Reversible Charge-Accelerated Oxy-Cope Rearrangements

Abstract

An asymmetric synthesis of the oxetane-containing norbornanone 23 and its coupling to trans-1-propenyllithium to give 24 are reported, in tandem with the preparation of the related alcohols 28 and 30. All three divinyl carbinols undergo anionic oxy-Cope rearrangement very rapidly at low temperature. Quenching of 24-K+ and 28-K+ under these conditions with water or various aqueous salt solutions results in protonation of the alkoxides. If these reaction mixtures are poured instead onto cold (0 °C) silica gel, their sigmatropically related ketones are isolated in very good yield. Whereas the 24-K+ ⇄ 25-K+ equilibrium pair is not reactive to molecular oxygen, 30-K+ is directly converted into an α-hydroperoxy ketone under comparable conditions. These and additional observations are rationalized in the context of atropisomerism involving conversion of oxygen-up enolates, formed reversibly under kinetically controlled conditions, into their thermodynamically favored, more reactive oxygen-down conformers.