STUDIES ON THE INTRAMOLECULAR CYCLOADDITION REACTION OF ISOMUNCHNONESDERIVED FROM N-ALKENYL SUBSTITUTED DIAZOIMIDES

A series of N-alkenyl substituted diazoimides were prepared and subjec ted to Rh(II) catalyzed decomposition, The initially generated rhodium carbenoid undergoes ready cyclization onto the neighboring amide carb onyl oxygen atom to generate an isomunchnone intermediate. Subsequent 1,3-dipolar cycloaddition across the pendant olefin affords the cycloa dduct in high yield. The stereochemical assignment of several of the c ycloadducts was deduced by X-ray crystallography. The stereochemical o utcome of the reaction is the consequence of an exo-cycloaddition of t he neighboring pi-bond across the transient isomunchnone dipole, Molec ular mechanics calculations were used to model energy differences betw een the exo and endo diastereomers. The calculations reveal that the e xo diastereomers are significantly (18-20 kcal/mol) lower in energy th an the corresponding endo isomers thereby providing a rationale for th e preferred exo-cycloaddition,