Competitive formation of peracetylated α-l-arabinopyranosides and β-l-arabinopyranose 1,2-(alkyl orthoacetates) in Koenigs-Knorr condensations

Abstract

Mixtures of peracetylated β-l-arabinopyranose 1,2-(alkyl orthoacetates) and the corresponding α-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-β-l-arabinosyl 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 acylglycosyl 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.