ethyl group which may have a substituent, an aralkyl group such as benzyl group and phenethyl group which may have a substituent or an aryl group such as phenyl group and naphthyl group which may have a substituent, each of R12 and R13 is an hydrogen atom, an alkyl group such as methyl group and ethyl group which may have a substituent, an aralkyl group such as benzyl group, naphthylmethyl group,