2,5‐Dioxabicyclo[2.2.2]octan‐Derivate: Konformation von Methyl‐2,6‐anhydro‐α‐ D ‐hexopyranosiden

2,5‐Dioxabicyclo[2.2.2]octane Derivatives: Conformation of Methyl 2,6‐Anhydro‐α‐ D ‐hexopyranosides Ring opening of methyl 2,6:3,4‐dianhydro‐α‐ D ‐altropyranoside ( 5 ) with KOH in water at 150°C yields almost exclusively the hitherto unknown methyl 2,6‐anhydro‐α‐ D ‐mannopyranoside ( 6 ). 1 H NMR spectra of this compound and of all other α‐anomers of this series suggest a conformation of the pyranose ring intermediate between the boat 2,5 B( D ) and the twist boat 5 S 1 ( D ). This was verified by X‐ray analysis of methyl 3,4‐di‐ O ‐acetyl‐2,6‐anhydro‐α‐ D ‐altropyranoside ( 2 ).