A treatment of 2,3,5-tri-O-benzyl-B-D-ribofuranosyl fluoride (1) with cyanotrimethylsilane in the presence of boron trifluoride diethyl etherate gave 2,3,5-tri-O-benzyl-α- (2α) and -β-D-ribofuranosyl (2β) cyanide in 46.2% and 46.6% yields, respectively. Confirmation of the corresponding isocyano isomer (3) formation and its conversion into2under boron trifluoride catalysis at -78°C made it possible to deduce that both2αand2βwere produced by way of3which was formed preponderantly in the initial stage of the reaction. On the other hand, the reaction of 2,3,4,6-tetra-O-benzyl-α-D-glucopyranosyl fluoride (4) with cyanotrimethylsilane in diethyl ether by the use of boron trifluoride diethyl etherate (0.05 mol. equiv.) gave 2,3,4,6-tetra-O-benzyl-α -D-glucopyranosyl cyanide (5α), 2,3,4,6-tetra-O-benzyl-α- (6α), and -β-D-glucopyranosyl isocyanide (6β) as a 30:61:9 mixture (94% yield) but that in dichloromethane by the use of the catalyst (1.0 mol. equiv.) gave5α(85% yield) as a sole product.