Therapeutically active compounds of the general formula: ##STR1## wherein Ac.sub.1 is a first acyl group, Ac.sub.2 is a second acyl group different from Ac.sub.1, A is a C--C bond or a divalent organic residue; R.sub.1, R.sub.2 are a hydrogen atom or a lower alkyl group, R.sub.1 and R.sub.2 together being able to take part in the formation of a ring having up to 6 atoms, A, R.sub.1 and R.sub.2 may be substituted by -OH groups esterified or not by acyl groups.