STUDIES ON THE INTRAMOLECULAR CYCLOADDITION REACTION OF MESOIONICS DERIVED FROM THE RHODIUM(II)-CATALYZED CYCLIZATION OF DIAZOIMIDES

A series of alkenyl-substituted imides were prepared by treating the appropriate amide with 2,2,6-trimethyl-4H-1,3-dioxen-4-one in xylene at 140-degrees-C to give the N-acetoacylated imides. Exposure of these imides to standard diazo transfer conditions gave the desired diazoimides. The carbenoid intermediate derived by treatment of the diazoimide with rhodium(II) acetate undergoes ready cyclization onto the neighboring amide carbonyl oxygen atom to generate an isomunchnone intermediate. Subsequent 1,3-dipolar cycloaddition across the pendant olefin affords the cycloadduct in high yield. The stereochemical assignment of several of the cycloadducts was deduced by X-ray crystallography. The stereochemical outcome of the reaction is the consequence of an endo cycloaddition of the neighboring pi-bond across the transient isomunchnone dipole. Molecular mechanics calculations were used to model energy differences between the endo and exo diastereomers. The calculations reveal that the endo diastereomers are significantly (8-12 kcal/mol) lower in energy than the corresponding exo isomers thereby providing a rationale for the preferred endo cycloaddition. Ring opening of the cycloadducts occurred readily producing a transient N-acyliminium ion which either lost a proton to give an enamide or was reduced by Et3SiH to afford a bicyclic piperidine ring.