A Novel Cycloaddition Reaction of α-Diazo-γ-amido Ketones Catalyzed by Rhodium(II) Acetate. Scope and Mechanistic Details of the Process

Abstract

α-Diazo ketones containing an amido group in the γ-position have been found to undergo a novel rhodium(II)-catalyzed cycloaddition reaction. Intramolecular cyclization of the keto carbenoid onto the oxygen atom of the amide group generates a carbonyl ylide dipole as a transient species. This highly stabilized dipole does not readily undergo 1,3-dipolar cycloaddition but rather transfers a proton to produce a cyclic ketene N,O-acetal. The ketene acetal is unstable to moisture and upon standing is readily hydrolyzed to a γ-keto δ-lactone and an amine. In the absence of any significant amount of water, the ketene N,O-acetal undergoes conjugate addition with the activated π-bond of the dipolarophile to give a zwitterion intermediate. The anionic portion of the zwitterion adds to the neighboring carbonyl group. This is followed by epoxide ring formation with charge dissipation leading to an amido-substituted spiro cyclopentyl epoxide. In certain cases a hydroxy lactone was also isolated and its formation can be attributed to the competitive hydrolysis of the zwitterionic intermediate. The Rh(II)-catalyzed reaction of the diazo ketoamide derived from N-benzylpiperidone with DMAD afforded two different types of cycloadducts. In addition to the spiro cyclopentyl epoxide, a product derived from trapping of the carbonyl ylide dipole was also obtained, thereby providing additional support for the proposed mechanism.