roup and a butylyl group; an aralkyl group having 7 to 8 carbon atoms such as a benzyl group and a phenethyl group; an aryloxy group such as a phenoxyl group and a tolyloxy group; an alkoxycarbonyl group having 2 to 7 carbon atoms such as a methoxycarbonyl group, an ethoxycarbonyl group, a propoxycarbonyl group and a butoxycarbonyl group; an aryloxycarbonyl group having 7 to 21 carbon atoms such a