The isoxazolidine derivative 6, prepared from an intermolecular 1,3-di
polar cycloaddition of the (5S)-unsaturated lactone 5 and N-methylnitr
one, was effectively used for the synthesis of [3.3.0] bicyclic isoxaz
olidinyl cytidine 4a and thymidine 4b. Copyright (C) 1996 Elsevier Sci
ence Ltd