The reaction of 1-[2-(diethylamino)ethylamino]-9-oxothioxanthene-4-carbaldehyde (I) with formic acid and formamide at 160 C gives N-[1-[2-(diethylamino)ethylamino]-9-oxothioxanthen-4-ylmethyl]formamide (II), which is hydrolyzed with hot aqueous HCl yielding the corresponding amine (III). Finally, this compound is acylated with methyl chloroformate and TEA in dichloromethane.