6 -C10)aryl group selected from phenyl, ??-naphthyl or ??-naphthyl; (C7 -C9)aralkyl group such as benzyl, 1-phenylethyl, 2-phenylethyl or phenylpropyl; or (CH2)n COOR7' where n=0-4 and R7' is selected from hydrogen; straight or branched (C1 -C3)alkyl selected from methyl, ethyl, n-propyl or 1-methylethyl; or (C6 -C10)aryl selected from phenyl, ??-naphthyl or ??-naphthyl; with the proviso that R5 a