Synthesis of Oxazolidin-2-ones by Tandem Cyclization of Propargylic Alcohols and Phenyl Isocyanate Promoted by Silver Catalysts as π-Lewis Acids

Abstract

Highly Z-selective syntheses of oxazolidin-2-ones from propargylic alcohols containing internal alkynes and phenyl isocyanate were achieved by using a combination of silver acetate and N,N-dimethylaminopyridine. The catalytic system was applied to propargylic alcohols containing alkyl-substituted alkyne groups. By considering the results in the presence and absence of an electron-withdrawing group on the aromatics, it was shown that the silver catalyst effectively activates the C≡C triple bond by acting as a π-Lewis acid to produce the corresponding oxazolidinones with high Z-selectivities.

Key words

silver - catalysis - cyclizations - oxazolidinones - tandem reactions - isocyanates

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• 6 (4Z)-4-Benzylidene-5,5-dimethyl-3-phenyl-1,3-oxazolidin-2-one; Typical Procedure A mixture of AgOAc (6.7 mg, 10 mol%) and DMAP (14.7 mg, 30 mol%) in 1,2-Cl₂C₆H₄ (0.5 mL) was stirred for 15 min under N₂, A solution of the propargylic alcohol 1a (64.1 mg, 0.4 mmol) in 1,2-Cl₂C₆H₄ (0.75 mL) was added, followed by a solution of PhNCO (65 μL, 1.5 equiv) in 1,2-Cl₂C₆H₄ (0.75 mL), and the mixture was stirred at 40 °C for 9 h. The reaction was quenched with H₂O, and the mixture was extracted with EtOAc (3 ×). The organic layers were combined, washed with brine, and dried (Na₂SO₄). The E/Z ratio was measured by GC analysis of the crude mixture. The solvent was then removed under reduced pressure, and the residue was purified by column chromatography (silica gel, hexane–EtOAc) to give oxazolidinone 3a as colorless solid; yield: 105.4 mg (94%); ¹H NMR (400 MHz, CDCl₃): δ = 1.73 (s, 6 H), 5.63 (s, 1 H), 6.66 (d, J = 7.3 Hz, 2 H), 6.82–6.92 (m, 3 H), 6.98–7.06 (m, 5 H).
• 7 It is also possible that isomerization of the vinylsilver intermediate produced the (E)-oxazolidinone, but the detail remains unclear.