Fluorination Reactions at C-5 of 3-O-Benzyl-6-deoxy-1,2-isopropylidenehexofuranoses

Article abstract of DOI:10.1246/bcsj.67.236

Fluorination reactions of 2-O-benzyl-6-deoxy-1,2-O-isopropylidene-α-D-gluco-, β-L-ido-, α-D-allo-, and β-L-talo-furanoses (1, 2, 3, and 4) were investigated.The reaction of 1 with diethylaminosulfur trifluoride (DAST) predominantly produced 2-O-benzyl-5,6-dideoxy-5-fluoro-1,2-O-isopropylidene-α-D-glucofuranose with the retention of the configuration at C-5.Both of the fluorides with retained and inverted configurations were obtained in the fluorination of 2 with DAST.In contrast, only the inversion of the configuration occurred when 3 and 4 were reacted with DAST.The reactions of the methanesulfonates of 3 and 4 with tetrabutylammonium fluoride (TBAF) and those of their trifluoromethanesulfonates with tris(dimethylamino)sulfonium difluorotrimethylsilicate (TASF) gave the 5-deoxy-5-fluoro derivatives with the inversion of each configuration and the 5,6-unsaturated derivative.However, the reactions of the sulfonates of 1 and 2 overwhelmingly produced the 4,5- and 5,6-eliminated derivatives.For the reaction in which the steric hindrance of the benzyloxyl group at C-3 and the electronic repulsion of ring oxygen to the S_N2 displacement with fluoride anion are significant, such as the fluorination of 1 with DAST, the S_Ni mechanism is reasonable.