The compound can be prepared by reacting mercaptopurine (I) with 2-chloroethylisocyanate (II) in an inert medium at a temperature from 0-50 C, preferably in the presence of an alkaline agent. Suitable inert media for the reaction are dimethylformamide, dimethylsulfoxide, phosphoric acid, hexamethyltriamide or tetramethylurea. Alkaline agents, preferably tertiary organic amines, accelerate the reaction and make the yield of the desired product higher.