Scheme 1.

The substrate scope of substituted benzaldehydes. Reaction condition: aryl aldehyde 1 (0.25 mmol), thiourea (0.5 mmol), H₂O (14 M), K₂S₂O₈ (0.5 mmol) and Py (5 equiv.) in DMF (1.5 mL) at 125 °C for 24 h in sealed tube. Isolated yields were given.