alkyl group having 1 to 6 carbon atoms or a straight chained, branched or cyclic alkoxy group having 1 to 6 carbon atoms; and R6 and R7 may form together a group of ???CO???O???CO??? or ???CO???NR10???CO???, wherein R10 is a hydrogen atom, a straight chained, branched or cyclic alkyl group having 1 to 6 carbon atoms or a straight chained, branched or cyclic alkoxy group having 1 to 6 carbon atoms