New Effective Transformations of Monosaccharides by Selective Inversion

Abstract

AbstractRegio‐ and stereoselective acetalizations of methyl α‐D‐lyxopyranoside (1), methyl β‐L‐arabinopyranoside (6), methyl α‐D‐galactopyranoside (11), and methyl β‐D‐galactopyranoside (13) with chloral, including an inversion of the configuration at C‐3, were achieved with dicyclohexylcarbodiimide as a coagent. Additionally, the formed methyl 3,4‐O‐(2,2,2‐trichloroethylidene)‐α‐D‐arabinopyranoside 2, methyl 2,3‐O‐(2,2,2‐trichloroethylidene)‐β‐L‐lyxopyranoside 7, methyl 2,3‐O‐(2,2,2‐trichloroethylidene)‐α‐D‐gulopyranoside 12, and methyl 2,3‐O‐(2,2,2‐trichloroethylidene)‐β‐D‐gulopyranosides 14 and 15 contain an N‐cyclohexylcarbamoyl group vicinal to the chloral acetal moiety; the compounds 12 and 15, moreover, a formyl group in 6‐position. The formyl and carbamoyl functions may be cleaved stepwise with methanolic sodium methoxide. Deformylation of the 6‐O‐formyl derivative 15 affords the urethane 16 with simultaneous migration of the carbamoyl group from the 4‐ to the 6‐position. The fluorination of 3 and 8 with anhydrous hydrogen fluoride/nitromethane/acetic anhydride, giving the glycosyl fluorides 4 and 9 without any cleavage of the cyclic chloral acetal functions, is described. BF3‐catalyzed glycosylations of chloro‐ethanol with the fluorides 4 and 9α/β yield the α‐glycoside 5 and the anomeric mixture 10α/β, respectively, showing that neither a cyclic chloral acetal function in 3,4‐O‐nor in 2,3‐O‐position affects such reactions.