β‐Eliminativer Abbau bei 4‐0‐substituierten Hexopyranosiduronat‐Derivaten

Abstract

Abstract Two types of endocyclic enol‐acetal forming β‐elimination were investigated on synthetic model compounds. In both types the 4‐O‐methanesulfonyl residue was chosen as leaving group. The a,e ‐β‐elimination was proved on 2,3‐benzyl ether protected D ‐glucopyranosiduronate derivatives I, and the a , a ‐β‐elimination on the analogous substituted D ‐galactopyranosiduronates XVII. Using a small excess of KOH in methanol at 25°, a quick elimination of a molecule of methanesulfonic acid was observed, and as reaction product the 4,5‐unsaturated 4‐deoxyhexopyranosiduronate derivative II was obtained. Only an unimportant stereoselectivity was found between the a,e ‐ and a,a ‐mesylate β‐eliminations. The 4,5‐unsaturated 4‐deoxyhexopyranosiduronates show a strong UV. maximum at 238 nm, and Cotton effects in the ORD. spectra. This stable ring system with an endocyclic enol‐acetal linkage is present in a half‐chair ( H ) conformation. The structure of the unsaturated deoxyhexopyranosiduronate obtained was established by structure‐ and stereo‐correlation with a 2‐deoxy‐ L ‐xylose derivative, showing that a ring contraction during the β‐elimination does not occur.