4,6-O-[1-Cyano-2-(2-iodophenyl)ethylidene] Acetals.Improved Second-Generation Acetals for the Stereoselective Formation of beta-D-Mannopyranosides and Regioselective Reductive Radical Fragmentation to beta-D-Rhamnopyranosides.Scope and Limitations

摘要

The [1-cyano-2-(2-iodophenyl)]ethylidene group is introduced as an acetal-protecting group for carbohydrate thioglycoside donors.The group is easily introduced under mild conditions,over short reaction times,and in the presence of a wide variety of other protecting groups by the reaction of the 4,6-diol with triethyl (2-iodophenyl)orthoacetate and camphorsulfonic acid,followed by trimethylsilyl cyanide and boron trifluoride etherate.The new protecting group conveys strong beta-selectivity with thiomannoside donors and undergoes a tin-mediated radical fragmentation to provide high yields of the synthetically challenging beta-rhamnopyranosides.The method is also applicable to the glucopyranosides when high alpha-selectivity is observed in the coupling reaction and alpha-quinovosides are formed selectively in the radical fragmentation step.In the galactopyranoside series,beta-glycosides are formed selectively on coupling to donors protected by the new system,but the radical fragmentation is unselective and gives mixtures of the 4- and 6-deoxy products.Variable-temperature NMR studies for the glycosylation step,which helped define an optimal protocol,are described.