Tautomerism and Regioselectivity in Ribosylation of Guanine

Abstract

N²-acetyl- and 9, N²-diacetylguanines were subjected to reaction with tetraacetylribose in the presence of p-toluenesulfonic acid. Unlike the ribosylation of diacetylguanine, which gives 7-riboside as a kinetic product, the reaction of monoacetylguanine produces directly a mixture of 7- and 9-ribosides. This reflects N⁷H ⇆ N⁹H tautomerism of the guanine substrate and supports the hypothesis that only the unsubstituted nitrogens of the imidazolium portion of guanine (either N7 or N9) react directly with a sugar cation.