An efficient two-step synthesis of 3-allylindoles
Graphical abstract
A two-step synthetic sequence for an efficient synthesis of 3-allylindoles is described.
Section snippets
Introduction and discussion
Several methods have been developed for the synthesis of indoles and substituted indoles as they are a key feature of many natural products and pharmaceutical compounds.1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 However, there is still a need to develop newer methods for their synthesis. Alkylation of indoles or salts of indoles under drastic conditions yields a complex mixture of 1-substituted, 1,3-disubstituted, 3-substituted indoles21, 22, 23, 24 and/or indolenines.
Experimental procedure for Claisen rearrangement/ring closure (3a)
To a stirred suspension of ferrous sulfate (16 g, 12 mmol) in diglyme–water (1:1, 5:5 ml), the allyl vinyl ether 2a (mixture of E and Z isomers) (1 g, 1 mmol) was added. After refluxing the mixture for 15–20 min (TLC check), excess ammonia was added. The resulting solution was refluxed for 24 h (TLC check). The reaction mixture as such was purified by silica gel column chromatography using the hexane–ethyl acetate (98:2) solvent system.
Acknowledgements
One of the authors S.D. would like to thank DST, New Delhi, for funding this project and for granting her a Fellowship as a Young Scientist. The authors A.D., D.G., A.B. and S.C. would like to thank CSIR, New Delhi, for Fellowships.
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