Cyclization-cycloaddition cascade of rhodium carbenoids using different carbonyl groups. Highlighting the position of interaction

A series of 3-diazoalkanediones, when treated with a catalytic quantity of a rhodium(II) carboxylate, were found to afford oxabicyclic dipolar cycload ducts derived by the trapping of a carbonyl ylide intermediate. The reactio n involves generation of the 1,3-dipole by intramolecular cyclization of th e keto carbenoid onto the oxygen atom of the neighboring keto group. Both f ive- and six-ring carbonyl ylides are formed with the same efficiency. A st udy of the tandem cyclization - cycloaddition cascade of several a-diazo ke toesters was also carried out, and the cascade sequence proceeded in high y ield. When the interacting keto carbonyl group was replaced by an imido gro up, the rhodium(II)-catalyzed reaction proceeded uneventfully. In contrast, alpha-diazo amidoesters do not undergo nitrogen extrusion on treatment wit h a Rh(II) catalyst. Instead, the; diazo portion of the molecule undergoes 1,3-dipolar cycloaddition with various dipolarophiles to give substituted p yrazoles as the final products.