Configuration anomeric center

Imidazole can be A -allylated. The A -glycosylimidazole 299 is prepared by regiospecific amination at the anomeric center with retention of configuration. Phenoxy is a good leaving group in this reaction[181]. Heterocyclic amines such as the purine base 300 are easily allylatedfl 82]. 

Both maltose and cellobiose have a free anomeric hydroxyl group that is not involved in a glycoside bond The configuration at the free anomeric center is variable and may be either a or (3 Indeed two stereoisomeric forms of maltose have been iso lated one has its anomeric hydroxyl group m an equatorial orientation the other has an axial anomeric hydroxyl... 

The structures of all products were determined by either conversion to the corresponding acetates or direct comparison with the corresponding natural p-glucosides. Identification of the -configuration of the anomeric center was easily achieved via the analysis of the C-H/C-H coupling constant. The synthetic (3-D-glucosides (30, 33, 34, 35 and 36) were identical with... 

The Tsuji-Trost reaction is the palladium-catalyzed allylation of nucleophiles [110-113]. In an application to the formation of an A-glycosidic bond, the reaction of 2,3-unsaturated hexopyranoside 97 and imidazole afforded A-glycopyranoside 99 regiospecifically at the anomeric center with retention of configuration [114], Therefore, the oxidative addition of allylic substrate 97 to Pd(0) forms the rc-allyl complex 98 with inversion of configuration, then nucleophilic attack by imidazole proceeds with a second inversion of configuration to give 99. 

Radical reduction of 1-nitro-C-glycosyl compounds. In 1983, Vasella and co-workers125 reported a stereoselective method for the synthesis of a-C-mannopyranosyl compounds by reduction of 1-nitro-C-mannopyranosyl derivatives with Bu3SnH in the presence of a,a -azoisobutyronitrile (AIBN) radical initiator. These reactions involve the formation of anomeric centered radicals. Thus, in the case of d-manno configuration as in 140, the 1,2-cts-C-pyranosyl compound 145 was obtained in 84% yield. The intermediate pyranosyl radicals 143 prefer a-attack by the tin hydride. Thus for D-glucopyranosyl derivatives, the corresponding l,2-tra x-C-pyranosyl compound 144 is obtained preferentially (Scheme 47). 

Carboxylic acids, being weaker acids, react with la-a with inversion of configuration at the anomeric center to yield /2-0-acyl compounds (1,53). This mild and convenient method for 1 -0-acylation of carbohydrates is also useful for pharmacological drug modification (54) or for the resolution of carboxylic acids (53). 

As the only difference between the two oxidation products is the configuration at the carbon atom that had been the anomeric center, it was possible to determine the configuration of C-l of other N-acetyl-hexopyranosylamines by comparison of the specific rotations of the dialdehydes formed by their oxidation with periodate. In this way, the anomeric configuration of N-acetyl-/3-D-glucopyranosylamine... 

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