Determination of the anomeric configuration in glycosides of 3-deoxy-2-aldulosonic acids by circular dichroism and X-ray crystallography

Abstract

Methyl (methyl 3-deoxy-D-arabino-2-heptulopyranosid)onate was obtained by acid-catalysed esterification and glycosidation of the free 2-aldulosonic acid. The molecular geometry of the glycosidic ester was determined by X-ray crystallography, and indicated both that the molecule was the α-anomer and that the ring oxygen and the carboxy-group were very nearly coplanar. The c.d. spectrum showed a negative n →π* transition band centred at 224 nm. The c.d. spectra of a number of methyl glycopyranosides derived from 6-, 7-, and 8-carbon 3-deoxy-2-aldulosonic acids demonstrated that all the compounds believed to be α-glycosides had negative maxima, and all those believed to be β-glycosides had positive maxima around 220 nm independent of their ²C₅ or ⁵C₂ conformation. The only known furanoside, believed to be a β-anomer, also had a positive maximum. The chirality of the anomeric carbon atom of this class of compound can thus be conveniently established by determining the sign of the Cotton effect at 220 nm.