Synthesis of 3-Oxazolines via SnCl₄-Promoted Formal [3+2] Cycloaddition of Donor–Acceptor Oxiranes and Nitriles

Abstract

A novel SnCl₄-promoted formal [3+2] cycloaddition of donor–acceptor oxiranes and nitriles via chemoselective C–C bond breakage was developed. This reaction provides an efficient and practical method for the preparation of 3-oxazolines in good yields (up to 89%).

Key words

synthesis - 3-oxazoline - formal [3+2] cycloaddition - oxirane - nitrile

• References and Notes

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• 12 Dimethyl 4-Phenyl-2-(p-tolyl)oxazole-5,5(2H)-dicarboxylate (3aa) – Typical Procedure To a mixture of dimethyl-3-p-tolyloxiranyl-2,2-dicarboxylate (1a, 0. 25 g, 1 mmol, 1 equiv) and benzonitrile (2a, 0.52g, 5 mmol, 5 equiv) was added SnCl₄ (0.12 mL, 1 mmol, 1 equiv) at 0 °C. The reaction mixture was then stirred at r.t. until the starting materials were consumed as indicated by TLC analysis. The mixture was poured into 50 mL aq sat. NaHCO₃ solution and extracted with CH₂Cl₂ (3 × 25 mL). The combined organic layer was rinsed with H₂O, brine, and dried over anhydrous MgSO₄. The solvent was evaporated off to get a residue which was then purified by flash chromatography to give pure product 3aa (269 mg, 76%). ¹H NMR (500 MHz, CDCl₃): δ = 8.11 (d, J = 7.8 Hz, 2 H), 7.54 (t, J = 7.5 Hz, 1 H), 7.46 (t, J = 7.5 Hz, 2 H), 7.19 (d, J = 7.9 Hz, 2 H), 7.11 (d, J = 7.9 Hz, 2 H), 6.15 (s, 1 H), 3.90 (s, 3 H), 3.20 (s, 3 H), 2.31 (s, 3 H). ¹³C NMR (125 MHz, CDCl₃): δ = 167.6, 166.0, 162.9, 138.2, 133.5, 132.2, 128.9, 128.9, 128.6, 128.0, 126.6, 90.7, 76.6, 53.8, 52.5, 21.3. HRMS: m/z calcd for C₂₀H₁₉NO₅ [M + H⁺]: 354.1336; found: 354.1330.
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