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Synlett 2019; 30(07): 809-812
DOI: 10.1055/s-0037-1612255
DOI: 10.1055/s-0037-1612255
letter
Direct Synthesis of β,γ-Unsaturated α-Keto Esters from Aldehydes and Pyruvates
We are grateful to the National Natural Science Foundation of China (Grant No. 21702185) and the Science Foundation of Zhejiang Sci-Tech University (Grant No. 16062189-Y).Further Information
Publication History
Received: 08 December 2018
Accepted after revision: 30 January 2019
Publication Date:
25 February 2019 (online)
Abstract
Herein, we describe two practical methods to synthesize β,γ-unsaturated α-keto esters directly from aldehydes and pyruvates promoted by BF3•Et2O in the presence of Ac2O or by Ti(OEt)4 under mild conditions. A variety of aromatic aldehydes was tolerated to afford the desired products in moderate to excellent yield. Moreover, aliphatic aldehydes and Isatin were also employed to give the γ-alkyl β,γ-unsaturated α-keto esters in moderate yield with use of the Ti(OEt)4 system.
Supporting Information
- Supporting information for this article is available online at https://doi.org/10.1055/s-0037-1612255.
- Supporting Information
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- 17 β,γ-Unsaturated α-Keto Esters (1a–u); General Procedure A To the solution of aldehyde (2 mmol) and pyruvate (2.4 mmol) in toluene (10 mL) were added BF3•Et2O (1 mmol) and Ac2O (3 mmol). After stirring for 48 h at 40 °C, the mixture was poured into saturated NaHCO3 (aq) solution (20 mL). The separated aqueous phase was extracted with ethyl acetate (20 mL) and the combined organic phases were washed by brine (20 mL), dried with Na2SO4, filtered, and concentrated in vacuum. The residue was filtered through a short pad of silica gel with petroleum ether/ethyl acetate (10:1) as the eluent to obtain a crude product, which was analyzed by 1H NMR spectroscopy to calculate the E/Z ratio. Further purification was carried out by column chromatography on silica gel with petroleum ether/ethyl acetate (20:1) as the eluent to afford the products 1. Ethyl (E)-4-Cyclohexyl-2-oxobut-3-enoate (1t) Pale yellow oil; yield: 176 mg (42%). 1H NMR spectrum of the crude product shows an E/Z ratio of >19:1. 1H NMR (400 MHz CDCl3): δ = 7.09 (dd, J = 16.0, 6.8 Hz, 1 H), 6.57 (dd, J = 16.0, 1.1 Hz, 1 H), 4.31 (q, J = 7.1 Hz, 2 H), 2.29–2.13 (m, 1 H), 1.78–1.64 (m, 5 H), 1.35 (t, J = 7.1 Hz, 3 H), 1.31–1.08 (m, 5 H). 13C NMR (101 MHz, CDCl3): δ = 183.8, 162.5, 159.6, 122.7, 62.2, 41.2, 31.4, 25.8, 25.6, 14.0.
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- 19 Ti(OEt)4 System; General Procedure B To the solution of aldehyde (2 mmol) and pyruvate (2.4 mmol) in toluene (10 mL) was added Ti(OEt)4 (2.4 mmol). After stirring for 72 h at 40 °C, the mixture was diluted with ethyl acetate (40 mL) and quenched with water (1 mL). After stirring for 0.5 h at rt, the mixture was dried with Na2SO4, filtered, and concentrated in vacuum. The residue was filtered through a short pad of silica gel with petroleum ether/ethyl acetate (10:1) as the eluent to obtain a crude product, which was analyzed by 1H NMR spectroscopy to calculate the E/Z ratio. Further purification was carried out by column chromatography on silica gel with petroleum ether/ethyl acetate (20:1) as the eluent to afford the products 1. Ethyl (E)-2-Oxohept-3-enoate (1s) Pale yellow oil; yield: 122 mg (36%). 1H NMR spectrum of the crude product shows an E/Z ratio of >19:1. 1H NMR (400 MHz, CDCl3): δ = 7.18 (dt, J = 15.8, 6.9 Hz, 1 H), 6.64 (d, J = 15.9 Hz, 1 H), 6.64 (d, J = 15.9 Hz, 1 H), 4.34 (q, J = 7.1 Hz, 2 H), 2.29 (td, J = 8.0, 1.0 Hz, 2 H), 1.59–1.47 (m, 2 H), 1.37 (t, J = 7.1 Hz, 3 H), 0.95 (t, J = 7.4 Hz, 3 H). 13C NMR (100 MHz, CDCl3): δ = 183.5, 162.4, 154.9, 125.3, 62.30, 35.1, 21.1, 14.0, 13.7.
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