Synthesis of [3.3.0] bicyclic isoxazolidinyl nucleosides

The isoxazolidine derivative 6, prepared from an intermolecular 1,3-dipolar cycloaddition of the (5S)-unsaturated lactone 5 and N-methylnitrone, was effectively used for the synthesis of [3.3.0] bicyclic isoxazolidinyl cytidine 4a and thymidine 4b.