Preferably, R3 is an isobutyl group and R4 is a hydrogen or R3 and R4 taken together are a group of the formula --(CH2)n -- in which n is an integer from 5 to 9 inclusive and R5 is hydrogen or R3 is isobutyl, R4 is methyl and R5 is carboxyl or C1 -C6 -alkoxy-carbonyl group, especially a carboxyl or ethoxycarbonyl.