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Chemistry of 1,2-Anhydro Sugars.

The 1,2-anhydro sugars are a class of valuable and versatile intermediates in carbohydrate chemistry. In the first part of this article, a review is given on preparation methods of 1,2-anhydro sugars that are suitably protected. Protected 1,2-anhydro sugars have been widely used as glycosyl donors for the synthesis of glycosyl compounds such as oligosaccharides and nucleosides. In the second part, a brief history and the chemistry of unprotected 1,2-anhydro sugars is described. In the past few years, our research group has developed protection-free methods for synthesis of glycosyl compounds through unprotected 1,2-anhydro sugars as reactive intermediates based on the concept of 'direct anomeric activation'. In this article, the one-step preparation of glycosyl compounds such as thioglycoside derivatives and glycosyl azides by using formamidinium-type dehydrating agents is presented. Furthermore, the initial results on the first detection of unprotected 1,2-anhydro sugar intermediates by NMR measurements are shown along with their full structure characterizations.