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 a ppropriate 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 i mides to standard diazo transfer conditions gave the desired diazoimid es. The carbenoid intermediate derived by treatment of the diazoimide with rhodium(II) acetate undergoes ready cyclization onto the neighbor ing amide carbonyl oxygen atom to generate an isomunchnone intermediat e. Subsequent 1,3-dipolar cycloaddition across the pendant olefin affo rds the cycloadduct in high yield. The stereochemical assignment of se veral of the cycloadducts was deduced by X-ray crystallography. The st ereochemical outcome of the reaction is the consequence of an endo cyc loaddition of the neighboring pi-bond across the transient isomunchnon e dipole. Molecular mechanics calculations were used to model energy d ifferences between the endo and exo diastereomers. The calculations re veal that the endo diastereomers are significantly (8-12 kcal/mol) low er in energy than the corresponding exo isomers thereby providing a ra tionale for the preferred endo cycloaddition. Ring opening of the cycl oadducts occurred readily producing a transient N-acyliminium ion whic h either lost a proton to give an enamide or was reduced by Et3SiH to afford a bicyclic piperidine ring.