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DOI: 10.1055/s-2006-926250
A New Efficient Method for the Total Synthesis of Linear Furocoumarins
Publication History
Publication Date:
20 February 2006 (online)
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Abstract
A new efficient method for the synthesis of linear furocoumarins by a Nef reaction and intramolecular cyclocondensation in one pot results in the construction of a benzofuran ring. This method provides a new strategy to furnish the benzofuran framework easily, and also allows the convenient synthesis of furocoumarin derivatives with different substituents on the coumarin ring by a subsequent Pechmann reaction. This strategy has also been applied to the preparation of four additional benzofuran derivatives.
Key words
Nef reaction - Pechmann reaction - furocoumarins - total synthesis - intramolecular cyclocondensation
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References and Notes
Benzofuran 4: Compound 2 (2 mmol) was added to an aq soln of 2 M NaOH (15 mL) and stirred at r.t. for 30 min under an N2 atmosphere. After cooling to 0 °C, the resultant solution was added dropwise to a well-stirred aq solution of 3 M H2SO4 (25 mL) at 0 °C. The reaction was stirred at r.t. for about 4 h until the reaction was complete. The solution was extracted with EtOAc and the organic phase was dried over anhyd Na2SO4. After removal of the solvent, the residue was purified by flash chromatography on silica gel to afford the desired compound 4. [28] Mp 54-55 °C. IR (KBr): 3423, 3122, 1602, 1446, 1132, 837 cm-1. 1H NMR (400 MHz, CDCl3): δ = 7.52 (d, J = 2.2 Hz, 1 H), 7.42 (d, J = 8.5 Hz, 1 H), 7.00 (m, 1 H), 6.79 (dd, J = 8.5 Hz, J = 2.2 Hz, 1 H), 6.69 (dd, J = 2.2 Hz, J = 0.9 Hz, 1 H), 5.16 (br s, 1 H). EI-MS: m/z (%) = 134 (M+, 100), 105(30), 78 (35), 51 (15). Anal. Calcd for C8H6O2: C, 71.64; H, 4.51. Found: C, 71.57; H, 4.52.
25
5b-f; General Procedure: To a mixture of 4 and an appropriate β-ketoester (ratio 1:1.2), an excess of MsOH (0.5 mL per mmol of 4) was slowly added, and stirred at 0 °C for 30 min. Stirring was continued at r.t. until the reaction was complete (monitored by TLC). The resulting mixture was poured into ice-water, stirred for 1 h at r.t., extracted with EtOAc, the organic phase was dried over anhyd Na2SO4, and then concentrated in vacuo. The residue was purified by flash chromatography over silica gel (PE-acetone) to afford the title compounds 5b-f.
5b Yield: 72%; mp 186-187 °C (acetone) (lit.
[29]
185-186 °C). IR (KBr): 3161, 1724, 1631, 1385, 746 cm-1. 1H NMR (400 MHz, CDCl3): δ = 7.82 (s, 1 H), 7.69 (d, J = 2.4 Hz, 1 H), 7.48 (d, J = 0.4 Hz, 1 H), 6.86 (dd, J = 2.4 Hz, J = 0.4 Hz, 1 H), 6.27 (s, 1 H), 2.51 (s, 3 H). EI-MS: m/z (%) = 200 (M+, 100), 172 (70), 171 (75), 144 (10), 115 (25).
5c Yield: 70%; mp 170-171 °C. IR (KBr): 3122, 2972, 1716, 1629, 1583, 1130 cm-1. 1H NMR (400 MHz, CDCl3): δ = 7.86 (s, 1 H), 7.69 (d, J = 2.4 Hz, 1 H), 7.49 (d, J = 0.8 Hz, 1 H), 6.84 (dd, J = 2.4 Hz, J = 0.8 Hz, 1 H), 6.28 (t, J = 1.2 Hz, 1 H), 2.89 (dq, J = 7.2 Hz, J = 1.2 Hz, 2 H), 1.38 (t, J = 7.2 Hz, 3 H). EI-MS: m/z (%) = 214 (M+, 75), 185 (30), 171 (100), 115 (15). Anal. Calcd for C13H10O3: C, 72.89; H, 4.71. Found: C, 72.97; H, 4.65.
5d Yield: 65%; mp 127-128 °C (lit.
[30]
128-129 °C). IR (KBr): 3112, 2955, 1708, 1630, 1576, 1161, 1112 cm-1. 1H NMR (300 MHz, CDCl3): δ = 7.84 (s, 1 H), 7.68 (d, J = 2.4 Hz, 1 H), 7.47 (s, 1 H), 6.84 (d, J = 2.4 Hz, 1 H), 6.25 (s, 1 H), 2.81 (t, J = 7.6 Hz, 2 H), 1.79 (m, 2 H), 1.08 (t, J = 7.6 Hz, 3 H). EI-MS: m/z (%) = 228 (M+, 100), 200(35), 185 (70), 171 (70), 115 (20).
5e Yield: 67%; mp 142-143 °C. IR (KBr): 3101, 1731, 1628, 1581, 1450, 1283, 1151 cm-1. 1H NMR (400 MHz, CDCl3): δ = 7.97 (q, J = 1.9 Hz, 1 H), 7.75 (d, J = 2.2 Hz, 1 H), 7.56 (d, J = 0.5 Hz, 1 H), 6.90 (dd, J = 2.2 Hz, J = 0.8 Hz, 1 H), 6.79 (s, 1 H). EI-MS: m/z (%) = 254 (M+, 90), 226 (100), 198 (20), 169 (20). Anal. Calcd for C12H5F3O3: C, 56.71; H, 1.98. Found: C, 56.67; H, 2.10.
5f Yield: 58%; mp 177-178 °C. IR (KBr): 3128, 3124, 1722, 1626, 1568, 1375, 1149 cm-1. 1H NMR (300 MHz, CDCl3): δ = 7.68 (s, 1 H), 7.67 (br s, 1 H), 7.57-7.47 (m, 6 H), 6.76 (dd, J = 2.2 Hz, J = 1.0 Hz, 1 H), 6.34 (s, 1 H). EI-MS: m/z (%) = 262 (M+, 90), 234 (100), 205 (35), 176 (30), 76 (20). Anal. Calcd for C17H10O3: C, 77.85; H, 3.84. Found: C, 77.94; H, 3.92.
Psoralen 5a: A mixture of ZnCl2 (210 mg, 1.5 mmol), ethyl propiolate (0.5 mL, 5 mmol), and compound 4 (134 mg, 1.0 mmol) was heated at 90 °C for 3 h under a N2 atmosphere. After cooling to r.t., 5% HCl (25 mL) was added to the resultant mixture, which was then extracted with EtOAc
(3 × 25 mL). The combined organic layer was washed with H2O (3 × 10 mL), brine (20 mL), and dried over anhyd Na2SO4. The solvent was removed on a rotary evaporator to yield the crude product which was purified by flash chromatography to give the desired compound 5a. Yield: 56%; mp 162-163 °C (lit.
[13a]
161-162 °C). IR (KBr): 3157, 1720, 1633, 1286, 1136, 750 cm-1. 1H NMR (400 MHz, CDCl3): δ = 7.81 (d, J = 9.6 Hz, 1 H), 7.70 (d, J = 1.2 Hz, 1 H), 7.69 (s, 1 H), 7.49 (s, 1 H), 6.84 (d, J = 1.2 Hz, 1 H), 6.39 (d, J = 9.6 Hz, 1 H). EI-MS: m/z (%) = 186 (M+, 100), 158 (90), 130 (30), 102 (40).
9a:
[31a]
1H NMR (400 MHz, CDCl3): δ = 7.62 (d, J = 1.6 Hz, 1 H), 7.57 (d, J = 10.0 Hz, 1 H), 7.51 (d, J = 8.4 Hz, 1 H), 7.30-7.27 (m, 2 H), 6.77 (d, J = 1.6 Hz, 1 H). EI-MS: m/z (%) = 118 (M+, 100), 78 (45).
9b:
[31b]
1H NMR (400 MHz, CDCl3): δ = 7.52 (d, J = 2.4 Hz, 1 H), 7.43 (d, J = 9.1 Hz, 1 H), 7.00 (s, 1 H), 6.81 (d, J = 9.1 Hz, 1 H), 6.69 (d, J = 2.4 Hz, 1 H), 3.78 (s, 3 H). EI-MS: m/z (%) = 148 (M+, 100), 134 (25), 78 (15).
9c:
[31c]
1H NMR (400 MHz, CDCl3): δ = 7.74 (s, 1 H), 7.63 (d, J = 2.3 Hz, 1 H), 7.39 (m, 2 H), 6.73 (d, J = 2.3 Hz, 1 H). EI-MS: m/z (%) = 197 (M+, 95), 196 (100), 78 (25).
9d:
[31d]
1H NMR (400 MHz, CDCl3): δ = 8.16 (d, J = 10.3 Hz, 1 H), 7.95 (d, J = 8.6 Hz, 1 H), 7.86 (d, J = 2.4 Hz, 1 H), 7.42-7.37 (m, 1 H), 6.95 (d, J = 2.4 Hz, 1 H). EI-MS: m/z (%) = 163 (M+, 100), 105 (25), 78 (35).