, --S-- or --NR6 -- in which R6 is a hydrogen atom or a lower alkyl group; ##STR51## Z is a hydrogen atom, an acyl group or an optionally substituted hydrocarbon group; and n is 1 or 2; with the following provisos:1) when n is 1, then A is ##STR52## Y is --S-- or --NR6 --, and at least one of R2, R3 and R4 is the formula --Y--R5 --, with the further proviso that when all Y's are --NR6 --, then at