Simple and Highly Efficient Synthesis of Indolo- and Pyrrolo[1,2-a]quinoxalines Promoted by Molecular Iodine

Abstract

A simple and highly efficient strategy is developed for the synthesis of indolo- and pyrrolo[1,2-a]quinoxalines from the corresponding 2-(1H-indol/pyrrol-1-yl)anilines promoted by molecular iodine in good to excellent yields.

Key words

nitrogen heterocycles - iodine - oxidation - cyclization - qunoxalines

• References and Notes

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• 24 Synthesis of Indolo- and Pyrrolo[1,2-a]quinoxalines; General Procedure: To a stirred solution of benzylamine (338 mg, 3.16 mmol) in MeCN (10 mL) was added iodine (800 mg, 3.16 mmol) followed by 1-(2-aminophenyl)pyrrole (250 mg, 1.58 mmol) at room temperature. The reaction mixture was heated to 80 °C and stirred for 5–8 h, with the progress of the reaction being monitored by TLC. Upon completion, the reaction mixture was allowed to cool to room temperature and excess iodine was quenched by the addition of sat. aq Na₂S₂O₃. The mixture was extracted with ethyl acetate (10 mL), and the organic layer was dried over anhydrous Na₂SO₄, filtered, and the solvent was removed to give the crude product, which was further purified by silica-gel column chromatography (hexane–EtOAc, 4:1) to afford the desired product. 4-Phenylpyrrolo[1,2-a]quinoxaline (3a): Yield: 347 mg (90%); pale-yellow solid; mp 117–119 °C; IR (KBr): 3435, 3064, 2918, 1602, 1532, 1522, 1475, 1445, 1416, 1370, 1322, 1251, 1167, 1096, 1073, 912, 933, 752, 710, 688 cm^–1; ¹H NMR (400 MHz, CDCl₃): δ = 6.87 (t, J = 3.0 Hz, 1 H), 6.98 (d, J = 3.2 Hz, 1 H), 7.55–7.42 (m, 5 H), 7.86 (d, J = 8.0 Hz, 1 H), 8.04–7.97 (m, 4 H); ¹³C NMR (100 MHz, CDCl₃): δ = 108.6, 113.6, 114.0, 114.5, 125.2, 125.4, 127.1, 127.4, 128.5, 128.6, 129.7, 130.2, 136.2, 138.4, 154.4; MS: m/z = 245.2 [M + H]; HRMS (ESI): m/z [M + H] calcd. for C₁₇H₁₃N₂: 245.1079; found: 245.1079.