Abstract: Compounds of the general formula I: ##STR1## in which R.sub.1 represents cycloalkyl, aralkyl, heterocyclic aryl, or substituted phenyl, in which the substituents on the phenyl group are one or more of the following groups, that is halogen, hydroxy, alkoxy (C.sub.1 -C.sub.4), alkyl (C.sub.1 -C.sub.4), nitro, dialkylamino or methylenedioxy;R.sub.2 represents a hydrogen atom, or together with the group R.sub.1 and the adjacent carbon atom represents a cycloalkyl group with 3 to 7 carbon atoms inclusive;R.sub.3 represents a group R.sub.5 CO--, or R.sub.5 CH(OH)-- in which R.sub.5 is methyl or ethyl;R.sub.4 is an alkyl group (C.sub.1 -C.sub.6) or a phenyl group; and in which where R.sub.4 is alkyl, R.sub.3 and R.sub.