STEREOSELECTIVE SYNTHESIS OF BRANCHED AND BICYCLO 2',3'-DIDEOXY-THREO-FURANOSYL NUCLEOSIDES FROM PYRANOSES USING A RING CONTRACTION REACTION AS THE KEY STEP

Bicycle nucleoside 21 and the branched-chain nucleoside 26 have been s tereoselectively synthesized (alpha:beta = 1:4) from a common intermed iate 14, which was obtained from methyl ene-2-deoxy-3-O-triflyl-alpha- D-arabino-pyranoside (13) through a ring contraction reaction. The bra nched nucleoside 26 was obtained with an alpha:beta selectivity of 1:4 using either electrophilic selenium or sulfur reagents. In contrast t he bicycle nucleoside 21 was only obtained using the phenylthio deriva tive 20. The selenium reagents resulted in predominant formation of th e alpha isomer.