Anomeric chain extensions

C-Glycosyl compounds are important molecular targets as they occur in Nature and have interesting biological properties (for reviews on C-glycosylation, see [72-75]). Chain extensions of aldonolactones have been employed to create C-C bond formation at the anomeric center. Claisen-type reactions of aldono-1,4-lactones (e.g., 9, 55) with acetone or acetophenone (Scheme 17) generate hemi-acetals of type 56a-c [76]. Similarly, lactone 55b reacts with CHsCN/NaH to give hemiacetal 57. 

Models of polymer chain extension were first used to compare the effect of the glycosidic linkage geometry of simple polysaccharide chains, eg cellulose and amylose (43). Both polymers are 1,4-linked glucans the only difference is in the anomeric configuration on the C-1 atom of the monomeric unit, a for amylose and for cellulose. The calculated data show a remarkable pseudohelical chain... 

The potentially useful insecticide milbemycin P3 (198) has been synthesized in racemic form by Smith and co-workers. Intramolecular closure of the hemiacetal (193), derived from the lactone (192), first gave, under anomeric control, the crystalline aldehyde (194). After chain extension to (195), a coupling reaction... 

The homoazasugars 33 and 34 have been synthesized and shown to be good inhibitors of almond p-glucosidase. The key steps in the syntheses include a two-carbon Wittig chain extension at the anomeric centre of a D-arabinopentose derivative and cyclization by attack of an amino fimction at an epoxide group (prepared via a Sharpless asymmetric epoxidation). ... 

As compared to the corresponding agalacto compound 6, the presence of Gal residues 6 and 6 introduces a change in the chemical shift of the H-l atoms of the peripheral GlcNAc residues 5 and 5 from 8 4.558 to 4.582. However, this extension has virtually no effect upon the chemical shifts of the H-l atoms of Man residues 3, 4, and 4. Interestingly, the H-l atom of Man-4 and also that of Man-4 each yields two separated doublets which must be ascribed to the two anomeric forms of 7. From the relative intensities of these doublets, it may be concluded that the anomers of this oetasaccharide occur in the ratio of ot /3 = 2 1. Apparently, the chain length does not affect the anomeric-equilibrium constant. 

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