Specifically, for example, in the case that R2 is a hydrogen atom in the compounds (1a), (1c) and (1d), the following methods come under the above-mentioned substituent conversions; that is, a method for converting R2 into a C1-6 alkyl group by performing a reductive amination reaction with aldehyde or ketone, a method in which, after a corresponding urea derivative is obtained as in <Step 1A-3> f