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
Date
2015
Authors
Willis, Anthony C.
Banwell, Martin G.
White, Lorenzo V.
Journal Title
Journal ISSN
Volume Title
Publisher
Japan Institute of Heterocyclic Chemistry
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 "-thiolactam 17 that incorporates the ABCE ring substructure of natural products 1-3. 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.
Description
Keywords
tetrahydrocarbazole 13, isothiocyanate 16, Photolysis, ABCE ring substructure, two-carbon homologation, azide 20, compound 13
Citation
Collections
Source
HETEROCYCLES
Type
Journal article