Treatment of N-Boc-beta-alanine (I) with bromochloromethane gives the chloromethyl ester (II), which is further converted to the iodomethyl analogue (III) under the Finkelstein halide exchange reaction conditions. Condensation of iodomethyl ester (III) with 6-mercaptopurine (IV) provides adduct (V). The N-Boc group of (V) is then removed employing trifluoroacetic acid to furnish the amine compound (VI). Finally, EDC-mediated coupling of amine (VI) with the polyethyleneglycol-bound aspartic acid (VII) affords the title mercaptopurine prodrug.