The silyl derivatives of 2-amino-6-chloropurine and 2-acetamido-6-chloropurine react readily with halo sugars in the presence of mercuric cyanide to afford nucleosides in high yields. These can be converted to guanine, thioguanine, and other related nucleosides by standard procedures. By this new and improved method and through a column chromatographic separation described here, a-2 '-deoxy tliioguanosine has been obtained in excellent yield and high anomeric purity, and 9-/3-D-xylofuranosylguanine has been obtained in almost twice the yield previously attainable. a-2 '-Deoxy thioguanosine ( -TGdR, a-52a) is the only a anomer of a nucleoside known to have antitumor activity.8 The ß anomer (/3-TGdR, ß-5) is also an interesting antitumor agent.3 4Studies have shown that both ß-and -TGdR can be phosphorylated and incorporated into the DNA of some murine tumors in vivoi and of some murine and human tumor extracts in vitro.1 Unlike the ß anomer, -TGdR is not phosphorylated to a significant extent by extracts of normal bone marrows ;5 hence, -TGdR is less toxic than the ß anomer.For these reasons, there is great interest in -TGdR, and large amounts are needed for additional studies. This article reports a new, improved synthesis of -TGdR and a means of separating its precursor from that of the ß anomer.The new method of synthesis seems generally useful for guanine, thioguanine, and other 2-amino 6-substituted purine nucleosides.6Of the general methods of guanine nucleoside synthesis,6 none could assure a better yield of a-5 and a (1) (a) This work was carried out under the auspices of

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