A Novel Skeleton Transformation of 2a-Substituted 2a,8b-Dihydro-3H-benzo[b]cyclobuta[d]pyran-3-ones to 2-(2-Oxoethyl)-2H-1-benzopyrans Promoted by a Combination of ZnI2, Organic Base and H2O

Navnath Dnyanoba Yadav; Masayuki Yamashita; Yumiko Saya; Ai Kurume; Ikuo Kawasaki; Shunsaku Ohta

doi:10.1055/s-2008-1032044

LETTER

A Novel Skeleton Transformation of 2a-Substituted 2a,8b-Dihydro-3H-benzo[b]cyclobuta[d]pyran-3-ones to 2-(2-Oxoethyl)-2H-1-benzopyrans Promoted by a Combination of ZnI₂, Organic Base and H₂O

Navnath Dnyanoba Yadav^a, Masayuki Yamashita*^a, Yumiko Saya^a, Ai Kurume^a, Ikuo Kawasaki^a,b, Shunsaku Ohta^a
^a Department of Functional Molecular Chemistry, 21st COE Program, Kyoto Pharmaceutical University, Misasagi-Nakauchi, Yamashina, Kyoto 607-8414, Japan
Fax: +81(75)5954795; e-Mail: yamasita@mb.kyoto-phu.ac.jp;
^b School of Pharmaceutical Sciences, Mukogawa Women’s University, Koshien Kyuban-cho, Nishinomiya 663-8179, Japan

Abstract

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Abstract

The novel skeleton transformation reaction of 2a-substituted 2a,8b-dihydrobenzo[b]cyclobuta[d]pyran-3-one derivatives into 2-(2-substituted 2-oxoethyl)-2H-1-benzopyran derivatives has been accomplished using Znl₂ in the presence of an organic base and water in excellent yields.

Key words

heterocycles - Lewis acids - photochemistry - rearrangements - ring opening

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References and Notes

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The cyclobutabenzopyrans 3 were relatively stable under these conditions except for 3a.

General Procedure: ZnI₂ (0.142 mmol) and piperidine (0.426 mmol) were added to a solution of 2a-substituted 2a,8b-dihydrobenzo[b]cyclobuta[d]pyran-3-one 3 (0.142 mmol) in THF-H₂O (3 mL:0.1 mL) at r.t., and the whole was refluxed. After disappearance of 3 was checked by TLC, the reaction mixture was acidified with 10% HCl. The mixture was extracted with EtOAc (3 × 15 mL). The combined extracts were washed, dried, and evaporated. The residue was chromatographed to give 2-(2-substituted 2-oxoethyl)-2H-1-benzopyran 6.¹²

5-Methoxy-2-(2-oxopropyl)-3-phenyl-2 H -1-benzopyran (6a): yellowish oil. ¹H NMR (300 MHz, CDCl₃): δ = 2.15 (s, 3 H), 2.34 (dd, J = 2.2, 15.9 Hz, 1 H), 3.10 (dd, J = 10.2, 15.9 Hz, 1 H), 3.84 (s, 3 H), 5.89 (dd, J = 2.3, 10.2 Hz, 1 H), 6.49 (d, J = 8.1 Hz, 1 H), 6.51 (d, J = 8.1 Hz, 1 H), 7.09 (t, J = 8.3 Hz, 1 H), 7.19 (s, 1 H), 7.24-7.39 (m, 3 H), 7.49-7.53 (m, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 31.0, 45.6, 55.6, 72.1, 103.7, 109.5, 111.8, 114.5, 125.1, 127.9, 128.8, 129.4, 132.3, 136.2, 151.4, 155.6, 206.1. IR (CHCl₃): 1708, 1594 cm^-1. MS: m/z (%) = 294 (20.3) [M⁺], 237 (100). HRMS: m/z [M⁺] calcd for C₁₉H₁₈O₃: 294.1256; found: 294.1257.3-Phenyl-2-(2-phenyl-2-oxoethyl)-2 H -1-benzopyran (6b): colorless crystals (recrystallized from EtOAc); mp 136-138 °C. ¹H NMR (300 MHz, CDCl₃): δ = 2.78 (dd, J = 2.2, 16.5 Hz, 1 H), 3.80 (dd, J = 9.6, 16.4 Hz, 1 H), 6.20 (dd, J = 2.2, 9.7 Hz, 1 H), 6.72 (d, J = 7.9 Hz, 1 H), 6.88 (s, 1 H), 6.93 (td, J = 1.1, 7.4 Hz, 1 H), 7.09 (dd, J = 1.7, 7.8 Hz, 1 H), 7.13 (td, J = 1.5, 7.2 Hz, 1 H), 7.29-7.42 (m, 5 H), 7.50-7.58 (m, 3 H), 7.82-7.86 (m, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 41.1, 72.7, 116.9, 120.0, 121.8, 122.5, 125.4, 127.0, 128.2, 128.3, 128.5, 128.9, 129.4, 133.2, 134.7, 136.1, 137.0, 151.0, 197.2. IR (CHCl₃): 1677, 1594 cm^-1. MS: m/z (%) = 326 (5.8) [M⁺], 207 (100), 178 (11.5). HRMS: m/z [M⁺] calcd for C₂₃H₁₈O₂: 326.1307; found: 326.1303. Anal. Calcd for C₂₃H₁₈O₂: C, 84.64; H, 5.56. Found: C, 84.51; H, 5.57.Methyl (3-Phenyl-2 H -1-benzopyran-2-yl)acetate (6c): colorless needles (recrystallized from EtOAc-n-hexane); mp 182-183 °C. ¹H NMR (300 MHz, CDCl₃): δ = 2.46 (dd, J = 2.9, 15.2 Hz, 1 H), 2.87 (dd, J = 9.9, 15.2 Hz, 1 H), 3.66 (s, 3 H), 5.89 (dd, J = 2.9, 10.1 Hz, 1 H), 6.82 (s, 1 H), 6.87 (d, J = 8.1 Hz, 1 H), 6.93 (td, J = 1.1, 7.4 Hz, 1 H), 7.11 (dd, J = 1.7, 7.5 Hz, 1 H), 7.16 (td, J = 1.7, 7.7 Hz, 1 H), 7.28-7.35 (m, 1 H), 7.36-7.42 (m, 2 H), 7.49-7.54 (m, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 38.1, 51.8, 73.0, 116.7, 120.2, 121.8, 122.2, 125.3, 127.0, 128.2, 128.9, 129.5, 134.1, 136.1, 150.8, 170.8. IR (CHCl₃): 1725, 1612 cm^-1. MS: m/z (%) = 280 (8.8) [M⁺], 207 (100), 178 (12.6). HRMS: m/z [M⁺] calcd for C₁₈H₁₆O₃: 280.1099; found: 280.1101. Anal. Calcd for C₁₈H₁₆O₃: C, 77.12; H, 5.75. Found: C, 76.50; H, 5.98.3-Butyl-2-(2-phenyl-2-oxoethyl)-2 H -1-benzopyran (6d): yellowish oil. ¹H NMR (300 MHz, CDCl₃): δ = 0.94 (t, J = 7.2 Hz, 3 H), 1.30-1.45 (m, 2 H), 1.45-1.65 (m, 2 H), 2.13 (t, J = 7.8 Hz, 2 H), 2.83 (dd, J = 3.1, 15.8 Hz, 1 H), 3.64 (dd, J = 9.0, 15.8 Hz, 1 H), 5.40 (dd, J = 3.1, 9.0 Hz, 1 H), 6.23 (s, 1 H), 6.62 (d, J = 8.1 Hz, 1 H), 6.86 (td, J = 1.1, 7.3 Hz, 1 H), 6.97 (dd, J = 1.7, 7.3 Hz, 1 H), 7.02 (td, J = 1.6, 7.7 Hz, 1 H), 7.35-7.45 (m, 2 H), 7.50-7.60 (m, 1 H), 7.85-7.90 (m, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 13.9, 22.3, 29.4, 32.5, 41.2, 74.2, 116.5, 118.8, 121.4, 122.5, 125.9, 128.3, 128.4, 128.5, 133.2, 137.1, 137.6, 150.7, 197.6. IR (CHCl₃): 1676 cm^-1. MS: m/z (%) = 306 (6.2) [M⁺], 187 (100), 157 (13.8). HRMS: m/z [M⁺] calcd for C₂₁H₂₂O₂: 306.1620; found: 306.1625.2-(3-Methyl-2-oxobutyl)-3-phenyl-2 H -1-benzopyran (6e): colorless needles (recrystallized from EtOAc-n-hexane); mp 75-77 °C. ¹H NMR (300 MHz, CDCl₃): δ = 1.00 (d, J = 7.0 Hz, 3 H), 1.05 (d, J = 7.0 Hz, 3 H), 2.28 (dd, J = 2.2, 16.2 Hz, 1 H), 2.51 (septet, J = 7.0 Hz, 1 H), 3.23 (dd, J = 10.0, 16.2 Hz, 1 H), 6.02 (dd, J = 2.2, 9.9 Hz, 1 H), 6.80 (d, J = 7.2 Hz, 1 H), 6.82 (s, 1 H), 6.92 (td, J = 1.0, 7.4 Hz, 1 H), 7.09-7.16 (m, 2 H), 7.28-7.41 (m, 3 H), 7.49-7.53 (m, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 17.6, 17.7, 41.8, 42.8, 72.6, 116.6, 119.7, 121.7, 122.5, 125.3, 126.9, 128.2, 128.9, 129.4, 134.6, 136.0, 150.9, 212.4. IR (CHCl₃): 1703, 1594 cm^-1. MS: m/z (%) = 292 (7.9) [M⁺], 207 (100), 178 (12.8). HRMS: m/z [M⁺] calcd for C₂₀H₂₀O₂: 292.1463; found: 292.1464. Anal. Calcd for C₂₀H₂₀O₂: C, 82.34; H, 6.89. Found: C, 82.34; H, 6.66.5-Methoxy-3-phenyl-2-(2-phenyl-2-oxoethyl)-2 H -1-benzopyran (6f): yellowish oil. ¹H NMR (300 MHz, CDCl₃): δ = 2.76 (dd, J = 2.2, 16.5 Hz, 1 H), 3.83 (dd, J = 9.7, 16.3 Hz, 1 H), 3.89 (s, 3 H), 6.17 (dd, J = 2.2, 9.6 Hz, 1 H), 6.38 (d, J = 8.3 Hz, 1 H), 6.49 (d, J = 7.7 Hz, 1 H), 7.04 (t, J = 8.3 Hz, 1 H), 7.22-7.33 (m, 2 H), 7.35-7.42 (m, 3 H), 7.38 (s, 1 H), 7.48-7.55 (m, 1 H), 7.55-7.60 (m, 2 H), 7.80-7.86 (m, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 40.7, 55.7, 72.1, 103.6, 109.9, 111.9, 114.5, 125.3, 127.9, 128.3, 128.5, 128.8, 129.3, 132.5, 133.2, 136.3, 137.0, 151.6, 155.5, 197.4. IR (CHCl₃): 1680, 1594 cm^-1. MS: m/z (%) = 356 (5.9) [M⁺], 237 (100), 222 (11.7). HRMS: m/z [M⁺] calcd for C₂₄H₂₀O₃: 356.1412; found: 356.1410.Methyl (5-Methoxy-3-phenyl-2 H -1-benzopyran-2-yl)acetate (6g): colorless oil. ¹H NMR (300 MHz, CDCl₃): δ = 2.45 (dd, J = 2.9, 15.2 Hz, 1 H), 2.89 (dd, J = 10.1, 15.2 Hz, 1 H), 3.67 (s, 3 H), 3.86 (s, 3 H), 5.86 (dd, J = 2.9, 10.1 Hz, 1 H), 6.49 (d, J = 8.3 Hz, 1 H), 6.53 (d, J = 8.1 Hz, 1 H), 7.10 (t, J = 8.3 Hz, 1 H), 7.21 (s, 1 H), 7.24-7.35 (m, 1 H), 7.35-7.40 (m, 2 H), 7.50-7.60 (m, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 37.8, 51.8, 55.7, 72.6, 103.8, 109.8, 111.8, 114.8, 125.3, 127.9, 128.8, 129.5, 132.0, 136.4, 151.4, 155.6, 170.9. IR (CHCl₃): 1726, 1593 cm^-1. MS: m/z (%) = 310 (10.8) [M⁺], 237 (100), 222 (15.6). HRMS: m/z [M⁺] calcd for C₁₉H₁₈O₄: 310.1205; found: 310.1201.Ethyl (5,7-Dimethoxy-3-phenyl-2 H -1-benzopyran-2-yl)acetate (6h): colorless oil. ¹H NMR (300 MHz, CDCl₃): δ = 1.25 (t, J = 7.2 Hz, 3 H), 2.44 (dd, J = 2.9, 15.4 Hz, 1 H), 2.88 (dd, J = 10.3, 15.4 Hz, 1 H), 3.78 (s, 3 H), 3.84 (s, 3 H), 4.06-4.23 (m, 2 H), 5.85 (dd, J = 2.9, 9.9 Hz, 1 H), 6.08 (d, J = 2.2 Hz, 1 H), 6.10 (d, J = 2.2 Hz, 1 H), 7.13 (s, 1 H), 7.23-7.28 (m, 1 H), 7.34-7.39 (m, 2 H), 7.50-7.52 (m, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 14.2, 37.9, 55.4, 55.6, 60.7, 73.0, 92.3, 94.3, 105.3, 114.7, 124.9, 127.4, 128.8 (2), 136.6, 152.6, 156.5, 161.5, 170.6. IR (CHCl₃): 1721, 1608 cm^-1. MS: m/z (%) = 354 (11.2) [M⁺], 267 (100). HRMS: m/z [M⁺] calcd for C₂₁H₂₂O₅: 354.1467; found: 354.1459.Methyl (3-Phenyl-5-trimethylsilyl-2 H -1-benzopyran-2-yl)acetate (6i): colorless oil. ¹H NMR (300 MHz, CDCl₃): δ = 0.26 (s, 9 H), 2.47 (dd, J = 2.8, 16.1 Hz, 1 H), 2.92 (dd, J = 9.2, 16.1 Hz, 1 H), 3.61 (s, 3 H), 6.06 (dd, J = 2.8, 9.2 Hz, 1 H), 6.78 (s, 1 H), 6.92 (t, J = 7.3 Hz, 1 H), 7.11 (dd, J = 1.5, 7.3 Hz, 1 H), 7.25 (dd, J = 1.8, 7.3 Hz, 1 H), 7.28-7.35 (m, 1 H), 7.35-7.43 (m, 2 H), 7.48-7.55 (m, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = -0.95, 38.1, 51.7, 73.0, 120.6, 121.0, 121.3, 125.4, 126.7, 128.1, 128.4, 128.9, 133.3, 135.1, 136.3, 155.8, 170.9. IR (CHCl₃): 1727, 1589 cm^-1. MS: m/z (%) = 352 (8.1) [M⁺], 279 (100). HRMS: m/z [M⁺] calcd for C₂₁H₂₄O₃Si: 352.1495; found: 352.1496.2-(2-Oxopropyl)-3-phenyl-5-trimethylsilyl-2 H -1-benzopyran (6j): colorless oil. ¹H NMR (300 MHz, CDCl₃): δ = 0.24 (s, 9 H), 2.09 (s, 3 H), 2.40 (dd, J = 2.1, 17.8 Hz, 1 H), 3.21 (dd, J = 9.0, 17.8 Hz, 1 H), 6.17 (dd, J = 2.0, 9.0 Hz, 1 H), 6.77 (s, 1 H), 6.92 (t, J = 7.2 Hz, 1 H), 7.10 (dd, J = 1.7, 7.3 Hz, 1 H), 7.24 (dd, J = 1.7, 7.3 Hz, 1 H), 7.26-7.33 (m, 1 H), 7.34-7.42 (m, 2 H), 7.44-7.52 (m, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = -0.94, 30.6, 46.4, 71.1, 120.2, 121.1, 121.3, 125.2, 126.8, 128.1, 128.4, 128.9, 133.8, 135.0, 136.1, 155.8, 205.1. IR (CHCl₃): 1714, 1579 cm^-1. MS: m/z (%) = 336 (8.4) [M⁺], 279 (100). HRMS: m/z [M⁺] calcd for C₂₁H₂₄O₂Si: 336.1546; found: 336.1545.3-Butyl-2-(2-phenyl-2-oxoethyl)-5-trimethylsilyl-2 H -1-benzopyran (6k): yellowish oil. ¹H NMR (300 MHz, CDCl₃): δ = 0.10 (s, 9 H), 0.92 (t, J = 7.2 Hz, 3 H), 1.33-1.41 (m, 2 H), 1.48-1.59 (m, 2 H), 2.08 (t, J = 7.7 Hz, 2 H), 2.95 (dd, J = 3.4, 17.0 Hz, 1 H), 3.63 (dd, J = 8.2, 17.1 Hz, 1 H), 5.59 (dd, J = 3.3, 8.3 Hz, 1 H), 6.19 (s, 1 H), 6.85 (t, J = 7.2 Hz, 1 H), 6.97 (dd, J = 1.7, 7.3 Hz, 1 H), 7.13 (dd, J = 1.7, 7.2 Hz, 1 H), 7.37-7.44 (m, 2 H), 7.51-7.53 (m, 1 H), 7.87-7.91 (m, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = -0.93, 13.9, 22.4, 29.3, 32.5, 41.5, 73.1, 119.0, 120.9, 121.4, 126.5, 127.3, 128.2, 128.5, 133.2, 133.9, 137.0, 137.1, 155.7, 197.1. IR (CHCl₃): 1682, 1594 cm^-1. MS: m/z (%) = 378 (5.18) [M⁺], 260 (23.7), 259 (100). HRMS: m/z [M⁺] calcd for C₂₄H₃₀O₂Si: 378.2015; found: 378.2013.2-Phenyl-(2-phenyl-2-oxoethyl)-3 H -naphtho[2,1- b ]pyran (6l): yellowish oil. ¹H NMR (300 MHz, CDCl₃): δ = 2.79 (dd, J = 2.2, 16.3 Hz, 1 H), 3.93 (dd, J = 9.8, 16.3 Hz, 1 H), 6.29 (dd, J = 2.1, 9.6 Hz, 1 H), 6.97 (d, J = 8.8 Hz, 1 H), 7.30-7.70 (m, 12 H), 7.75-7.85 (m, 3 H), 8.13 (d, J = 8.6 Hz, 1 H). ¹³C NMR (75 MHz, CDCl₃): δ = 40.3, 72.8, 115.2, 115.8, 118.4, 121.3, 123.9, 125.4, 126.8, 128.2, 128.3, 128.5, 128.7, 129.0, 129.6, 129.8, 123.0, 133.1, 133.2, 136.4, 137.0, 149.0, 197.3. IR (CHCl₃): 1721, 1600 cm^-1. MS: m/z (%) = 376 (6.4) [M⁺], 257 (100), 228 (7.0). HRMS: m/z [M⁺] calcd for C₂₇H₂₀O₂: 376.1463; found: 376.1457.