ChemInform Abstract: Improved Preparations of Some Per‐O‐Acetylated Aldohexopyranosyl Cyanides.

Abstract

Abstract 3,4,6-Tri- O -acetyl-1,2- O -[1-( exo -, endo -cyano)ethylidene]-α- d -galacto- ( 1a/b ), -α- d -gluco- ( 2a/b ), and -β- 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-α- d -manno- and -gluco-pyranosyl bromide with mercuric cyanide in nitromethane generates, in the ratio of ∼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-α- and -β- d -galacto, -α- and -β- d -gluco-, -α- d -manno-, and -2-deoxy-2-phthalimido-β- 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-α- and -β- d -mannopyranosyl cyanide (48 and 16% total yield, respectively) and -α- and -β- d -glucopyranosyl cyanide (12 and 39% total yield, respectively) were synthesized by fusion of the corresponding -α- d -glycosyl bromides with mercuric cyanide.