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Synlett 2009(12): 1985-1989  
DOI: 10.1055/s-0029-1217543
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Synthesis of 3-Allenylindoles and 3-Dienylindoles by Brønsted Acid Catalyzed Allenylation of 2-Arylindoles with Tertiary Propargylic Alcohols

Roberto Sanz*a, Mukut Gohaina, Delia Miguela, Alberto Martíneza, Félix Rodríguezb
a Departamento de Química, Área de Química Orgánica, Facultad de Ciencias, Universidad de Burgos, Pza. Misael Bañuelos s/n, 09001-Burgos, Spain
Fax: +34(947)258831; e-Mail: rsd@ubu.es;
b Instituto Universitario de Química Organometálica ‘Enrique Moles’, Universidad de Oviedo, C/ Julián Clavería 8, 33006-Oviedo, Spain
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Publication History

Received 5 March 2009
Publication Date:
03 July 2009 (online)

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Abstract

The Brønsted acid catalyzed direct nucleophilic substitution of tertiary propargylic alcohols with 2-aryl-substituted indoles has been studied. A competitive allenylation process takes place with appropriate substituents on the alkynol moiety. Starting from 2-arylindoles, new 3-allenyl and 3-dienylindole derivatives have been easily synthesized.

Key words

catalysis - indoles - allenes - dienes - nucleophilic substitution

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13

We have observed that the presence of an alkyl substituent at the C-2 of the indole nucleus does not give rise to significant competitive allenylation processes.

14

The 1Z,3E-isomer was mainly formed. Variable amounts of the 1Z,3Z-isomer were detected in the crude product. After column chromatography we usually isolated the major isomer slightly contaminated with the minor one, probably due to further isomerization under the purification conditions. The stereochemistry was assessed by NOESY experiments on 4cb, and the rest of the compounds were assigned by inference. Moreover, the structure of 4af was confirmed by single-crystal X-ray diffraction analysis (CCDC 729740 contains the supplementary crystallographic data for this compound. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif).

16

We have checked that the presence of alkyl or aryl groups at the nitrogen atom of the starting 2-phenylindole derivative mainly gives rise to propargylated products regardless the substituents of the alkynol.