Photoinduced Synthesis of Benzannulated Spiroketals

Abstract

A range of benzannulated spiroketals were prepared through photoinduced enone isomerization and spiroketalization as key steps. The simple protocol requires no additional reagents or catalyst, and tolerates both moisture and air.

Key words

benzannulated spiroketal - catalyst-free - isomerization - photochemistry - enones

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• 16 Photoinduced Synthesis of Benzannulated Spiroketals; General Procedure: To a 5 mL flask was added substrate (0.1 mmol) and MeCN (2 mL). The mixture was stirred under irradiation from a household 40 W fluorescent light bulb at ambient temperature. The progress of the reaction was monitored by TLC. Upon consumption of starting materials, the mixture was concentrated under reduced pressure and the residue was purified by silica gel column chromatography to afford the spiro product. 4′,5′-Dihydro-3′H-spiro[chromene-2,2′-furan] (7a): Yield: 14.8 mg (93%); colorless liquid. ¹H NMR (400 MHz, CDCl₃): δ = 7.18 (td, J = 8.0, 1.6 Hz, 1 H), 7.13 (dd, J = 7.2, 1.2 Hz, 1 H), 6.95–6.90 (m, 2 H), 6.71 (d, J = 9.6 Hz, 1 H), 5.75 (d, J = 9.6 Hz, 1 H), 4.17–4.12 (m, 1 H), 4.00–3.94 (m, 1 H), 2.42–2.32 (m, 2 H), 2.09–2.00 (m, 2 H). ¹³C NMR (100 MHz, CDCl₃): δ = 151.7, 129.2, 126.9, 126.8, 122.5, 121.2, 120.3, 116.4, 105.3, 68.2, 39.1, 24.5. HRMS: m/z [M + H]⁺ calcd for C₁₂H₁₃O₂: 189.0916; found: 189.0909. 3′,4′,5′,6′-Tetrahydrospiro[chromene-2,2′-pyran] (20a): Yield: 10 mg (90%); colorless liquid. ¹H NMR (400 MHz, CDCl₃): δ = 7.21 (td, J = 8.4, 2.0 Hz, 1 H), 7.14 (dd, J = 7.2, 1.2 Hz, 1 H), 7.01 (d, J = 8.0 Hz, 1 H), 6.95 (td, J = 7.6, 1.2 Hz, 1 H), 6.64 (d, J = 9.6 Hz, 1 H), 5.73 (d, J = 9.6 Hz, 1 H), 4.03–3.965 (m, 1 H), 3.64–3.60 (m, 1 H), 2.17–2.11 (m, 2 H), 1.80–1.63 (m, 4 H). ¹³C NMR (100 MHz, CDCl₃): δ = 151.4, 129.1, 127.0, 126.0, 125.4, 121.4, 121.2, 116.5, 95.4, 61.7, 35.0, 24.7, 18.5. HRMS: m/z [M + H]⁺ calcd for C₁₃H₁₅O₂: 203.1072; found: 203.1067. 3H-Spiro[benzofuran-2,2′-chromene] (22b): Yield: 12.8 mg (80%); white solid. ¹H NMR (400 MHz, CDCl₃): δ = 7.27–7.17 (m, 4 H), 7.00 (td, J = 7.6, 1.2 Hz, 1 H), 6.98–6.91 (m, 2 H), 6.87 (d, J = 9.6 Hz, 1 H), 6.78 (d, J = 7.6 Hz, 1 H), 5.97 (d, J = 9.6 Hz, 1 H), 3.58 (d, J = 16.8 Hz, 1 H), 3.48 (d, J = 16.8 Hz, 1 H). ¹³C NMR (100 MHz, CDCl₃): δ = 157.1, 150.9, 129.8, 128.4, 127.5, 127.0, 125.3, 124.4, 122.0, 121.3, 119.7, 116.7, 109.8, 107.7, 43.7. HRMS: m/z [M + H]⁺ calcd for C₁₆H₁₃O₂: 237.0916; found: 237.0909. 6,6′-Diethyl-2,2′-spirobi[chromene] (29c): Yield: 20.7 mg (85%); colorless liquid. ¹H NMR (400 MHz, CDCl₃): δ = 7.04–7.02 (m, 4 H), 6.81 (d, J = 9.2 Hz, 2 H), 6.77 (d, J = 8.8 Hz, 2 H), 5.96 (d, J = 9.6 Hz, 2 H), 2.61 (q, J = 7.6 Hz, 4 H), 1.23 (t, J = 7.6 Hz, 6 H). ¹³C NMR (100 MHz, CDCl₃): δ = 148.8, 137.5, 129.3, 126.6, 126.1, 121.7, 119.4, 116.5, 95.2, 28.1, 15.8. HRMS: m/z [M + H]⁺ calcd for C₂₁H₂₁O₂: 305.1542; found: 305.1540.