Abstract
As part of our studies on the synthesis of conformationally restricted nucleosides of types 1 and 2, where X = CH2, O or S, we required access to differentially substituted D-psicofuranosyl nucleosides such as 3. As shown in the table, we have developed a convenient approach to such compounds that depends on the direct condensation of the 1,2:3,4-di-O-isopropylidene-β-D-psicofuranose derivative 4 with an appropriate silylated purine or pyrimidine base.2 Although the α and β anomers of 3 are formed in a 1:1 ratio, the yields of the β anomers are generally comparable with earlier condensation methods that use psicofuranosyl- halide2, 2-benzoates4 or 2-nitro derivative5. However, the present method has the advantage that the starting sugar 4 is more readily accessible. The precursor 6′-alcohol can be prepared in very large amounts from D-fructose using the method of Prisbe et al.4