Acidic Opening of 4,6-O-Orthoesters of Pyranosides

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Abstract
Acid-catalyzed opening of five-membered ring orthoesters of pyranosides are well explored and often used in synthetic pathways.1 The opening gives the acyl group preferentially on the axial oxygen contrary to most other regioselective methods which give the equatorial ester.2 Opening of six-membered ring orthoesters are much less investigated. We have made ortho-acetates and -benzoates of different 4,6-diols of glucose and galactose and studied their acid-catalyzed opening. These reactions each gave a high yield mixture of the respective 4- and 6-O-mono-esters. With the orthoacetates, the 6-O-acetyl derivatives were always in excess, and by treating the 4-O-acetyl compounds under conditions that gave base-catalyzed acetyl migration from O-4 to O-6, high overall yields of the 6-O-acetates were obtained. With the ortho-benzoates, the ratio between the two products was mainly determined by the protecting groups used at O-2 and −3.