In order to synthesize isoquinoline derivative having amino radical in 4-position, 1-phenyl-4-benzoylamino-3, 4-dihydro-6, 7-dimethoxyisoquinoline was synthesized. Benzylurethane was obtained from 3, 4-dimethoxyphenylsuccinic acid by way of its ester, hydrazide and azide by the usual method and converted into diamine by the Sugasawa's improved Curtius' degradation. It was led to dibenzoyl-3, 4-dimethoxyphenyl-ethylenediamine, by the Schotten-Baumann reaction, and cyclized to the above-mentioned compound by POCl₃. In this case, it was difficult to cyclize by using P₂O₅, and toluene gave a better yield than benzene as a solvent. Analytical results indicated that imidazolidone-type urethane had been formed as a by-product, besides dibenzylurethane. The saponification of benzyl derivatives will be reported later.