Cycloaddition chemistry of 2-vinyl-substituted indoles and related heteroaromatic systems.J Org Chem. 2005 Mar 18; 70(6):2206-18.JO
[reaction: see text] The intramolecular Diels-Alder cycloaddition reaction (IMDAF) of several N-phenylsulfonylindolyl-substituted furanyl carbamates containing a tethered pi-bond on the indole ring were examined as an approach to the iboga alkaloid catharanthine. Only in the case where the tethered pi-bond contained two carbomethoxy groups did the [4 + 2]-cycloaddition occur. Push-pull dipoles generated from the Rh(II)-catalyzed reaction of diazo imides, on the other hand, undergo successful intramolecular 1,3-dipolar cycloaddition across both alkenyl and heteroaromatic pi-bonds to provide novel pentacyclic compounds in good yield and in a stereocontrolled fashion. The facility of the cycloaddition was found to be critically dependent on conformational factors in the transition state. Ligand substitution in the rhodium(II) catalyst markedly altered the product ratio between [3 + 2]-cycloaddition and intramolecular C-H insertion. The variation in reactivity reflects the difference in electrophilicity between the various rhodium carbenoid intermediates. Intramolecular C-H insertion is enhanced with the more electrophilic carbene generated using Rh(II) perfluorobutyrate.