Some preferred embodiments are shown below as Formula 2: ##STR4## Wherein R3 and R4 is H; an alkyl group; a methyl group; or an ethyl group; --CH2 --CO2 --Y; or an N substituted methylene-carbonyl-amino group; were Y is an alkyl group; a methyl group; or an ethyl group; and R5 is an acyl-oxy group; or an acyl-amino group; and wherein the benzyl ring may optionally be substituted with inert groups;