Synthetic routes to 2′,3′-cyclopropanated and 2′,3′-unsaturated nucleosides

Abstract

Cyclopropyl glycopyranosides which solvolyze via a stabilized cyclopropyl carbinyl-oxocar-bonium ion react under neutral conditions with nucleoside bases in nitromethane at 200 °C to give good yields of the corresponding cyclopropanated nucleosides. Under similar conditions hex-2-enopyranosides (which solvolyze by allyl-oxocarbonium ions) do not react. However, under catalysis by mercuric bromide or acetic acid, the corresponding 2′,3′ unsaturated nucleosides are formed although accompanied by appreciable amounts of 1′,2′-unsaturated-3′-nucleosides. Formation of the latter may be avoided if the hex-2-enopyranosides are first converted to the hex-2-enopyranosyl acetates by treatment with boron trifluoride and acetic anhydride. The derived acetates react with the nucleoside bases without catalysis to give anomeric mixtures of nucleosides, there being no trace of the unwanted 3′-nucleosides.