Organofluorine compounds and fluorinating agents. Part 11. Glycosyl fluorides from acetal-protected sugars

2-O-(2-Acetoxyethyl)-3,4,6-tri-O-methyl-α-D-glucosyl fluoride (5) was obtained by partial cleavage of the 1,2-O-ethanediyl-β-d-glucopyranose derivative 4 at the glycosidic position by means of the HF/nitromethane/acetic anhydride system. The same medium allows the selective cleavage of the 1,2-isopropylidene function in α-d-glucofuranosides (7, 11) and of the 5,6-O-isopropylidene group in acetyl 2,3:5,6-di-O-isopropylidene-d-manno-furanoside (15) with simultaneous conversion into the corresponding acetylated glycosyl fluorides (8, 12, 16). The second acetal function in 11 and 15 is not cleaved by the weakened HF medium. 2-O-Benzyl-protected sugars such as methyl 3,5,6-tri-O-methyl-2-O-benzyl-d-glucofuranoside (13) react intramolecularly in the presence of HF/nitromethane/acetic anhydride to form a cyclic C-glycoside (14).