摘要:
A stereoselective method is described for the synthesis of [3.3.0] fused lactones (gamma-butyrolactones) of ribonucleosides at the 2' and 3' positions of the furanose ring by intramolecular addition of radicals onto the alpha-position of alpha,beta-unsaturated esters. A new chiral center is formed at an off-template site of the ribofuranose ring, with good diastereoselectivity. These gamma-butyrolactones of nucleosides are useful chiral synthons for the preparation of branched-chain nucleosides.;Opening of the lactone ring afforded highly functionalized 2'-C- and 3'-C-branched-chain nucleosides which are difficult to synthesize by currently available methods. Conformational analysis of the gamma-butyrolactones of nucleosides has indicated an unusual ribose ring conformation, probably induced by the fused gamma-lactone ring.