Glucose anomeric position

Pyridone also catalyzes epimerization of the anomeric position of the tetramethyl ether of glucose. The mechanism involves two double proton transfers. The first leads to a ring-opened intermediate, and the second results in ring closure to the isomerized product ... 

Sucrose (Section 25.14) A disaccharide of glucose and fructose in which the two monosaccharides are joined at their anomeric positions. 

Fig. 18. A variant of the STARFISH 10-mer in which the Pk antigen is linked to the central glucose core at the anomeric position.166...

Several methods were described for the selective de-S-acetylation of 0-acetyl protected 1-thioglycoses. Sodium methoxide in methanol at low temperature (below -20 °C) was known to afford mainly the de-S-acetylated compound [16] or exclusively this compound when the reaction was quenched at low temperature by adding H-l- resin [17]. Demercuration of tetra-O-acetyl-l-phenylmercury(II)-thio- -D-glucopyranose (12) (Scheme 4) obtained by treatment of (8e) with phenylmercury(II)acetate afforded a convenient synthesis of tetra-0-acetyl-l-thio-/3-D-glucose (8a) [18]. This sequence applied to the a-anomer (10a) (Scheme 3) led to the expected de-S-acetylated compound (10b) [19]. Chemoselective deprotection of thioacetate at the anomeric position of peracetylated 1-thioglycoses was also achieved in good yield by action of cysteamine in acetonitrile or hydrazinium acetate in DMF [20,11]. 

Thioglucobioses (69a) and (70a) were easily prepared from 2-0-triflyl-man-nose tetraacetate (68) and 1-thio-glucoses (8a) or (10b). However, Zemplen 0-deacetylation led to an intractable mixture of compounds. Thus, prior to methoxide treatment, the readily cleavable allyl group was introduced at the anomeric position (Scheme 22) [20] of the acetylated disaccharides (69a) and (70a). Benzyl -thiokojibioside (70c) was also prepared by condensation of (68) and (16a) [29bj. 

To explain the above results, glucose is assumed to exist as a cyclic hemiacetal in equilibrium with a small amount of the open-chain aldehyde. In the hemiacetal, C is chiral and two diastereomers (anomers) are possible they are called a- and jS-glucosides. The C of C=0 is always the anomeric position. Cyclic hemiacetals form when an OH and a CHO group in the same molecule can form a five- or six-membered ring that is more stable than the open chain. 

Derivatives of ortho-nitrobenzyl alcohol were also used to protect the hydroxy groups of carbohydrates, including in the anomeric position. Thus, the following acetal was quantitatively photolysed to glucose (Scheme 13.5) [27]. 

Labeling starch with the 14C isotope of carbon at the anomeric position of the reducing terminal D-glucose unit was achieved by treatment with labeled cyanide followed by alkaline hydrolysis of the resulting cyanohydrin.17 Labeled maltose could be prepared from such labeled starch.18 In addition, starches modified by substitution might be labeled in the substituent being introduced, as shown by Kratz and Kaufmann,19 who prepared starch acetylsalicylate with 14C in the acetylsalicylate moiety. 

According to the reactivity scale proposed by Heyns et al. [2-4], oxidation at the anomeric position at C] is easier than for primary or secondary alcohol functions. Selectivity higher than 95 % for aldonic acids were indeed observed in the catalytic oxidation of o-glucose, and of reducing disaccharides such as lactose and maltose. Palladium catalysts or palladium-bismuth catalysts are more selective than platinum because they are less active in the oxidation of primary or secondary alcohol functions. 

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