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Synlett 2009(4): 661-663  
DOI: 10.1055/s-0028-1087814
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© Georg Thieme Verlag Stuttgart ˙ New York

Phase-Transfer-Catalyzed Enantioselective Alkylation of α-Benzoyloxy-β-Keto Ester

Takuya Hashimotoa, Keigo Sasakia, Kazuhiro Fukumotoa, Yuichi Murasea, Takashi Ooib, Keiji Maruoka*a
a Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502, Japan
Fax: +81(75)7534041; e-Mail: maruoka@kuchem.kyoto-u.ac.jp;
b Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Chikusa, Nagoya 464-8603, Japan
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Publication History

Received 10 October 2008
Publication Date:
16 February 2009 (online)

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Abstract

Highly enantioselective alkylation of tert-butyl 2-benzoyloxy-3-oxobutanoate could be realized by the use of N-spiro chiral quaternary ammonium salt as phase-transfer catalyst, as a complementary approach to the asymmetric hydroxylation of α-alkyl-β-keto esters.

Key words

phase-transfer catalysis - alkylations - asymmetric catalysis - esters - alkyl halides

    References and Notes

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7

Typical Procedure for the Catalytic Asymmetric Alkylation of tert -Butyl 2-Benzoyloxy-3-oxobutanoate under Phase-Transfer Conditions
To a solution of phase-transfer catalyst (S,S)-1c (0.002 mmol, 2.2 mg) and 2a (0.20 mmol, 55.7 mg) in toluene (2 mL) was added benzyl bromide (0.24 mmol, 28.5 µL) and the mixture was cooled to -20 ˚C. After adding freshly powdered KOH (0.3 mmol, 19.8 mg) to this mixture, the reaction mixture was vigorously stirred until the completion of the reaction. The mixture was then poured into sat. aq NH4Cl and extracted with EtOAc. The organic layer was dried over Na2SO4 and concentrated in vacuo. The residue was purified by column chromatography on SiO2 to give tert-butyl 2-benzoyloxy-2-benzyl-3-oxobutanoate as a colorless oil [91% (67.6 mg), 90% ee]. Enantiomeric purity was determined by HPLC analysis [(Daicel Chiralpak AD-H, hexane-2-PrOH (100:1), flow rate = 0.5 mL/min, t R = 22.3 min(major) and 25.0 min(minor)].
¹H NMR (400 MHz, CDCl3): δ = 8.00 (2 H, d, J = 7.3 Hz, ArH), 7.61 (1 H, t, J = 7.6 Hz, ArH), 7.46 (2 H, app t, J = 7.8 Hz, ArH), 7.21-7.22 (3 H, m, ArH), 7.12 (2 H, m, ArH), 3.65 (2 H, s, PhCH2), 2.25 (3 H, s, COCH3), 1.40 (9 H, s, t-Bu). ¹³C NMR (100 MHz, CDCl3): δ = 202.2, 165.3, 165.2, 134.4, 133.5, 130.2, 129.8, 129.3, 128.5, 128.3, 127.1, 89.0, 83.4, 38.8, 27.6, 27.4. IR (neat): 2978, 2932, 1755, 1721, 1279, 1152, 1107, 1094, 1069, 1026 912, 845, 733, 700 cm-¹. ESI-HRMS: m/z calcd for C22H24O5: 391.1516 [M + Na]+; found: 391.1516 [M + Na]+. [α]D ²8 -46.5 (c 1.0, CHCl3; 90% ee).