Scheme 3. Synthesis of the 4,5-Diarylisoxazole-3-carboxylic Acids 1–4.

Reagents and conditions: (i,) NaOH, MeOH, THF, rt, 1 h (1, 65%; 2, 60%; 3, 50%; 4, 55%); (ii) NBS, TFA, heat under reflux 48 h (64%); (iii) ArB(OH)₂, Na₂CO₃, DMF, bis(triphenylphosphine)palladium(II) dichloride or Pd(Ph₃P)₄, 90 °C, 1–3 h (16, 80%; 17, 42%; 18, 49%).