Structural requirements for the thallium(III)-mediated cyclisation of unsaturated alcohols. A novel fragmentation reaction producing 19-norsteroids

Abstract
The unsaturated alcohol (1) is readily cyclised to the hydroxytetrahydrofuran (3) by means of thallium(III), whereas its congener (4) has been found to give (7) as the product of a novel, stereoelectronically controlled, fragmentation; the scope of the tandem electrophilic cyclisation/solvolysis is discussed; the structure of (7) has been determined by X-ray crystallography.