engleski

Competitive formation of peracetylated alpha-L-arabinopyranosides and beta-L-arabinopyranose 1, 2-(alkyl orthoacetates) in Koenigs-Knorr condensations

Mixtures of peracetylated beta-L-arabinopyranose 1, 2-(alkyl orthoacetates) and the corresponding alpha-L-arabinopyranosides, in ratios as high as 1:1.3, have been obtained from primary, secondary, and tertiary alcohols and 2, 3, 4-tri-O-acetyl-beta-L-arabinopyranosyl bromide, by reaction in dichloromethane-diethyl ether (3:1) in the presence of silver oxide and anhydrous calcium sulfate. Orthoester formation decreased when dichloromethane was replaced by the less-polar chloroform or carbon tetrachloride, and also when diethyl ether was replaced by the more bulky dibutyl or di-isopropyl ethers. The product distribution, which is unusual for Koenigs-Knorr condensations involving 1, 2-cis acylglucosyl halides in the presence of silver oxide, may be ascribed to competitive inhibition of glycoside formation by complexation of the intermediate glycosyl cation with ether and to solvent polarity effects.

glycoside synthesis ; sugar orthoester ; solvent polarity ; aglycone and solvent stereochemistry

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