Asymmetric Conjugate Addition of O-Benzylhydroxylamine to α,β-Unsaturated 3-Acyloxazolidin-2-ones Catalyzed by Sc(OTf)₃/i-Pr-Pybox Complex

Abstract

The asymmetric conjugate addition of O-benzylhydroxylamine to α,β-unsaturated 3-acyloxazolidin-2-ones was smoothly catalyzed by the Sc(OTf)₃/2,6-bis[(S)-4-isopropyloxazolin-2-yl]pyridine (i-Pr-pybox) complex in the presence of MS 4 Å to give the corresponding β-amino acids in good conversions with high enan­tioselectivities (up to 91% ee).

Key words

additional reactions - amino acids - asymmetric catalyst - lanthanides - scandium

References and Notes

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9

The bidentate-type substrate was known to produce a good enantioselectivity for the reaction using rare-earth Lewis acids, [14] and the corresponding product, the β-amino-3-acyloxazolidin-2-one derivative, could be easily transformed into more useful compounds.

11

The reason of this phenomenon was thought to be the failure of the regeneration of the complex between Sc(OTf)₃ and i-Pr-pybox at cold temperatures.

12

Typical Procedures (Table 2). Under a nitrogen atmosphere, to the suspension of Sc(OTf)₃ (10.34 mg, 0.021 mmol) and MS 4 Å (75 mg), which were predried at 180 °C for 3 h under reduced pressure, in CH₂Cl₂ (5 mL) was added pybox (1a, 6.95 mg, 0.023 mmol) in CH₂Cl₂ (3 mL) at 0 °C. After stirring for 0.5 h, a solution of 3-crotonoyloxazolidin-2-one (2, 63.61 mg, 0.41 mmol) in CH₂Cl₂ (3 mL) was added and stirred for an additional 0.5 h. Then, O-benzylhydroxylamine (3, 0.45 mmol), which was used as a CH₂Cl₂ solution (0.5 M, 0.9 mL, 0.45 mmol), was slowly added dropwise at the same temperature. After 1 h, the reaction was quenched with sat. aq Na₂CO₃ and the mixture was extracted with CH₂Cl₂. The combined organic layer was dried over Na₂SO₄. This organic layer was filtered and evaporated under reduced pressure. The residue was purified by recycling preparative HPLC (GPC column, CHCl₃ as eluent) to give the desired products. The enantioselectivity was determined by an HPLC analysis using a chiral column: Chiralpak AD (0.46 cm × 25 cm).
Compound 4a: ¹H NMR (300 MHz, CDCl₃): δ = 1.17 (d, 3 H, J = 6.45 Hz), 2.87 (dd, 1 H, J = 4.71 Hz, 16.4 Hz), 3.21 (dd, 1 H, J = 8.22 Hz, 16.4 Hz), 3.58-3.67 (m, 1 H), 4.19-4.33 (m, 2 H), 4.67 (s, 2 H), 5.77 (s, 1 H), 7.28-7.37 (m, 5 H). ¹³C NMR (300 MHz, CDCl₃): δ = 18.1, 39.4, 42.2, 52.9, 61.8, 76.3, 127.6, 128.2, 128.3, 137.6, 153.5, 171.9.
Compound 5a: ¹H NMR (300 MHz, CDCl₃): δ = 1.09 (d, 3 H, J = 6.45 Hz), 2.31 (dd, 1 H, J = 4.08 Hz, 15.2 Hz), 2.36 (dd, 1 H, J = 4.71 Hz, 15.8 Hz), 3.29-3.38 (m, 1 H), 4.52 (s, 2 H), 4.88 (s, 2 H), 5.39 (s, 1 H), 7.24-7.37 (m, 10 H), 9.08 (s, 1 H). ¹³C NMR (300 MHz, CDCl₃): δ = 17.7, 37.7, 52.7, 76.3, 77.9, 128.0, 128.3, 128.4, 128.6, 128.7, 129.1, 135.6, 137.3, 169.2.