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DOI: 10.1055/s-2005-871953
Total Synthesis of (R)-(+)-Kavain via (MeCN)2PdCl2-Catalyzed Isomerization of a cis Double Bond and Sonochemical Blaise Reaction
Publication History
Publication Date:
12 July 2005 (online)
Abstract
From the chiral source 2,3-O-isopropylidene-d-glyceraldehyde 2, the natural product (R)-(+)-kavain 1a was efficiently synthesized in a total yield of 25% via (MeCN)2PdCl2-catalyzed isomerization of the cis double bond of an olefin as the key step and sonochemical Blaise reaction. The chiral center adjacent to the cis double bond was retained without protection of the free allylic hydroxy during the isomerization process.
Key words
kavain - total synthesis - (MeCN)2PdCl2-catalyzed - isomerization - cis double bond - sonochemical Blaise reaction
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References
Isomerization of the cis double bond: A solution of nitriles 5 (145 mg, 0.84 mmol) and bis(acetonitrile)palladium(II) chlorine (17.5 mg, 0.07 mmol) in benzene (5 mL) was stirred at reflux for ca. 5 h until the cis form of 5 was fully consumed (monitored by TLC). After removal of the solvent in vacuo, the residue was dissolved in EtOAc (50 mL) and filtered to remove the insoluble solid. The EtOAc soluble fraction was then purified by flash column chromatography on silica gel (EtOAc-petroleum ether, 1:4) to afford the desired pure trans form of 5 (132 mg, 91%) as a colorless oil. 1H NMR (400MHz, CDCl3): δ = 2.25 (br, 1 H), 2.63-2.69 (dd, J = 6.2, 16.7 Hz, 1 H), 2.69-2.75 (dd, J = 5.5, 16.7 Hz, 1 H), 4.652 (m, 1 H), 6.25 (dd, J = 6.7, 15.9 Hz, 1 H), 6.74 (d, J = 15.9 Hz, 1 H), 7.27-7.40 (m, 5 H); EI-MS: m/z =173 (M+).
14Sonochemical reactions: A mixture of the trans form of 5 (205 mg, 1.18 mmol) and activated Zn (0.80 g, 12.3 mmol) in anhyd THF (10 mL) was subjected to ultrasonic irradiation at 50 °C for 10 min and then methyl bromoacetate (0.70 ml, 5.86 mmol) was added dropwise under a nitrogen atmosphere. The resultant mixture was irradiated under the same conditions for another 5 h, then acidified with 2 M HCl (to pH 3), stirred at r.t. for 10 min, and diluted with EtOAc (60 mL). The organic phase was washed with sat. NaHCO3 (20 mL), brine (20 mL), and then dried over anhyd Na2SO4. After evaporation of the solvent in vacuo, the residue was purified by column chromatography on silica gel (petroleum ether-EtOAc, 10:3) to afford the desired product 6 (206 mg, 70%), as a pale yellow oil. [α]D 20 +19.4 (c 1.15, CHCl3), {lit.15 [α]D 25 +20.2 (c 1.0, CHCl3)}. 1H NMR (400MHz, CDCl3): δ = 2.88 (d, J = 6.1 Hz, 2 H), 3.53 (s, 2 H), 3.75 (s, 3 H), 4.79 (m, 1 H), 6.22 (dd, J = 6.2, 15.9 Hz, 1 H), 6.67 (dd, J = 1.1, 15.9 Hz, 1 H), 7.25-7.39 (m, 5 H). EIMS: m/z = 248 (M+).