oup, an ethyl group, an isopropyl group and a t-butyl group; an aralkyl group having 7 to 20 carbon atoms such as a benzyl group and a phenethyl group; an alkoxyl group having 1 to 10 carbon atoms such as a methoxyl group, an ethoxyl group, a propoxyl group and a butoxyl group; or a monovalent aromatic heterocyclic group having a hetero atom and 3 to 20 carbon atoms (e.g., a furyl group, a thienyl