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Paper | Special issue | Vol 90, No. 1, 2015, pp.298-315
Published online, 18th April, 2014
DOI: 10.3987/COM-14-S(K)19
ABC → ABCE/D Based Approaches to the Pentacyclic Ring System of the Vinca Alkaloids Using Intramolecular Hetero-[2+2]cycloaddition and Gold(I)-Catalyzed 6-endo-dig Cyclisation Protocols

Lorenzo V. White, Martin G. Banwell,* and Anthony C. Willis

*Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia

Abstract

The angularly substituted tetrahydrocarbazole 13, which is readily obtained from cyclohexane-1,4-dione monoethylene ketal (6) using Fischer indole chemistry, has been converted into the isothiocyanate 16. Photolysis of this last compound affords, via an intramolecular hetero-[2+2]cycloaddition reaction, the pentacyclic β-thiolactam 17 that incorporates the ABCE ring substructure of natural products 13. Attempts to effect a two-carbon homologation of the four-membered ring within compound 17, and thereby establish the D-ring, failed. The azide 20, also obtained from compound 13, forms the cyclic imine 21 on thermolysis in refluxing toluene and the readily derived enamide 23 engages in a Au(I)-catalysed 6-endo-dig cyclisation reaction to give compound 24 embodying the ABCDE ring system of the title alkaloids.