FLUORINATION REACTIONS AT C-5 OF O-BENZYL-6-DEOXY-1,2-O-ISOPROPYLIDENEHEXOFURANOSES

Fluorination reactions of enzyl-6-deoxy-1,2-O-isopropylidene-alpha-D-g luco-, beta-L-ido-, alpha-D-allo-, and beta-L-talo-furanoses (1, 2, 3, and 4) were investigated. The reaction of 1 with diethylaminosulfur t rifluoride (DAST) predominantly produced -fluoro-1,2-O-isopropylidene- alpha-D-glucofuranose with the retention of the configuration at C-5. Both of the fluorides with retained and inverted configurations were o btained in the fluorination of 2 with DAST. In contrast, only the inve rsion of the configuration occurred when 3 and 4 were reacted with DAS T. The reactions of the methanesulfonates of 3 and 4 with tetrabutylam monium fluoride (TBAF) and those of their trifluoromethanesulfonates w ith tris(dimethylamino)sulfonium difluorotrimethylsilicate (TASF) gave the 5-deoxy-5-fluoro derivatives with the inversion of each configura tion and the 5,6-unsaturated derivative. However, the reactions of the sulfonates of 1 and 2 overwhelmingly produced the 4,5- and 5,6-elimin ated derivatives. For the reaction in which the steric hindrance of th e benzyloxyl group at C-3 and the electronic repulsion of ring oxygen to the S(N)2 displacement with fluoride anion are significant, such as the fluorination of 1 with DAST, the S(N)i mechansim is reasonable.