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[摘要]：The recyclization of (2-aminophenyl)bis(5-tert-butyl-2-furyl)methanes under acidic conditions was studied. The extent of reaction of these substrates depends on the substituent at the N atom of the aniline moiety. N-Tosyl derivs. 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 derivs. However, the behavior of the N-benzoyl analogs depends on the reaction conditions. At room temp. 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 expts. showed that the reaction mechanism consists of 3 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. |
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