A Novel Route for the Synthesis of Deoxy Fluoro Sugars and Nucleosides

The reaction of (diethylamino)sulfur trifluoride (DAST) with methyl 5-O-benzoyl-β-D -xylofuranoside ( 1 ) followed by column chromatography afforded the riboside 2 (62%) and the ribo-epoxide 3 (18%) (Scheme 1). Under similar reaction conditions, the α-D -anomer 4 gave the riboside 5 and the difluoride 6 in 60 and 9% yield, respectively. Treatment of the β-D -xyloside 10 with DAST gave, after chromatographic purification, the riboside 11 as the principal product (48%; Scheme 2). These results suggest that the C(3)−O−SF₂NEt₂ derivatives were initially formed in the case of the xylosides studied. The distinctive feature of the reaction of DAST with the β-D -arabinoside 12 consists in the formation of a 3- or 5-benzylideneoxoniumyl-substituted intermediate on one of the consecutive transformations, which finally give rise to the inversion of the configuration at C(3) affording the xylosides 17 (18%) and 18 (55%); the lyxoside 14 was also isolated from the reaction mixture in a yield of 25% (Scheme 3). In the presence of the non-participating 5-O-trityl group, i.e., from the reaction products of 21 with DAST, the compounds 23 and 24 were isolated in 16 and 52% yield, respectively (Scheme 4). It may be thus reasonable to conclude that, in the case of the β-D -arabinosides 12 and 21 , the principal route of the reaction is the formation of the intermediate C(2)−O−SF₂NEt₂ derivative. Unlike 21 , the α-D -arabinoside 26 was converted to the lyxo-epoxide 25 (53%) and the lyxoside 27 (14%), which implies the intermediate formation of the C(3)−O−SF₂NEt₂ derivative (Scheme 5).