Synthesis of 5-methylidene-2-thio- and 2-selenohydantoins from isothiocyanates or isoselenocyanates and l-serine

3-Aryl-5-methylidenethiohydantoins and 3-aryl-5-methylidene-selenohydantoins were obtained by one-pot reaction of aryl isothiocyanates or isoselenocyanates with l-serine in NaOH–H₂O–Py mixture followed by room temperature HCl-promoted dehydration.