The acid-catalyzed oxido-reduction of spiroketals. Evidence for stereoelectronic control in hydride transfer to cyclic oxenium ions

Abstract

Tricyclic spiroketal 1 undergoes an acid-catalyzed oxidation–reduction reaction which yields equatorial bicyclic ether aldehyde 5 specifically. Similarly, spiroketals 2, 3, and 4 give equatorial bicyclic ether ketone 12. These results are interpreted by invoking an internal hydride transfer from an alcohol function to a cyclic oxenium ion which takes place with stereoelectronic control. The reduction of tricyclic ketals 1 and 22 with sodium cyanoborohydride under acidic conditions is also reported.