. . . "tuted aralkyl group or a hydrogen atom, R5 denotes a substituted or unsubstituted alkyl group, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted aryl group, a substituted or unsubstituted aralkyl group or a hydrogen atom, R5' is a hydrogen atom or methyl group provided that if R5' is a methyl group, then R5 is a hydrogen atom, and R6 to R8 independently mean a substit" . . . .