A NEW TYPE OF CARBOHYDRATE ORTHOESTER: SIDE REACTIONS OF THE KÖNIGS-KNORR SYNTHESIS

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Abstract
A trisaccharide orthoester (IV) is one of the products formed by decomposition of 2,3,4,6-tetra-O-acetyl α-D-mannopyranosyl bromide in the presence of silver oxide. This new type of carbohydrate derivative contains three units of D-mannose linked together through the functional groups of a molecule of acetoacetic acid. The carboxyl group of the latter takes part in the orthoester structure of IV, with one D-mannose unit closing a 1,2-orthoester ring, and another unit comprising the —OR group through its anomeric center; the keto group of the acetoacetate forms a 1,2-ketal structure with the third D-mannose unit. The n.m.r. spectra characteristics of the trisaccharide and some related compounds are described. The following sequence of reactions is consistent with formation of the trisaccharide: 1,2-trans-acetoxyl bromide [Formula: see text] 1,2-cyclic acetoxonium ion (IX)[Formula: see text] 1,2-cyclic ketene-acetal; ion IX and 2,3,4,6-tetra-O-acetyl D-mannose then added to the ketene derivative.Selective degradation of trisaccharide IV yields a novel type of ketal, i.e., 1,2,O-[1-(carbomethoxymethyl)-ethylidene]-β-D-mannose.