Synthesis of Optically Active Oxazolines by an Organocatalytic Isocyanoacetate Aldol Reaction with α-Keto Esters

Abstract

An enantioselective [3+2] cyclization is reported for the construction of a chiral oxazoline skeleton in moderate yield and up to 97% ee. The reactivity and stereochemical discrimination originate from the noncovalent interaction and orientation of a bifunctional catalyst. The novel combination of an α-keto ester and an α-isocyanoacetate establishes an oxazoline which could be a potential chiral ligand for metal-mediated catalysis, and also could be easily converted into an optically active β-hydroxy-α-amino acid.

Key words

organocatalysis - bifunctional catalyst - noncovalent interaction - asymmetric cyclization - chiral oxazoline

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• 16 General Procedure for the Synthesis of Chiral Oxazoline: Phenylglyoxylate (1; 0.1 mmol), tert-butyl isocyanoacetate (2d; 11.9 mg, 0.12 mmol) and bifunctional catalyst 3c (5.9 mg, 0.01 mmol) were stirred in toluene (0.5 mL) at 26 °C for 24 h. The mixture was separated by silica gel chromatography (10% EtOAc/petroleum ether) and gave product 4. Analysis data for compound 4a: ¹H NMR (400 MHz, CDCl₃): δ = 1.20–1.24 (m, 3 H), 1.52 (s, 9 H), 4.11–4.24 (m, 2 H), 4.88 (d, J = 2.0 Hz, 1 H), 7.14 (d, J = 2.0 Hz, 1 H), 7.31–7.56 (m, 2 H), 7.58–7.60 (m, 2 H). ¹³C NMR (100 MHz, CDCl₃): δ = 14.0, 28.0, 62.5, 79.7, 83.0, 89.4, 125.3, 129.0, 138.7, 155.5, 168.2, 168.6. HPLC (Chiralcel IB, hexane/i-PrOH = 95:5, flow rate: 1.0 mL/min, λ = 210 nm): t _R (major) = 18.08 min, t _R (minor) = 20.22 min. ESI–HRMS: m/z [M + H] calcd for C₁₇H₂₁NO₅: 320.1498; found: 320.1492.