Stereoselective synthesis of a [3.3.0]-fused γ-butyrolactone. Application in the preparation of bicyclic nucleosides

Stereoselective rearrangement of 6,7-epoxy-3-oxabicyclo[3.2.0]heptan-2-ones 2 and 3 in water afforded a cis-fused butyrolactone as a mixture of two epimers 1a–b. These were used as the starting materials to prepare new bicyclonucleosides 8a–b.