R7 is selected from hydrogen; straight or branched (C1 -C3)alkyl group selected from methyl, ethyl, n-propyl or 1-methylethyl; (C6 -C10)aryl group selected from phenyl, ??-naphthyl or ??-naphthyl; (C7 -C9)aralkyl group such as benzyl, 1-phenylethyl, 2-phenylethyl or phenylpropyl; a heterocycle group selected from a five membered aromatic or saturated ring with one N, O, S or Se heteroatom optional