Studies on the intramolecular cycloaddition reaction of isomunchnones derived from N-alkenyl substituted diazoimides

A series of N-alkenyl substituted diazoimides were prepared and subjected to Rh(II) catalyzed decomposition, The initially generated rhodium carbenoid 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 exo-cycloaddition of the neighboring pi-bond across the transient isomunchnone dipole, Molecular mechanics calculations were used to model energy differences between the exo and endo diastereomers. The calculations reveal that the exo diastereomers are significantly (18-20 kcal/mol) lower in energy than the corresponding endo isomers thereby providing a rationale for the preferred exo-cycloaddition,