C-GLYCOSIDE SYNTHESES .1. GLYCOSYL CYANIDES AND ISOCYANIDES FROM GLYCOSYL FLUORIDES WITH FULL ACETAL PROTECTION

2,3:5,6-Di-O-isopropylidenemannofuranose, 2,3:4,6-di-O-isopropylidenemannopyranose, and 2,3,4,6-tetra-O-benzylglucopyranose each have been converted with 2-fluoro-1-methylpyridinium tosylate into anomerically pure pairs of glycosyl fluorides. Reaction of each anomeric mannopyranosyl and mannofuranosyl fluoride with Et2AlCN in THF gave only the two (four component) anomeric mixtures of mannopyranosyl, respectively mannofuranosyl cyanides and isocyanides. The pyranosidic four component mixture (alpha-CN, alpha-NC, beta-CN, and beta-NC) was completely separated by a combination of flash chromatography, crystallization, and/or preparative HPLC to give the individual components; in the furanose series, only the crystalline two component mixture of alpha-furano cyanide and isocyanide could not be resolved. Isocyanides show two absorption maxima in their UV spectra (195 and 230 nm) while cyanides show only the first. Cyanides, being C-glycosides, char more slowly on heated TLC plates than isocyanides.