Anhydroaldonic acids and their derivatives are widely used in the syntheses of C-glycosyl heterocycles. General precursors of compounds of this class are gly-cosyl cyanides (anhydro-aldononitriles), whose cyanide group can be cyclized or further functionalized with groups such as CHO, CH=N N H Ts, CH=NNHCOR,CH=NNHC(=NH)R, COOH, COOR, CONH CSNH2, C(= NH)OEt, C(=NH)SEt, C(=NH)NH2, C(= NH)NHNH2, C(=NH)NHNHTs, C(=NH)NHOH, and CH2NH2, which are suitable for other transformations. Glycosyl cyanides are most frequently prepared as O-peracylated derivatives with a 1,2-trans conguration; hydroxy protection of glycosyl cyanides with base-stable groups is much less common. Although some O-alkyl and O-isopropyl derivatives are known, access to such compounds suffers from the lack of stereoselectivity in the cyanide substitution step due to the nonparticipating nature of the protecting groups. These features prevent wide use of anhydroaldonic acid derivatives in het-erocyclizations under basic conditions, which are otherwise extensively applied in the construction of several types of heterocyclic rings.
