Formal Halo-Meyer-Schuster Rearrangement of Propargylic Acetates through a Novel Intermediate and an Unexampled Mechanistic Pathway.

A formal, highly stereoselective halo-Meyer-Schuster rearrangement of inactivated propargylic acetates to (Z)-α-haloenones has been reported, under metal free conditions. This cascade process involves a new class of intermediate, i.e., α,α-dihalo-β-acetoxyketones and mechanism to generate the α-haloenones, employing water as Lewis base. The outcome of the reaction is temperature-dependent, as room temperature, selectively provides α,α-dihalo-β-acetoxyketones whereas reactions at 100 °C give direct access to α-haloenones. Either type of product can be obtained in excellent yield. A suitable rationale for the observed high Z-selectivity for α-haloenones (based on conformational population) and distinct reaction rates for various N-halosuccinimide (NXS) reagents (based on C-X bond strengths) has also been provided.