Compounds of structure (I), and when for use in therapy: where R.sub.1 is an acyl group derived from a C.sub.16-30 fatty acid with two or more cis or trans double bonds and particularly an n-6 or n-3 series EFA or conjugated linoleic acid, or columbinic acid, or parinaric acid and R.sub.2 is as R.sub.1 the same or different, or any other nutrient, drug or other bioactive residue released as the active in the body and R.sub.3 is either hydrogen, fully hydrocarbon, or containing heteroatoms, preferably an alkyl group particularly a C.sub.1 -C.sub.4 alkyl group. ##STR1##