Asymmetric Total Synthesis of (+)-Brefeldin A: Intramolecular Epoxide-Opening/RCM Strategy

Abstract

A highly efficient asymmetric total synthesis of (+)-brefeldin A was accomplished by using intramolecular epoxide opening of an epoxy allylsilane and RCM reaction for the constructions of the cyclopentane and macrocyclic lactone rings of (+)-brefeldin A, respectively.

Key words

(+)-brefeldin A - cyclizations - epoxides - metathesis - ­total synthesis

References and Notes

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• 12b

  β-Lactone opening: ref 3j

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17

We were not able to isolate the minor Z-isomers. Only
E-isomers were isolated after column chromatography.

18

Lewis Acid Mediated Intramolecular Epoxide Opening: To a solution of 5 (365 mg, 1.14 mmol) in toluene (230 mL) was added BF₃˙OEt₂ (0.21 mL, 2.3 mmol) at -78 ˚C for 2 h using a syringe pump. The reaction mixture was treated with sat. aq NaHCO₃ (20 mL) at -78 ˚C and the layers were separated. The aqueous layer was extracted with EtOAc (2 × 20 mL) and the combined organic layers were dried over MgSO₄, filtered, concentrated in vacuo, and the residue was purified by silica gel chromatography (hexane-EtOAc, 6:1) to give 8 and the cis isomer (trans/cis = 88: 12, 203 mg, 72%) as a colorless oil. The ratios were determined by HPLC.
8: R _f 0.29 (hexane-EtOAc, 3:1); [α]^²4 _D -51 (c 0.125, CHCl₃). ^¹H NMR (400 MHz, CDCl₃): δ = 6.76-6.86 (m, 4 H), 5.78-5.89 (m, 1 H), 4.94-5.09 (m, 2 H), 4.65-4.72 (m, 1 H), 3.76 (s, 3 H), 3.68-3.75 (m, 1 H), 3.55-3.63 (m, 1 H), 2.26-2.42 (m, 2 H), 2.12-2.22 (m, 1 H), 2.03-2.12 (m, 1 H), 1.69-1.79 (m, 2 H), 1.36 (t, J = 5.5 Hz, 1 H). ^¹³C NMR (100 MHz, CDCl₃): δ = 154.06, 152.42, 142.55, 116.70, 114.81, 65.19, 55.88, 46.18, 40.30, 36.33. IR (film, neat): 3447, 3066, 2923, 2353, 1735, 1631, 1584, 1497, 1463, 1436, 1355, 1221, 1177, 1104, 1035, 992, 905, 823, 737 cm^-¹. HRMS: m/z [M]⁺ calcd for C₁₅H₂₀O₃: 248.1412; found: 248.1417.
cis-Isomer of 8: R _f 0.29 (hexane-EtOAc, 3:1). ^¹H NMR (400 MHz, CDCl₃): δ = 6.76-6.86 (m, 4 H), 5.85-5.97 (m, 1 H), 5.04-5.16 (m, 2 H), 4.74-4.81 (m, 1 H), 3.77 (s, 3H), 3.65 (dd, J = 11.2, 7.0 Hz, 1 H), 3.55 (dd, J = 11.2, 6.0 Hz, 1 H), 3.03 (q, J = 4.2 Hz, 1 H), 2.47-2.58 (m, 1 H), 2.00-2.13 (m, 2 H), 1.88-1.97 (m, 1 H), 1.78-1.88 (m, 1 H). 1.40 (t, J = 5.5 Hz, 1 H). ^¹³C NMR (100 MHz, CDCl₃): δ = 139.13, 116.77, 115.62, 114.82, 78.52, 64.06, 55.08, 43.73, 43.70, 38.39, 35.39.
Macrolactone Synthesis by Ring-Closing Metathesis: A solution of 2 and 4-epi-2 (32 mg, 0.060 mmol) and 7 (10 mol%) in CH₂Cl₂ (60 mL) was refluxed at 55 ˚C for 8 h under N₂. The solvent was removed in vacuo and the residue was purified by silica gel chromatography (hexane-EtOAc, 30:1) to give the macrocyclic lactones (25 mg, 81%) as a colorless oil. These lactones (22.6 mg, 0.045 mmol) in THF (0.5 mL) were treated with 1 M TBAF in THF (2.19 mL, 2.19 mmol) at r.t. for 3 h. The solvent was removed in vacuo and the residue was purified by silica gel chromatography (hexane-EtOAc, 5:1) to give 14 (7 mg) and 4-epi-14 (7 mg) in 78% yield.
14: mp 178-179 ˚C; R _f 0.26 (hexane-EtOAc, 2:1); [α]^²0 _D +42.6 (c 0.9, CHCl₃). ^¹H NMR (400 MHz, CDCl₃): δ = 7.38 (dd, J = 15.7, 3.2 Hz, 1 H), 6.81 (m, 4 H), 5.91 (dd, J = 15.6, 1.8 Hz, 1 H), 5.70 (ddt, J = 16.4, 11.4, 5.8 Hz, 1 H), 5.29 (dd, J = 15.2, 9.2 Hz, 1 H), 4.81-4.91 (m, 1 H), 4.63-4.71 (m, 1 H), 4.10-4.18 (m, 1 H), 3.76 (s, 3 H), 2.25-2.48 (m, 3 H), 2.00 (q, J = 9.3 Hz, 2 H), 1.80-1.91 (m, 2 H), 1.70-1.80 (m, 3 H), 1.63-1.70 (m, 1 H), 1.47-1.55 (m, 1 H), 1.26 (d, J = 6.2 Hz, 3 H), 0.90-1.00 (m, 1 H). ^¹³C NMR (100 MHz, CDCl₃): δ = 166.31, 154.02, 151.69, 136.72, 130.67, 117.76, 116.94, 114.81, 78.49, 76.39, 71.89, 55.90, 52.60, 44.50, 40.31, 39.00, 34.33, 31.93, 26.85, 21.01. IR (film, neat): 3496, 2925, 2851, 1709, 1681, 1506, 1443, 1377, 1268, 1227, 1032, 969, 824 cm^-¹. HRMS: m/z [M]⁺ calcd for C₂₃H₃₀O₅: 386.2093; found: 386.2090.
4-epi-14: mp 105-106 ˚C; R _f 0.31 (hexane-EtOAc, 2:1); [α]^²0 _D +40.3 (c 0.65, CHCl₃). ^¹H NMR (400 MHz, CDCl₃): δ = 7.10 (dd, J = 15.6, 8.4 Hz, 1 H), 6.81 (s, 4 H), 5.78 (d,
J = 16.0 Hz, 1 H), 5.66 (ddt, J = 14.9, 10.2, 4.6 Hz, 1 H), 5.28 (dd, J = 14.9, 9.6 Hz, 1 H), 4.89-4.99 (m, 1 H), 4.63-4.70 (m, 1 H), 4.42-4.48 (m, 1 H), 3.76 (s, 3 H), 2.86 (q, J = 4.8 Hz, 1 H), 2.18-2.28 (m, 1 H), 2.11-2.18 (m, 1 H), 2.02-2.11 (m, 1 H), 1.92-2.02 (m, 2 H), 1.75-1.87 (m, 2 H), 1.61-1.75 (m, 3 H), 1.48-1.53 (m, 1 H), 1.26 (d, J = 6.4 Hz, 3 H), 0.89-1.00 (m, 1 H). ^¹³C NMR (100 MHz, CDCl₃): δ = 166.43, 153.91, 151.97, 151.09, 137.78, 130.17, 119.89, 116.89, 114.80, 79.08, 72.04, 71.91, 55.87, 50.24, 40.33, 39.72, 35.63, 34.58, 32.01, 26.51, 20.93. IR (film, neat): 3475, 2929, 2846, 2364, 1714, 1506, 1457, 1355, 1260, 1228, 1038, 974, 825 cm^-¹. HRMS: m/z [M]⁺ calcd for C₂₃H₃₀O₅: 386.2093; found: 386.2090.
Synthesis of (+)-Brefeldin A (1): To a solution of 14 (8 mg, 0.02 mmol) in MeCN-H₂O (4:1, 0.2 mL) was added CAN (ceric ammonium nitrate; 22 mg, 0.040 mmol) in MeCN-H₂O (4:1, 0.1 mL) at 0 ˚C. The reaction mixture was warmed to r.t. and stirred for 1 h. A solution of sat. aq NH₄Cl (2.5 mL) was added. The layers were separated and the aqueous layer was extracted with EtOAc (2 × 10 mL). The combined organic layers were dried over MgSO₄, filtered, concentrated in vacuo, and the residue was purified by silica gel chroma-tography (CH₂Cl₂-MeOH, 20:1) to give 1 (4.8 mg, 84%) as a white solid; mp 202-204 ˚C; R _f 0.23 (CH₂Cl₂-MeOH, 10:1); [α]^²² _D +86.6 (c 0.9, MeOH). ^¹H NMR (400 MHz, CD₃OD): δ = 7.49 (dd, J = 15.6, 3.0 Hz, 1 H), 5.85 (dd, J = 15.6, 1.9 Hz, 1 H), 5.79 (ddd, J = 15.2, 10.2, 4.7 Hz, 1 H), 5.20 (dd, J = 15.0, 9.6 Hz, 1 H), 4.75-4.83 (m, 1 H), 4.18-4.25 (m, 1 H), 4.01-4.07 (m, 1 H), 2.39 (q, J = 8.7 Hz, 1 H), 2.12 (ddd, J = 13.6, 8.6, 5.4 Hz, 1 H), 1.96-2.06 (m, 3 H), 1.72-1.92 (m, 6 H), 1.52-1.63 (m, 2 H), 1.40-1.48 (m, 1 H), 1.27 (d, J = 6.3 Hz, 3 H), 0.85-1.00 (m, 1 H). ^¹³C NMR (100 MHz, CD₃OD): δ = 168.38, 155.12, 138.13, 131.41, 117.78, 76.63, 73.21, 72.98, 53.18, 45.46, 44.09, 41.85, 34.99, 32.97, 28.01, 21.06. IR (film, neat): 3364, 2972, 2927, 2857, 1715, 1442, 1255, 1111, 1077, 1000, 975 cm^-¹. HRMS: m/z [M]⁺ calcd for C₁₆H₂₄O₄: 280.1675; found: 280.1681.