The solvent to be employed here is not particularly limited so long as it does not affect the present reaction, and may be a halogenated hydrocarbon such as dichloromethane, chloroform or carbon tetrachloride; an ether such as diethyl ether, tetrahydrofuran or dioxane; or an ester such as ethyl acetate or propyl acetate, and is preferably a halogenated hydrocarbon (particularly dichloromethane), a