Purines. LXX. An Extension of the "Phenacylamine Route" to the Syntheses of the 7-N-Oxides of 6-Mercaptopurine and 6-Methylthiopurine, and Antileukemic Activity of Some Purine N-Oxides

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A full account is given of the first syntheses of 6-mercaptopurine 7-N-oxide (4) and 6-methylthiopurine 7-N-oxide (5). The synthesis of 4 followed a "phenacylamine route", which started from condensation of 4, 6-dichloro-5-nitropyrimidine (15) with N-(4-methoxybenzyl)phenacylamine to form the phenacylaminopyrimidine derivative (11) and proceeded through conversion into the mercapto derivative, intramolecular cyclization between the NO₂ nitrogen atom and the phenacyl carbanion to give 6-mercapto-9-(4-methoxybenzyl)purine 7-N-oxide (12), and removal of the 4-methoxybenzyl group. S-Methylation of 12 and removal of the 4-methoxybenzyl group afforded 5. The location of the oxygen function in 4, 5, and 12 was confirmed by X-ray crystallographic analysis of 5·H₂O, which was shown to exist in the N(7)-OH form (19). A UV spectroscopic approach suggested that the neutral species of 4 exists in H_O as the N(7)-OH tautomer (21), whereas that of 5 exists as an equilibrated mixture of the N(7)-oxide (5) and the N(7)-OH (19) tautomers. In the in vitro bioassay of antileukemic activity against murine L5178Y cells, the N-oxides 4 and 12 were found to be weakly cytotoxic.