Intramolecular Dipolar Cycloaddition Reactions to Give Substituted Indoles – A Formal Synthesis of Deethylibophyllidine

Abstract

AbstractA four‐step route to the tetracyclic ring system found in a number of indole alkaloids is described using stereoselective dipolar cycloaddition reactions of azomethine ylides. The ylides were derived from N‐protected 2‐butenylindole‐3‐carbaldehydes, which were prepared in three steps from 2‐methylindole. Condensation of these aldehydes with a variety of alkylamino esters or amino acids or with N‐methylhydroxylamine gave the required azomethine ylides or nitrone that undergo intramolecular cycloaddition onto the pendant, unactivated alkene. The cycloaddition sets up two new rings and up to three new stereocentres stereoselectively. After cycloaddition with N‐allylglycine, N‐deallylation provided the N‐unsubstituted product and this constitutes a formal synthesis of the alkaloid deethylibophyllidine. The cycloaddition chemistry was also shown to be amenable as a route to the alkaloid ibophyllidine or epiibophyllidine, although the use of 2‐(allylamino)butyric acid to prepare the azomethine ylide can favour competing iminium ion isomerisation and hydrolysis.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)