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Synlett 2019; 30(11): 1317-1320
DOI: 10.1055/s-0037-1611849
DOI: 10.1055/s-0037-1611849
letter
Enantioselective Protonation of Silyl Enol Ethers Catalyzed by a Chiral Pentacarboxycyclopentadiene-Based Brønsted Acid
Fundamental Research Funds for the Central Universities, (Grant/Award Number: XK-1802-6, 12060093063); National Natural Science Foundation of China, (Grant/Award Number: 21402005).Further Information
Publication History
Received: 14 April 2019
Accepted after revision: 13 May 2019
Publication Date:
29 May 2019 (online)
Abstract
The enantioselective protonation of silyl enol ethers was realized in the presence of a pentacarboxycyclopenta-1,3-diene-based chiral Brønsted acid catalyst with water as an achiral proton source to give the corresponding α-aryl ketones in good yields and up to 75% ee.
Key words
protonation - Brønsted acids - asymmetric catalysis - organocatalysis - carbonyl compounds - silyl enol ethersSupporting Information
- Supporting information for this article is available online at https://doi.org/10.1055/s-0037-1611849.
- Supporting Information
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References and Notes
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- 9 (2R)-2-Phenylcyclohexanone (3a); Typical Procedure PCCP catalyst 1b (19.3 mg, 0.02 mmol, 10 mol%) was weighed into a cryogenic vial. The vial was cooled to –10 °C, and H2O (4 μL 1.1 equiv) and anhyd xylenes (2 mL) were added. After 15 min at –10 °C, silyl enol ether 2a (49 mg, 0.2 mmol, 1.0 equiv) was added dropwise to the stirred mixture. After 8 h, the reaction was quenched by the addition of sat. aq NaHCO3 (3 mL) and the mixture was extracted with CH2Cl2 (25 mL). The organic layer was washed with brine (5 mL), dried (NaSO4), filtered, and concentrated in vacuo. The residue was purified by flash column chromatography [silica gel, hexanes–EtOAc (100:1 to 10:1)] to give a white solid; yield: 35 mg (99%, 74% ee); [α]D 25 +50.7 (c = 0.4, CHCl3). HPLC: Chiralpak Lux 5u cellulose-1 [hexanes–i-ProH (99:1), flow rate = 1.0 mL/min; λ = 215 nm], t R (minor, S) = 14.8 min; t R (major, R) = 16.2 min. 1H NMR (400 MHz, CDCl3): δ = 7.33 (t, J = 7.4 Hz, 2 H), 7.25 (t, J = 7.4 Hz, 1 H), 7.14 (d, J = 7.4 Hz, 2 H), 3.60 (dd, J = 12.1, 5.4 Hz, 1 H), 2.63–2.36 (m, 2 H), 2.26–2.30 (m, 1 H), 2.21–2.09 (m, 1 H), 2.09–1.90 (m, 2 H), 1.90–1.71 (m, 2 H).