Pd-Catalyzed Intramolecular Cyclization–Thiocarbonylation Cascade Using Thioesters

Abstract

A Pd-catalyzed intramolecular cyclization–thiocarbonylation cascade using thioesters is described. The developed cascade reaction afforded chromane, coumaran, indoline, and oxindole derivatives with a chiral quaternary carbon atom at the benzylic position in high to excellent yields. Relative to those observed in the previously reported relevant cascade using TIPSSPh, the yields with thioesters are almost the same or higher, depending on the substrate. Moreover, the use of thioesters significantly reduces the reaction time to less than one hour. Therefore, AcSPh is advantageous over TIPSSPh in terms of reaction time, atom economy, and cost effectiveness.

Key words

cascade reactions - cyclization - palladium - thiocarbonylation - thioesters

• References and Notes


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• 14 Synthesis of compound 2: A 10 mL test tube was charged with 1 (24.6 mg, 0.0854 mmol, 1.0 equiv), AcSPh (0.0174 mL, 0.128 mmol, 1.5 equiv), Pd(PPh₃)₄ (9.9 mg, 0.0086 mmol, 0.1 equiv), Cs₂CO₃ (41.7 mg, 0.128 mmol, 1.5 equiv), and anhydrous toluene (1.7 mL, 0.05 mol/L). The reaction mixture was well degassed and then stirred at 100 °C under a CO atmosphere. After an hour, 1 M aq. HCl (1.7 mL) was added to the reaction mixture at room temperature and the aqueous layer was extracted with ethyl acetate (1.7 mL × 3). The combined organic layer was dried over Na₂SO₄, filtered, and concentrated under reduced pressure. The residue was purified by flash silica gel column chromatography (hexane/ethyl acetate, 200:1) to afford 2 (23.7 mg, 93%) as a colorless oil: R _f = 0.56 (hexane/ethyl acetate, 4/1). ¹H NMR (400 MHz, CDCl₃): δ = 7.39–7.45 (m, 3 H), 7.34–7.39 (m, 2 H), 7.25 (dd, J = 7.7, 1.6 Hz, 1 H), 7.09–7.15 (m, 1 H), 6.89–6.94 (m, 1 H), 6.82 (dd, J = 8.2, 1.1 Hz, 1 H), 4.15–4.30 (m, 2 H), 3.03 (d, J = 14.6 Hz, 1 H), 2.99 (d, J = 14.6 Hz, 1 H), 2.29–2.36 (m, 1 H), 1.89–1.97 (m, 1 H), 1.52 (s, 3 H). ¹³C NMR (125 MHz, CDCl₃): δ = 195.2, 153.8, 134.4, 129.5, 129.3, 129.2, 127.9, 127.8, 126.8, 120.6, 117.3, 62.7, 54.6, 34.2, 34.1, 28.4. HRMS (ESI): m/z [M + Na]⁺ calcd for C₁₈H₁₈NaO₂S: 321.0925; found: 321.0919.