Improved preparations of some per-O-acetylated aldohexopyranosyl cyanides

3,4,6-Tri-O-acetyl-1,2-O-[1-(exo-, endo-cyano)ethylidene]-alpha-D- galacto- (1a/b), -alpha-D-gluco- (2a/b), and -beta-D-manno-pyranose (3a/b) were stereoselectively isomerized to the corresponding per-O-acetylated 1,2-trans-aldohexopyranosyl cyanides in 75, 16, and 62% yield, respectively, by treatment with boron trifluoride etherate in dry nitromethane. The corresponding per-O-acetylated 1,2-cis-aldohexopyranosyl cyanides were obtained concurrently in respective yields of 1.9, 0.9, and 4.8%. The per-O-acetylaldohexopyranosyl cyanide products were found stable to the reaction conditions and were readily isolated following completion of the rearrangement. It had previously been proved that reaction of 2,3,4,6-tetra-O-acetyl-alpha-D-manno- and -gluco-pyranosyl bromide with mercuric cyanide in nitromethane generates, in the ratio of approximately 1:1, the desired 1,2-trans-glycosyl cyanides and the corresponding 1,2-O-(1-cyanoethylidene) isomers (3a/b and 2a/b, respectively). Treatment of these reaction-mixtures with boron trifluoride etherate in nitromethane effected the rearrangement of 3a/b and 2a/b, thereby facilitating the isolation, and increasing the overall yields, of the per-O-acetylated 1,2-trans-D-manno- and -gluco-pyranosyl cyanides (58 and 30% total yield, respectively) relative to the earlier procedures. The boron trifluoride etherate-mediated reaction of per-O-acetyl-alpha- and -beta-D-galacto-, -alpha- and -beta-D-gluco-, -alpha-D-manno-, and -2-deoxy-2-phthalimido-beta-D-gluco-pyranoses with trimethylsilyl cyanide in nitromethane was also investigated. This reaction provides a "one-flask" synthesis of the corresponding per-O-acetylated 1,2-trans-aldohexopyranosyl cyanides in which 1,2-O-(1-cyanoethylidene) derivatives are isomerized in situ. Finally, improved preparations of the (not readily accessible) per-O-acetylated 1,2-cis-D-manno- and -gluco-pyranosyl cyanides are described. Thus, 2,3,4,6-tetra-O-acetyl-alpha- and -beta-D-mannopyranosyl cyanide (48 and 16% total yield, respectively) and -alpha- and -beta-D-glucopyranosyl cyanide (12 and 39% total yield, respectively) were synthesized by fusion of the corresponding alpha-D-glycosyl bromides with mercuric cyanide.