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Synlett 2010(11): 1674-1678  
DOI: 10.1055/s-0030-1258081
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Efficient Synthesis of Phenanthridines Using Hendrickson Reagent Initiated Cascade Reaction under Mild Conditions

Jie Xia,b, Qing-Li Donga, Guan-Sai Liua, Shaozhong Wanga, Lin Chen*b, Zhu-Jun Yao*a
a Nanjing National Laboratory of Microstructures and State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, 22 Hankou Road, Nanjing, Jiangsu 210093, P. R. of China
Fax: +86(25)83593732; e-Mail: yaoz@nju.edu.cn;
b School of Chemistry and Biotechnology, Yunnan Nationalities University, Kunming, Yunnan 650031, P. R. of China
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Publication History

Received 16 April 2010
Publication Date:
10 June 2010 (online)

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Abstract

A number of variously substituted phenanthridines have been synthesized using the newly developed methodology under mild conditions. The Hendrickson reagent initiated cascade annulation, which is composed of a mild conversion of stable amide precursor to highly reactive imido-carbonium intermediate and a subsequent intramolecular Friedel-Crafts reaction, successfully served as the key method.

Key words

phenanthridine - cascade reaction - Hendrickson reagent - carbocation - Friedel-Crafts reaction

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Representative Procedure for the Synthesis of 6-Methyl-phenanthridine (2a) To a solution of Ph3PO (260 mg, 0.9 mmol, 3 equiv) in anhyd CH2Cl2 (5 mL) was added Tf2O (0.078 mL, 0.45 mmol, 1.5 equiv) dropwise under nitrogen atmosphere at 0 ˚C. After 15 min, amide 1a (64 mg, 0.3 mmol, 1 equiv) in anhyd CH2Cl2 (2 mL) was added. The reaction was then warmed to r.t. and stirred until completion. The reaction was quenched by addition of sat. aq NaHCO3. The mixture was extracted with CH2Cl2 (3 × 10 mL). The combined extracts were washed with brine, dried over anhyd Na2SO4, and concentrated. The crude product was purified by column chromatography on silica gel using a mixture of PE and EtOAc (5:1 to 2:1, v/v) as the eluent to afford a light-yellow solid 2a (51 mg, 95%).4n ¹H NMR (300 MHz, CDCl3): δ = 8.64 (1 H, d, J = 8.4 Hz), 8.55 (1 H, d, J = 7.8 Hz), 8.24 (1 H, d, J = 7.8 Hz), 8.11 (1 H, d, J = 7.8 Hz), 7.88-7.61 (4 H, m), 2.98 (3 H, s). MS (EI): m/z = 193 [M+].