Abstract: A compound of the formula: ##STR1## wherein R is an unsubstituted benzyl group, a p-nitrobenzyl group, a benzyl group in which the phenyl group is substituted by one or two substituents selected from the group consisting of C.sub.1 -C.sub.4 alkyl groups and C.sub.1 -C.sub.4 alkoxy groups, a phenyl group which is substituted by a C.sub.1 -C.sub.4 alkylenedioxy group or by one or two substituents selected from the group consisting of C.sub.1 -C.sub.4 alkyl groups and C.sub.1 -C.sub.4 alkoxy groups, a C.sub.1 -C.sub.4 trialkylsilyl group, a mono-substituted C.sub.1 -C.sub.4 alkyldiarylsilyl group, a triarylmethyl group, a C.sub.1 -C.sub.4 acyl group, a substituted or unsubstituted benzoyl group, a C.sub.1 -C.sub.4 alkoxycarbonyl group, a substituted or unsubstituted aryloxycarbonyl group, a tetrahydropyranyl group, a benzyloxymethyl group, or a methoxymethyl group.