A Simple and Efficient Synthesis of Dibenzothiophene via BF₃·OEt₂-Promoted Intramolecular Annulation

Abstract

A simple and efficient protocol promoted by BF₃·OEt₂ for synthesizing functionalized dibenzothiophenes (DBTs) has been demonstrated. The annulation condition can tolerate a variety of functional groups.

Key words

dibenzothiophenes - cyclization - Lewis acid - triazene

• References and Notes

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• 20 Representative Procedure for the Synthesis of 3a: BF₃·OEt₂ (2.0 equiv) was added dropwise to the stirred solution of 2a (1 mmol) in anhyd MeCN at 45 °C. The mixture was stirred for 4 h. After the completion of the reaction detected by TLC, it was washed with sat. aq NaHCO₃ and extracted with EtOAc. The combined organic extracts were dried over anhyd Na₂SO₄ and removed under reduced pressure. The product was purified by column chromatography to give 3a as a white solid (80% yield); mp 68–71 °C. ¹H NMR (400 MHz, CDCl₃): δ = 8.84 (s, 1 H), 8.22–8.26 (m, 1 H), 8.11–8.14 (m, 1 H), 7.84–7.90 (m, 2 H), 7.48–7.51 (m, 2 H), 4.46 (q, J = 6.8 Hz, 2 H), 1.47 (t, J = 7.2 Hz, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 166.7, 144.2, 139.6, 135.5, 135.2, 127.3, 126.8, 124.8, 123.1, 122.8, 122.5, 121.9, 61.1, 14.4. IR (thin film): 3065, 2924, 1710, 1267, 1100, 1025 cm^–1. MS (EI): m/z (%) = 256 (97) [M⁺], 241 (14), 228 (39), 211 (100), 183 (58), 139 (54). HRMS (EI–TOF): m/z [M⁺] calcd for C₁₅H₁₂O₂S: 256.0558; found: 256.0566.