Reactions, glycoside-forming

More success has been achieved in syntheses which are applicable only to polysaccharides of simple repeating structure. Condensation polymerization has been used most frequently, a variety of glycoside-forming reactions have been employed (//O) and a few have been modestly successful in producing products of reasonably pure anomeric configuration (Table 1). 

Mechanism 27.1 explains why a single anomer forms two glycosides. The reaction proceeds by way of a planar carbocation, which undergoes nucleophilic attack from two different directions to give a mixture of diastereomers. Because both a- and (i-D-glucose form the same planar carbocation, each yields the same mixture of two glycosides. 

Deoxyribonucleoside (Section 27.14B) An iV-glycoside formed by the reaction of D-2-deoxyribose with certain amine heterocycles. 

The comparable galactose derivative (Table III, no. 6) was notably solvent-insensitive, and in glycosidation reactions gave less than 40% of the a-anomer. However, when the C-6, and C-4 substituents were both IJ-phenylcarbamate,the proportion of a-glycoside formed in ether was markedly greater (no. 7), the derivative was sensitive to solvent structure, and in dimethoxyethane, glycosidation was comparable in a-selectivity to the best case found with glucose (no. 9). A more extensive... 

Reaction conditions have been established under which any of the four methyl D-allosides can be prepared in good yield, in one step, from D-allose by varying the temperature and the concentration of acid, in the presence or absence of strontium chloride. When none of the methyl glycosides form a complex, the dimethyl acetal of the acyclic form can sometimes be obtained in the presence of calcium chloride. Normally, this acetal is formed in only a very small proportion. 

In a final example demonstrating the utility of unfunctionalized olefins, Levy, et al.,43 reported a novel adaptation in which the formed C-glycoside cyclizes to an adjacent oxygen with loss of a benzyl group. As shown in Scheme 2.3.17, l-0-acetyl-2,3,4-tri-0-benzyl-L-fucose was treated with isobutylene and an excess of TMSOTf giving the illustrated fused ring C-glycoside. This reaction proceeded in >70% yield in both methylene chloride and acetonitrile. Additionally, borontrifluoride etherate could also be used. The proposed mechanism, shown in Scheme 2.3.17, involves the initial elimination of the... 

The yield (6%) of methyl /3-D-eri/product from methyl /3-D-xylopyranoside), was somewhat higher than that from chromium trioxide oxidation, but the yield was obviously limited because of secondary oxidation and degradation reactions. In unpublished experi-... 

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