Synlett 2011(14): 2080-2084  
DOI: 10.1055/s-0030-1260986
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Organocatalytic Asymmetric Hetero-Diels-Alder Reaction of Oxindoles under High Pressure [¹]

Kaori Moria, Tomoyuki Yamauchia, Jacques Maddalunob, Keiji Nakanoa, Yoshiyasu Ichikawaa, Hiyoshizo Kotsuki*a
a Laboratory of Natural Products Chemistry, Faculty of Science, Kochi University, Akebono-cho, Kochi 780-8520, Japan
Fax: +81(888)448359; e-Mail: kotsuki@kochi-u.ac.jp;
b IRCOF, CNRS UMR 6014 & FR 3038, Université de Rouen, 76821 Mont Saint-Aignan Cedex, France
Further Information

Publication History

Received 14 May 2011
Publication Date:
10 August 2011 (online)

Abstract

A general and efficient protocol for the high-pressure-promoted asymmetric hetero-Diels-Alder reactions of oxindoles has been developed. These reactions can be realized by using chiral thiourea-derived organocatalysts, and the desired adducts are obtained in good to high yields with good to moderate enantioselectivity.

    References and Notes

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    see Ref. 3b.

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8

Most of these catalysts are known. The new catalysts, 1a, 1b, 1d, 1f, and 1g, were simply prepared by condensation of 3,5-bis(trifluoromethyl)phenylisothiocyanate with chiral amine precursors. 1a: mp 130.5-133 ˚C (hexane-CH2Cl2), [α]D ²6
-16.4 (c = 1.0, CHCl3); 1b: mp 48-50 ˚C (hexane-CH2Cl2), [α]D ²0 -93.7 (c = 1.0, CHCl3); 1d: mp 129.0-132.0 ˚C (hexane-CH2Cl2), [α]D ²¹ +48.8 (c = 0.90, CHCl3); 1f: mp 151-153 ˚C (hexane-CH2Cl2), [α]D ²6 +3.96 (c = 0.15, CHCl3); 1g: mp 149.5-151 ˚C (hexane-CH2Cl2), [α]D ²6 +10.3 (c = 0.16, CHCl3).

9

General Procedure: A mixture of diene 2 (1.0 mmol) and ketone 3 (0.25 mmol) in the presence of 1a (0.1 mmol) in toluene or CH2Cl2 (ca. 2.5 mL) was placed in a Teflon reaction vessel, and the mixture was allowed to react at 1.0 GPa and r.t. for 12 h. After the pressure was released, the mixture was concentrated and purified by silica gel column chromatography (elution with hexane-Et2O) to afford the pure adduct 4.

10

At lower pressures, the yield and enantioselectivity were both decreased for the reaction of 2a with 3a using 30 mol% of 1e in CH2Cl2: at 0.6 GPa, r.t., 4.5 d, 31% yield, exo-4a (49% ee, R), endo-4a (62% ee, R).

11

Not optimized.

14

In general, the exo-adducts were less polar than the endo-adducts. TLC data (hexane-EtOAc, 4:1); exo-4a: R f 0.42 and endo-4a: R f 0.34.
Compound exo-4a: colorless oil. FTIR (neat): 1777, 1733, 1606, 1478, 1467 cm. ¹H NMR (400 MHz, CDCl3): δ = 1.65 (s, 9 H), 2.26 (dd, J = 17.6, 6.0 Hz, 1 H), 2.71 (ddd, J = 17.6, 6.0, 2.0 Hz, 1 H), 3.25 (s, 3 H), 5.34 (d, J = 2.0 Hz, 1 H), 6.00-6.04 (m, 1 H), 6.15-6.20 (m, 1 H), 7.12 (t, J = 8.0 Hz, 1 H), 7.34 (m, 1 H), 7.70 (d, J = 7.6 Hz, 1 H), 7.86 (d,
J = 8.0 Hz, 1 H). ¹³C NMR (100 MHz, CDCl3): δ = 28.1 (3 ×), 31.2, 55.2, 73.8, 84.7, 95.7, 114.8, 124.3, 125.2, 125.9, 126.1, 129.4, 129.7, 139.0, 149.1, 173.9.
Compound endo-4a: colorless oil. FTIR (neat): 1780, 1731, 1606, 1478, 1467 cm. ¹H NMR (400 MHz, CDCl3): δ = 1.63 (s, 9 H), 2.32-2.37 (m, 1 H), 2.68-2.73 (m, 1 H), 3.43 (s, 3 H), 5.25 (s, 1 H), 5.98-6.02 (m, 1 H), 6.19-6.23 (m, 1 H), 7.17 (t, J = 8.0 Hz, 1 H), 7.39 (t, J = 8.0 Hz, 1 H), 7.43 (d, J = 8.0 Hz, 1 H), 7.91 (d, J = 8.0 Hz, 1 H). ¹³C NMR (100 MHz, CDCl3): δ = 28.1 (3 ×), 30.0, 55.2, 74.1, 84.5, 96.0, 115.3, 123.9, 124.6, 126.2, 126.4, 128.9, 130.2, 139.4, 149.4, 172.5.

15

Chiral HPLC analysis results: Chiralpak AD-H column, 0.46 × 25 cm, hexane-2-propanol (80:20), flow rate: 1.0 cm³/min, λ = 254 nm; t R (S-isomer) = 7.8 min; t R (R-isomer) = 9.5 min. See also ref. 4i.