Anomeric position, of sugars

Allenylsilanes react with acetals, as they do with aldehydes, by addition, but a simple elimination step completes the substitution reaction (Scheme 48). Propargylsilanes likewise react with acetals in the presence of Lewis acids (Scheme 49). The reaction has en used intramolecularly (Scheme 50), where the first step is likely to be acetal or hemiacetal formation followed by ring closure, and in reactions at the anomeric position of sugars with high levels of axial attack giving allenes. ... 

Interestingly, when the role of the partners is reversed, e. g., when the radical is formed on a non-anomeric position of one of the sugars as in 262 (O Scheme 54), the addition occurs equatorially to an anomeric exomethylene, resulting in an anomeric radical that abstracts hydrogen from the a face. 

Sugar phosphates and a nucleoside diphosphate sugar. When the anomeric position of a sugar is not substituted, the configuration at this position is not specified because the sugar can exist in either the a- or j6-anomeric form. 

The aqueous Diels-Alder reaction has been widely exploited at the preparative level since the first studies of Breslow. Two reviews [4, 6] were in part devoted to this aspect. In order to increase the water-solubility, dienes have been attached at the anomeric position of a carbohydrate. The sugar moiety, which induces chirality, can easily be removed after the cycloaddition either via acidic or enzymic hydrolysis [37]. 

This procedure enables direction of the reaction toward the formation of either the a ox p anomer. Control is achieved during attachment of the trichloroacetimi-date group to the anomeric position of the sugar. If the reaction is performed in the presence of NaH, 96 % of the trichloroacetimidated product (9) is obtained with an aip ratio of 96 4. In the presence of Na2C03, 80 % of the product is obtained with an alp ratio of 25 75. Substitution of the trichloroacetimidate by the alkyl group occurs with inversion of the configuration. 

Figure 2 (a) Single RCT spectrum of 1. Labelled peaks indicate the relayed coherence transfers between anomeric protons of sugar residues with their respective 3-position protons, (b) Double RCT Sj tnim of 1. Labelled peaks indicate the relayed coherence transfers between ancmieric protons with their respective 4-position protons, and the coherence transfer between Rha.Hi with Rha.H3. 

Hgure 3. Triple RCT specinim of 1. Labelled peaks indicate correlations between anomeric protons of sugars with respective S-position protons. ... 

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