Add time:07/12/2019         Source:sciencedirect.com

Publisher SummaryThe neighboring-group reactions studied most intensively have been those of acyloxy groups, which constitute the complex neighboring-groups. The intermediate formation of a dioxolanylium ion is invoked. The formation of such a ring between C-1 and C-2 of aldoses determines the stereochemical outcome of glycosylation reactions from glycosyl halides. Neighboring-group reactions of acyloxy groups involve a synchronous process whereby the nucleophilic oxygen atom of the carbonyl group becomes attached at the same time as the nucleophilic leaving group is split off. The intermediate dioxolanylium ion is exceedingly reactive. The chapter presents reactions that can generate the dioxolanylium and dioxanylium ring-systems selectively in carbohydrate derivatives, in the absence of a nucleophile and in the presence of a difficultly polarizable complex anion. The various rearrangement reactions such as rearrangement of acyloxonium ions in polyol systems and monosaccharides, rearrangement of esters of polyols and monosaccharides in liquid hydrogen fluoride, rearrangement of cyclitols with acetic acid-sulfuric acid, and rearrangement of saccharides with other lewis acids are studied in the chapter.

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