Monosaccharides hemiacetal formation from

A typical procedure calls for reaction of the hemiacetal donor with dicydohexyl carbodiimide and copper(I) chloride (0.1 equiv) at 80 °C, followed by an addition of the acceptor and continued heating. As an early demonstration of this protocol, oc-riboside 86 was prepared in moderate yield but with exclusive stereoselectivity [141]. Further measures were required for the glycosylation of monosaccharide acceptors, such as addition of p-toluenesulfonic add (0.1 equiv) to promote the formation of disaccharide 87 [144]. The method was more suitably applied to the synthesis of O-acyl glycopeptides, as evidenced by the formation of 88 in 60% yield [143,144]. Various peptides with non-nudeophilic side chains were found to be amenable to this stereoselective reaction. The [3-selectivity was suggested to arise from a preponderance of the a-isourea intermediate 85 in the activation step. 

Monosaccharides normally exist as cyclic hemiacetals rather than as open-chain aldehydes or ketones. The hemiacetal linkage results from reaction of the carbenyl group witli an -OH group three or four carhou atoms away. A nvu-mrfn]b r cyclic hemiacetal is called a furanose, and a six-mcnibered cyclic hemiacetal is called a pyranose. Cycltzation leads to th formation of a new chirality center and production of two dia tereomerlc hemiacetaU, called a and p anomers. 

The compound 3,5-dihydroxy-2-methyl-5,6-dihydropyran-4-one (V in Formula 4.67) is also formed from the pyranoid hemiacetals of l-deoxy-2,3-hexodiulose (Formula 4.75). In comparison, maltol is preferentially formed from disaccharides like maltose or lactose (Formula 4.76) and not from dihydroxypyranone by water elimination. The formation of maltol from monosaccharides is negligible. A comparison of the decomposition of 1-deoxyosones from the corresponding cyclic pyranone structure clearly shows (cf. Formula 4.75 and 4.76) that the glyco-sidically bound carbohydrate in the disaccharide directs the course of water elimination in another direction (Formula 4.76). It is the stabilization of the intermediates to quasi-aromatic maltol which makes possible the cleavage of the glycosidic bond with the formation of maltol. Parallel to the formation of maltol, isomaltol derivatives which still contain the second carbohydrate molecule are also formed from disaccharides (Formula 4.77). Indeed, the formation of free isomaltol is possible by the hydrolysis of the... 

The aldehydic or hemiacetal group is the most easily oxidised portion of the monosaccharide molecule. All aldopyranoses (CLVIII) are oxidised with aqueous buffered bromine solution (pH 5.6). The initial product is the 1,5-lactone (CLIX) which then hydrolyses to the aldonic acid (CLX). Evaporation of the aqueous solution yields the y-lactone (CLXI), but from certain solvents such as p uidine which inhibit lactone formation the... 

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