Abstract: A compound of formula I or a pharmaceutically acceptable salt thereof ##STR1## where --R.sub.1 is hydrogen or methyl;--X-- is --CO-- or --CH.sub.2 --;--Y-- is >CH--A or its vinylogous group >C.dbd.CH--CH.sub.2 --A, wherein --A is a --NR.sub.4 R.sub.5 group, where each R.sub.4 and R.sub.5, equal or different between them, is C.sub.1 -C.sub.4 -alkyl or a phenyl-substituted C.sub.1 -C.sub.4 -alkyl; or --A is the N-radical of a 5- or 6-membered, uncharged (i.e. neither quaternary nor zwitterionic) monocyclic ring, either aromatic or non-aromatic, said ring containing one N atom, two N atoms, three N atoms or an N/O pair of atoms, all other atoms in the ring being carbon, and said ring being optionally mono- or di-substituted by groups selected from C.sub.1 -C.sub.4 -alkyl, benzyl, 2-furanylmethyl and 2-thienylmethyl;--R.sub.2 is hydrogen or a radical C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.4 -alkoxy, di-(C.sub.1 -C.sub.