Acid-catalysed opening of five- and six-membered cyclic orthoesters of hexopyranosides is well explored and often used in synthetic pathways.1-7Opening of a five-membered ring orthoacetate or benzoate (formed fromcis-vic-diols) gives the axial ester,1,2contrary to most other regioselective methods which give preferentially the equatorial ester, whereas opening of six-membered ring 4,6-orthoesters give a mixture of 4- and 6-esters, the ratio depending in part on the substituent at O-3 and on the amount of acyl migration during the reaction conditions.3,4Since chlorinated acetates (mono-, di- and tri-) are selectively removable in the presence of acetates and benzoates,8their introduction by means of the acidic opening of the corresponding orthoacetate could give a route to a flexible protecting group scheme with regioselective introduction of the selectively removable chloroacetate. This would open up a suitable way for synthesis of,inter alia, branched oligosaccharides. Therefore, mono-, di- and trichloroorthoacetates (both five- and six-membered) of different monosaccharide derivatives were synthesized and their acidic opening studied.