Studies on the Intramolecular Cycloaddition Reaction of Mesoionics Derived from the Rhodium(II)-Catalyzed Cyclization of Diazoimides

Article abstract of DOI:10.1021/jo00085a035

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 deg 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 isomuenchnone 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 ?-bond across the transient isomuenchnone 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 occured 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.