A compound of formula: wherein R6 and R7 are lower alkyl or benzyl or R6 and R7 taken together are ?CH2CH2-, -CH2CH2CH2-, or is CH2CH2CH2CH2-, R8 is C1-C21 saturated or monounsaturated alkyl optionally substituted, R25 is hydrogen or is C (O) NR27R28 wherein R27 and R28 taken together with the nitrogen to which they are attached form a pyrrolidinyl group or a piperidinyl group and R26 is lower alkyl, phenyl or benzyl, where do I book inferior? It represents alkyl of straight or branched chain containing from 1 to 7 carbon atoms.