiazole ring, form a condensed ring and A is a lower alkylene group.