INTRAMOLECULAR CYCLOADDITION OF ISOMUNCHNONE DIPOLES TO HETEROAROMATIC PI-SYSTEMS

A series of furanyl-, thienyl-, and indole-substituted diazo imides we re prepared by treating the appropriate amides with diketene to give t he N-acetoacylated imides. Exposure of the imides to standard diazo tr ansfer conditions afforded the desired diazo imides. Treatment of thes e diazo imides bearing tethered heterocyclic rings with rhodium(II) ac etate affords transient isomunchnone dipoles. The mesoionic dipoles ar e formed by cyclization of the rhodium carbenoid onto the neighboring amide carbonyl oxygen atom. The scope and limitations of the intramole cular 1,3-dipolar cycloaddition of the isomunchnones across a tethered furan and thiophene ring were studied. The facility of the internal c ycloaddition is influenced by the length and nature of the tether conn ecting the dipole and dipolarophile functionalities. The reaction is c ritically dependent on conformational factors in the transition state. In addition, the first examples of intramolecular cycloaddition of is omunchnones to indole dipolarophiles are reported. Cycloadditions of t his type generate highly functionalized polyheterocyclic systems with complete relative stereocontrol at the newly formed stereocenters.