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Synthesis 2008(18): 2943-2952  
DOI: 10.1055/s-2008-1067248
PAPER
© Georg Thieme Verlag Stuttgart ˙ New York

Recyclization of (2-Aminophenyl)bis(5-tert-butyl-2-furyl)methanes into Indole Derivatives: Unusual Dependence on Substituent at Nitrogen Atom

Alexander V. Butin*a, Sergey K. Smirnova, Fatima A. Tsiunchika, Maxim G. Uchuskina, Igor V. Trushkovb
a Research Institute of Heterocyclic Compounds Chemistry, Kuban State University of Technology, Moskovskaya st. 2, Krasnodar 350072, Russian Federation
Fax: +7(861)2596592; e-Mail: alexander_butin@mail.ru; av_butin@yahoo.com;
b Department of Chemistry, M.V. Lomonosov Moscow State University, Leninskie Gory 1/3, Moscow 119991, Russian Federation
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Publication History

Received 20 May 2008
Publication Date:
04 September 2008 (online)

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Abstract

The recyclization of (2-aminophenyl)bis(5-tert-butyl-2-furyl)methanes under acidic conditions was studied. It was found that the extent of reaction of these substrates depends on the substituent at the nitrogen atom of the aniline moiety. N-Tosyl derivatives were converted into the corresponding 3-(5-tert-butyl-2-furyl)-2-(4,4-dimethyl-3-oxopentyl)-1-tosyl-1H-indoles. Indole formation was followed by furan ring opening in the case of N-unsubstituted substrates leading to 3-(5,5-dimethyl-1,4-dioxohexyl)-2-(4,4-dimethyl-3-oxopentyl)-1H-indoles. The same products were obtained from N-acetyl derivatives. However, the behavior of the N-benzoyl analogues depends on the reaction conditions: at room temperature 1-benzoyl-3-(5-tert-butyl-2-furyl)-1H-indoles were formed, but debenzoylation and furan ring opening proceed with heating. These data and results of control experiments showed that the reaction mechanism consists of three successive steps: recyclization itself, deacylation of the resulting N-acylindole, and furan ring opening in N-unsubstituted 3-(2-furyl)indoles. The last step can be realized for N-unsubstituted indoles only, but the furan ring is stable for N-acylindoles. This was explained by transformation of N-unsubstituted 1H-indoles under the reaction conditions into 3H-indole tautomers. These tautomers can be considered as 2,5-dialkylfurans, which have much lower stability against acids than 2-aryl-5-alkylfurans. This tautomerization is impossible for N-acylindoles. The high acidic stability of 2-(5-tert-butyl-2-furyl)indoles supports this conclusion.

Key words

furans - ring opening - ring closure - fused ring system - indole - triketoindole

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