NEW DIASTEREOSPECIFIC SYNTHESIS OF 2',3'-DIDEOXY-2' OR 3'-C-2 BRANCHED OR 2',3'-ALPHA-FUSED-ISOXAZOLIDINE NUCLEOSIDES DIRECTLY FROM THE SECONUCLEOSIDE

The first diastereospecific syntheses of [3.3.0]-alpha-fused-isoxazoli dine nucleosides 20 or 24, and 2'- or 3'-C-2-branched-2',3'dideoxynucl eosides 14 or 16 have been reported starling directly from 2',3'seconu cleoside 3. The key steps involve tile unsymmetrical modification of t he 2'- or 3'-hydroxyl in the seconucleoside 3 to give pure 8 (3 --> 4 --> 7 --> 81 or 11 (3 --> 5 --> 10 --> 11) and their diastereospecific recyclisation to the furanose moiety to give the title compounds eith er by radical or [2+3] cycloaddition reaction.