Synlett 2017; 28(13): 1530-1543
DOI: 10.1055/s-0036-1589494
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© Georg Thieme Verlag Stuttgart · New York

Enantioselective Approaches to 3,4-Annulated Indoles Using Organocatalytic Domino Reactions

Lorenzo Caruana
,
Mariafrancesca Fochi*
Department of Industrial Chemistry “Toso Montanari” & INSTM RU Bologna, Alma Mater Studiorum - University of Bologna, V. Risorgimento 4, 40136 Bologna, Italy   Email: mariafrancesca.fochi@unibo.it   Email: luca.bernardi2@unibo.it
,
Department of Industrial Chemistry “Toso Montanari” & INSTM RU Bologna, Alma Mater Studiorum - University of Bologna, V. Risorgimento 4, 40136 Bologna, Italy   Email: mariafrancesca.fochi@unibo.it   Email: luca.bernardi2@unibo.it
› Author Affiliations
Further Information

Publication History

Received: 20 February 2017

Accepted after revision: 20 March 2017

Publication Date:
19 April 2017 (online)


Abstract

Organocatalytic domino reactions of 4-substituted indoles are summarized in this account. Two reactions have been developed, one with enals, activated by secondary amine catalysts via iminium ions, and one with nitroethene, using a phosphoric acid catalyst. Both reactions required solving the challenge posed by the very low nucleo­philicity of the indole substrates, which bear an electron-withdrawing Michael acceptor at C4. DFT calculations were used to shed light on the unique reaction pathway followed by the phosphoric acid catalyzed transformation, wherein a bicoordinated nitronic acid intermediate was found to evolve preferentially through an intramolecular nitro-Michael reaction, instead of the common tautomerization pathway. These reactions provide new and efficient entries to 3,4-ring-fused indoles in dia­stereo- and enantioenriched form. In more detail, the structures obtained feature a 1,3,4,5-tetrahydrobenzo[cd]indole core, which is present in the structural framework of ergot alkaloids. Indeed, the preparation of an intermediate previously used in ergot alkaloid (6,7-secoagroclavine) synthesis was possible from one of the catalytic adducts.

1 Introduction

2 Reactions of 4-Substituted Indoles with α,β-Unsaturated Aldehydes Catalyzed by Secondary Amines

3 Reactions of 4-Substituted Indoles with Nitroethene Catalyzed by Brønsted Acids

4 Conclusion