|
|
a) Generation of the organozinc species 8At room temperature, MnBr2 (40.7 mg, 0.189 mmol,15 mol %) was dissolved in DMPU (3 mL) under anargon atmosphere. CuCl (12.5 mg, 0.126 mmol,10 mol %), <strong>[110556-33-7]methyl (R)-(+)-3-bromo-2-methylpropionate</strong>(19, 482 muL, 3.787 mmol, 3.0 equiv) and Et2Zn(3.47 mL, 3.472 mmol, 2.75 equiv) were added in rapid succession. The resulting, dark redsuspension was stirred at room temperature for 4 h. Then, it was used for the cross couplingreaction without further purification or filtration.b) Negishi couplingAt room temperature, the iodoalkene 18 (454.2 mg, 1.261 mmol, 1.0 equiv) andPd(dppf)Cl2 · DCM (103.1 mg, 0.126 mmol, 10 mol %) were dissolved in THF (20 mL) underan argon atmosphere. Then, the suspension of organozinc species 8 obtained in a) wasslowly added and the mixture was stirred at room temperature for 15 h. It was then dilutedwith MTBE (30 mL) and water (30 mL), the phases were separated and the aqueous phasewas further extracted with MTBE (2 × 40 mL). The combined organic phases were washedwith H2O and saturated aqueous NaCl (25 mL each), dried over MgSO4 and the solventswere removed in vacuo. After column chromatography [silica, PE/EA (30:1)], the protectedpolyketide 20 (314.4 mg, 0.940 mmol, 75%) was obtained as a colorless oil. TLC [PE/EA (20:1)]: Rf = 0.34. []22 = +15.5 (c = 1.19, MeOH). 1H NMR (600 MHz, CDCl3):delta = 6.83-6.79 (m, 4H, C(5?)H, C(6?)H), 4.98 (dq, J = 9.6 Hz, J = 1.1 Hz, 1H, C(5)H), 4.19 (dq,J = 7.3 Hz, J = 6.0 Hz, 1H, C(8)H), 3.77 (s, 3H, C(8?)H3), 3.64 (s, 3H, C(9?)H3), 2.67-2.61 (m,1H, C(2)H), 2.56-2.48 (m, 1H, C(6)H), 2.36 (ddd, J = 13.5 Hz, J = 7.6 Hz, J = 1.0 Hz, 1H,C(3)H2), 2.04 (ddd, J = 13.5 Hz, J = 7.6 Hz, J = 0.9 Hz, 1H, C(3)H2), 1.72 (ddd, J = 13.6 Hz,J = 8.7 Hz, J = 5.9 Hz, 1H, C(7)H2), 1.62 (d, J = 1.4 Hz, 3H, C(2?)H3), 1.43 (ddd, J = 13.4 Hz,J = 7.4 Hz, J = 6.0 Hz, 1H, C(7)H2), 1.24 (d, J = 6.0 Hz, 3H, C(9)H3), 1.12 (d, J = 6.9 Hz, 3H,C(1?)H3), 0.92 (d, J = 6.7 Hz, 3H, C(3?)H3). 13C NMR (150.9 MHz, CDCl3): delta = 177.0 (1C,C(1)O), 153.7 (1C, C(7?)), 152.0 (1C, C(4?)), 133.1 (1C, C(5)H), 131.0 (1C, C(4)), 117.2 +114.6 (4C, C(5?)H, C(6?)H), 73.0 (1C, C(8)H), 55.7 (1C, C(8?)H3), 51.4 (1C, C(9?)H3), 44.1(1C, C(3)H2), 44.0 (1C, C(7)H2), 38.0 (1C, C(2)H), 29.2 (1C, C(6)H), 21.3 (1C, C(3?)H3), 19.8(1C, C(9)H3), 16.7 (1C, C(1?)H3), 15.9 (1C, C(2?)H3). IR (ATR): = 2953 cm-1 (m), 2930 (m),2871 (w), 1736 (m), 1504 (s), 1457 (m), 1376 (m), 1287 (w), 1225 (s), 1165 (m), 1107 (m),1038 (m), 942 (w), 825 (m), 737 (m), 581 (w). UV (CH2Cl2): lambdamax (lg epsilon) = 292 nm (3.34), 230(3.84). MS (EI, 70 eV): m/z (%) = 334 (13) [M]+?, 211 (3), 179 (12), 151 (8), 124 (100), 109(26), 95 (11), 81 (8), 55 (5). HREIMS: calc. for C20H30O4 [M]+? 334.21386; found 334.21487. When using the mixed fraction from 3.1.4 for further synthesis, the (2S,6S,8S,E)diastereomer SI-3 could also be obtained and separated from the main diastereomer:TLC [PE/EA (30:1)]: Rf = 0.21. []22 = +35.3(c = 0.57, MeOH). 1H NMR (600 MHz, CDCl3):delta = 6.81-6.78 (m, 4H, C(5?)H, C(6?)H), 4.91 (dq,J = 9.7 Hz, J = 1.0 Hz, 1H, C(5)H), 4.14 (dqd,J = 9.2 Hz, J = 6,.0 Hz, J = 3.0 Hz, 1H, C(8)H), 3.76 (s, 3H, C(8?)H3),3.65 (s, 3H, C(9?)H3), 2.72-2.64 (m, 1H, C(6)H), 2.63-2.57 (m, 1H, C(2)H), 2.36 (ddd,J = 13.5 Hz, J = 7.5 Hz, J = 0.7 Hz, 1H, C(3)H2), 1.99 (ddd, J = 13.5 Hz, J = 7.5 Hz,J = 0.7 Hz, 1H, C(3)H2), 1.73 (ddd, J = 13.7 Hz, J = 9.5 Hz, J = 4.0 Hz, 1H, C(7)H2), 1.42 (d,J = 1.3 Hz, 3H, C(2?)H3), 1.31 (ddd, J = 13.7 Hz, J = 10.2 Hz, J = 3.1 Hz, 1H, C(7)H2), 1.19(d, J = 6.1 Hz, 3H, C(9)H3), 1.10 (d, J = 7.0 Hz, 3H, C(1?)H3), 0.95 (d, J = 6.7 Hz, 3H,C(3?)H3). 13C NMR (150.9 MHz, CDCl3): delta = 177.0 (1C, C(1)), 153.6 (1C, C(7?)), 152.2 (1C,C(4?)), 133.3 (1C, C(5)H), 131.6 (1C, C(4)), 117.2 + 114.5 (4C, C(5?)H, C(6?)H), 72.7 (1C,C(8)H), 55.7 (1C, C(8?)H3), 51.5 (1C, C(9?)H3), 45.0 (1C, C(7)H2), 44.0 (1C, C(3)H2), 38.0(1C, C(2)H), 29.1 (1C, C(6)H), 21.6 (1C, C(3?)H3), 20.1 (1C, C(9)H3), 16.7 (1C, C(1?)H3), 15.8(1C, C(2?)H3). |