References
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1
1b
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1c See also: Huisgen R.
Jakob F.
Liebigs Ann. Chem.
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1d For a recent review of asymmetric electrophilic α-aminations of carbonyl compounds, see: Greck C.
Drouillat B.
Thomassigny C.
Eur. J. Org. Chem.
2004,
1377
2a
Marigo M.
Juhl K.
Jørgensen KA.
Angew. Chem. Int. Ed.
2003,
42:
1367
2b For a related auxiliary-controlled α-amination reaction using a Ru catalyst, see: Lumbierres M.
Marchi C.
Moreno-Mañas M.
Sebastián RM.
Vallribera A.
Lago E.
Molins E.
Eur. J. Org. Chem.
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3a
List B.
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2002,
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3b
Bøgevig A.
Juhl K.
Kumaragurubaran N.
Zhuang W.
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Angew. Chem. Int. Ed.
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41:
1790
3c
Vogt H.
Vanderheiden S.
Bräse S.
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2448
4
Kumaragurubaran N.
Juhl K.
Zhuang W.
Bøgevig A.
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5a During the preparation of this manuscript, Jørgensen and co-workers reported a similar approach to α-aminations of β-keto esters using a quinidine-derived β-isocupreidine catalyst: Saaby S.
Bella M.
Jørgensen KA.
J. Am. Chem. Soc.
2004,
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5b With substrates 1 and 5, the enantioselectivities obtained with cinchonine were similar to those obtained by Jørgensen with di-tert-butyl azodicarboxylate for similar substrates; however, the reaction rates were much faster in the present study.
6
General Experimental Procedure. To a solution of the azodicarboxylate (32.0 mg, 0.105 mmol, 105 mol%) in CH2Cl2 (1 mL) at -25 °C was added the β-keto ester or β-keto lactone substrate (0.1 mmol, 100 mol%) and the catalyst (0.02 mmol, 20 mol%). The resulting yellow solution was stirred at this temperature until the yellow color of the azodicarboxylate faded and the reaction was judged complete by TLC. The reaction mixture was partitioned between CH2Cl2 (3-5 mL) and 0.5 M HCl (2 mL). The aqueous layer was extracted with CH2Cl2 (3 × 1 mL). The combined organic extracts were dried (Na2SO4) and concentrated to give the crude product which was typically >90-95% pure by 1H NMR but could be purified further by flash chromatography.
Basification of the aqueous layer (1 M NaOH) and extraction with CH2Cl2 afforded the recovered catalyst in quantitative yield and >90% purity.
7 The racemic samples were prepared either with KOAc catalyst or by a mixing the cinchonine- and cinchonidine-derived products.
8 All new compounds gave satisfactory analytical and spectral data. Selected characterization data:
N
,
N
′-Bis(benzyloxycarbonyl)-1-hydrazino-2-oxocyclo-pentanecarboxylic acid, ethyl ester (Table
[2]
, entry 3): Colorless glass, R
f
= 0.46 (1:1 MTBE-hexanes); [α]D +2.0 (c = 1.1, CH2Cl2, 88% ee); lit.2a [α]D +3.7 (c 1.1, CH2Cl2, 99% ee). IR (thin film): 3307, 3034, 2962, 1728, 1498, 1455, 1399, 1343, 1224, 1118, 1027, 742, 697 cm-1. 1H NMR (400 MHz, d
2-tetrachloroethane, 80 °C): δ = 7.35-7.29 (m, 10 H), 6.80 (s, 1 H), 5.14 (d, 4 H, J = 7.7 Hz), 4.18 (q, 2 H, J = 7.1 Hz), 2.68 (m, 2 H), 2.46 (m, 1 H), 2.34 (m, 1 H), 2.16 (m, 1 H), 1.95 (m, 1 H), 1.22 (t, 3 H, J = 7.1 Hz). 13C NMR (100 MHz, d
2-tetrachloroethane, 80 °C): δ = 205.6 (br), 167.4, 155.5, 155.2, 135.3, 135.1, 128.3, 128.26, 128.1, 128.06, 127.7, 127.6, 76.3, 68.5, 67.6, 62.1, 36.4, 33.1, 18.2, 13.7. The enantioselectivity was determined by HPLC (Daicel Chiralcel AS, 20% i-PrOH in hexanes, 1 mL/min, λ = 254 nm, τmajor = 17.2 min, τminor = 36.1 min.
N
,
N
′-bis(benzyloxycarbonyl)-2-hydrazino-2-acetyl-γ-butyrolactone (Table
[2]
, entry 10): Colorless glass, R
f
= 0.46 (1:1 MTBE-hexanes); [α]546 -0.3 (c = 3.0, CHCl3, 42% ee). IR (thin film): 3307, 3034, 1726, 1498, 1456, 1399, 1342, 1220, 1184, 1027, 744, 697 cm-1. 1H NMR (400 MHz, d
2-tetrachloroethane, 90 °C): δ = 7.40-7.29 (m, 10 H), 6.93 (s, 1 H), 5.22-5.11 (m, 4 H), 4.36 (dt, 1 H, J = 3.8, 8.9 Hz), 4.27 (q, 1 H, J = 8.1 Hz), 3.19 (ddd, 1 H, J = 3.8, 7.6, 13.3 Hz), 2.77 (m, 1 H), 2.30 (s, 3 H). 13C NMR (100 MHz, d
2-tetrachloroethane, 90 °C): δ = 196.7, 168.7, 155.5, 154.9, 135.0, 134.6, 128.3, 127.8, 127.7, 77.8, 69.1, 68.2, 66.4, 29.1, 24.1. HRMS (ESI+): m/z calcd for C22H22N2O7Na [M + Na]+: 449.1325; found: 449.1321, Δ = 0.8 ppm. The enantioselectivity was determined by HPLC (Chiralcel OD, 3% i-PrOH in hexanes, 1 mL/min, λ = 254 nm, τmajor = 106 min, τminor = 124 min.
9 In addition to 3, α-methyl, α-benzyl and α-allyl-substituted ethyl acetoacetates were also screened as substrates, all affording the products in good yields (70-85%) but poor enantioselectivity (<35% ee).
10 Compound 4 was readily prepared by transesterification from 1 (BnOH, 175 °C). See: Taniguchi M.
Koga K.
Yamada S.
Chem. Pharm. Bull.
1972,
20:
1438
11 Compounds 7 and 8 were obtained by acylation of γ-buty-rolactone [LDA, THF, -78 °C, then isobutyroyl chloride (7) or pivaloyl chloride (8)].
12
Brown HC.
Brewster JH.
Shechter H.
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1954,
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467
13
Hoffmann RW.
Chem. Rev.
1989,
89:
1841
14 Control experiments with 20 mol% 6-methoxyquinoline and quinoline showed that these alone are poor catalysts for the reaction. Presumably, both the basic amine moiety and the free OH group are required for effective catalysis. Studies are in progress to determine the full mechanistic picture of these reactions.