One-pot alpha-glycosylation pathway via the generation in situ of alpha-glycopyranosyl imidates in N,N-dimethylformamide.

Divergent pathways are disclosed in the activation of 2-O-benzyl-1-hydroxy sugars by a reagent combination of CBr4 and Ph3P, all of which afford one-pot alpha-glycosylation methods. When this reagent is used in CH2Cl2, the 1-hydroxy sugar is converted to the alpha-glycosyl bromide in a conventional way and leads to the one-pot alpha-glycosylation method based on a halide ion-catalytic mechanism. In either DMF or a mixture of DMF and CHCl3, however, alternative alpha-glycosyl species are generated. From the 1H and 13C NMR study of the products, as well as the reactions using Vilsmeier reagents [(CH3)2N+=CHX]X- (X=Br and Cl), these were identified as cationic alpha-glycopyranosyl imidates having either Br- or Cl- counter ion. The cationic alpha-glycosyl imidate (Br-), derived specifically in the presence of DMF, is more reactive than the alpha-glycosyl bromide and thus is responsible for the accelerated one-pot alpha-glycosylation. The one-pot alpha-glycosylation methodology performed in DMF was assessed also with different types of acceptor substrates including tertiary alcohols and an anomeric mixture of 1-OH sugars.