Direct Dehydrative Glycosylation of C1-Alcohols

Due to the central role played by carbohydrates in a multitude of biological processes, there has been a sustained interest in developing effective glycosylation methods to enable more thorough investigation of their essential functions. Among the myriad technologies available for stereoselective glycoside bond formation, dehydrative glycosylation possesses a distinct advantage given the unique properties of C1-alcohols such as straightforward preparation, stability, and a general reactivity compatible with a diverse set of reaction conditions. In this Focus Review, a survey of direct dehydrative glycosylations of C1-alcohols is provided with an emphasis on recent achievements, pervading limitations, mechanistic insights, and applications in total synthesis.

[Dehydrative glycosidation and partially benzylated sugar derivatives]

Dehydrative glycosidation reactions reported by the authors' group are reviewed. The authors' efforts were concentrated on developing reagent systems usable for one-stage-one-pot glycosidation. Such systems could simplify the glycosidation step using 1-OH sugar derivatives, since any preactivation stage for the hemiacetal OH group could be omitted. The systems, utilizing the dehydration potential of sulfonyl chloride, such as the p-nitrobenzenesulfonyl chloride-silver trifluoromethanesulfonate-triethylamine system as well as the p-nitrobenzenesulfonyl chloride-silver trifluoromethanesulfonate-N,N-dimethylacetamide-triethylamine system, were useful for the syntheses of many kinds of oligosaccharides. As a system free from any metals, the authors developed the trimethylsilyl trifluoromethanesulfonate-pyridine (TP) system. During the study of the system containing cobalt (II) bromide, the authors found that the bromide converts 1-OH sugar into the corresponding 1-Br derivative, which is then activated with the cobalt salt to undergo glycosidation with alcohol. To prepare partially benzylated sugar derivatives used as acceptors in the authors' studies, controlled benzylation and forced tritylation were carried out. Short syntheses of a variety of useful sugar derivatives using such convenient procedures are described. As a novel protecting group for the hemiacetal OH group, the authors used the 2-methoxyethyl group. Many kinds of trehalose-type disaccarides we prepared.