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DOI: 10.1055/s-0029-1217347
Enantioselective Protonation in the Aza-Michael Reaction Using a Combination of Chiral Pd-µ-Hydroxo Complex with an Amine Salt
Publication History
Publication Date:
02 June 2009 (online)
Abstract
A highly enantioselective protonation of enolate intermediates in aza-Michael reaction was achieved by using the combination of a bifunctional chiral Pd-µ-hydroxo complex with aromatic amine salts. The reaction proceeded smoothly to give the desired β-amino carbonyl compounds bearing a stereogenic carbon center at the α-position in good yield with excellent enantioselectivity (up to 97% ee). Although reactions with salts of electron-deficient amines were slow, the introduction of free amine as an additive promoted the reaction to a synthetically useful level.
Key words
asymmetric catalysis - protonations - palladium - aza-Michael additions - β-amino acids
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References and Notes
General Procedure
The
starting material 9 (0.1 mmol), amine salts 7 (0.15 mmol), and the Pd complex 1 (5 mol%) were dissolved in THF
(0.2 mL). In the case of 7c, the additive 15 (0.05 mmol) was included. The resulting
solution was stirred at ambient temperature for the time shown in
Tables
[¹]
-
[³]
. For quench-ing, cold
sat. aq NaHCO3 (2 mL) was added under ice-bath cooling.
Usual workup, followed by flash column chromato-graphy (Si2O,
hexane-EtOAc system) gave the pure products.
Analytical Data of 10ca
¹H
NMR (400 MHz, CDCl3): δ = 3.38 (dd, J = 5.5, 13.0
Hz, 1 H), 3.74 (s, 3 H), 3.84 (dd, J = 8.7,
13.0 Hz, 1 H), 4.48 (br s, 1 H), 5.08 (s, 2 H), 6.58 (d, J = 8.8 Hz,
2 H), 6.78 (d, J = 8.8
Hz, 2 H), 7.26-7.39 (m, 10 H), 7.55 (br s, 1 H).
¹³C
NMR (100 MHz, CDCl3): δ = 48.0, 51.2,
55.7, 67.9, 114.7, 115.0, 128.0, 128.4, 128.5, 128.6, 128.7, 129.1, 134.7,
136.0, 141.2, 150.6, 152.5, 172.4. LRMS-FAB (mNBA): m/z = 404 [M+],
405 [M + H]+. HRMS
(PEG 400/mNBA): m/z calcd for C24H24N2O4 [M]+ 404.1736;
found: 404.1739. [α]D
²5 +64.6
(c 0.82, CHCl3; 93% ee).
HPLC (DAICEL CHIRALPAK AD-H, hexane-2-PrOH = 3:1,
1.0 mL/min, 254 mn): t
R(minor) = 17.2
min, t
R(major) = 20.9 min.