Hydrochloric Acid → 3,4-di-O-nicotinoyl-N-carbobenzoxydopamine

19.0 g (66 mmol) of N-carbobenzoxydopamine obtained in Synthetic Example 1-(1) and 37.2 g (209 mmol) of hydrochloride of nicotinoyl chloride were added to 200 ml of pyridine. After stirring at room temperature for 12 hours, the mixture was allowed to react at 35°-40° C. for 2 hours. Then, the reaction mixture was poured into ice and a sodium carbonate solution followed by extraction with chloroform. The resulting extract was dried over anhydrous magnesium sulfate and the solvent was distilled off to obtain crude crystals. The thus-obtained crystals were recrystallized from ethyl acetate/diethyl ether (1:5 by volume) to obtain 23.5 g (yield: 71.6%) of the desired compound in the form of white crystals (m.p. 77°-78° C.).