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Synlett 2003(11): 1707-1709
DOI: 10.1055/s-2003-40989
LETTER
© Georg ThiemeVerlag Stuttgart ˙ New York

Cation Radical Imino Diels-AlderReaction: A New Approach for the Synthesis of Tetrahydroquinolines

Xiaodong Jia, Hechun Lin, Congde Huo, Wei Zhang, Jianming Lü, Li Yang, Guangyu Zhao, Zhong-Li Liu*
National Laboratory of Applied OrganicChemistry, Lanzhou University, Lanzhou,Gansu 730000, China
Fax: +86(931)8625657; e-Mail: liuzl@lzu.edu.cn;
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Publication History

Received 6 June 2003
Publication Date:
05 August 2003 (online)

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Abstract

Cycloaddition of N-arylimines with α-methylstyrenesor 2,3-dihydrofuran was efficiently catalyzed by tris(4-bromophenyl)aminiumhexachloroantimonate (Ar3N + ˙SbCl6 - ) producing tetrahydroquinolinederivatives in excellent yields. The reaction was controlled sensitivelyby the oxidation potentials of the imine and the dienophile.

Key words

cation radical - imines - tetrahydroquinolines - cycloaddition

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RepresentativeSpectral Data of the Products. syn-3b: Yellow needles, mp 174-176 °C(uncor.). HR-ESI-MS: 379.1211 (calcd for C22H19N2O2Cl + H+:379.1208). 1H NMR (400 MHz, CDCl3): δ = 1.63(s, 3 H, CH3), 1.98 (dd, J = 2.7,13.4 Hz, 1 H, H-3e), 2.24 (dd, J = 11.4,13.4 Hz, 1 H, H-3a), 4.74 (dd, J = 2.7,11.4Hz, 1 H, H-2), 6.61 (d, J = 8.5Hz, 1 H, H-8), 6.67 (d, J = 2.3Hz, 1 H, H-5), 7.0 (dd, J = 2.3,8.5 Hz, 1 H, H-7), 7.22-7.31 (m, 5 H, Ph), 7.62 (d, 2 H, J = 8.7 Hz,Ar), 8.19 (d, 2 H, J = 9.19,Ar). 13C NMR (100.08 MHz, CDCl3): δ = 29.6(CH3), 42.3 (C-4), 48.9 (CH2), 53.8 (CH),116.5 (C-8), 123.8 (C-6), 126.4 (C-5), 127.1 (2 C, Ph), 127.6 (1C, Ph), 128.0 (2 C, Ph), 128.3 (C-7), 129.3 (2 C, Ph), 129.7 (2C, Ph), 132.0 (1 C, Ph), 142.0 (1 C, Ph), 147.5 (1 C, Ph), 148.4(C-10), 150.6 (C-9). anti -3b:Yellow needles, mp 152-153 °C (uncor.).HR-ESI-MS: 379.1210 (calcd. for C22H19N2O2Cl + H+:379.1208). 1H NMR (400 MHz, CDCl3): δ = 1.76(s, 3 H, CH3), 2.08 (dd, J = 12.0,13.1 Hz, 1 H, H-3a), 2.25 (dd, J = 3.0, 13.1, 1 H, H-3e),4.13 (dd, J = 3.0,12.0 Hz, 1 H, H-2), 6.62 (d, J = 8.5 Hz,H-8), 6.81 (dd, J = 7.0,7.0 Hz, H-6), 7.18 (d, J = 2.2Hz, 1 H, H-5), 7.19-7.35 (m, 5 H, Ph), 7.46 (d, J = 7.2 Hz,2 H, Ar), 8.16 (d, J = 7.2Hz, 2 H, Ar). 13C NMR (100.08 MHz, CDCl3): δ = 29.3(CH3), 41.7 (C-4), 47.5 (CH2), 53.9 (CH), 116.1(C-8), 122.4 (C-6), 123.7 (C-5), 126.3 (2 C, Ph), 126.6 (2 C, Ph),127.5 (2 C, Ph), 127.8 (2 C, Ph), 128.1 (C-7), 128.5 (2 C, Ph),142.1 (1 C, Ph), 147.5 (1 C, Ph), 144.6 (C-10), 150.4 (C-9). Thecoupling constants of H-2 suggest its axial conformation in both syn- and anti-3b. The significant low-field shift ofH-3e and up-field shift of H-2 of anti-3b in comparison with those of syn-3b suggestsan axial 4-phenyl group in anti-3b while an equatorial 4-phenyl group in syn-3b. Thestereochemistry was confirmed by their NOESY spectra which showa clear cross peak between the 4-methyl and H-2 in syn-3b, while no such correlation in anti-3b.

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The oxidation peak potential was determinedvs. SCE by cyclic voltammetry in MeCN using a glassy carbon electrode.

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The structures of the products wereidentified by comparing their 1H and 13CNMR data with those reported in the literature. See ref. [4]