Abstract Entecavir, labeled as 1H-[13 C 4] purin-6 (9H)-one, was prepared from commercially available [13 C] guanidine HCl, 1 and diethyl [1, 2, 3-13 C 3] malonate, 2. The reagents were condensed together to give 2-amino-4, 6-dichloro [2, 4, 5, 6-13 C 4] pyrimidine 3, which in turn was coupled to an optically active amino cyclopentanol derivative, 9. A further sequence of eight reaction steps completed the constructions of the ...