The reported cycloaddition reaction of isocyanates and symmetric carbodiimides is reinvestigated in the presence of pyridine. The Huisgen zwitterion generated by the 1:1 addition of pyridine to arylsulfonyl isocyanate is trapped by dialkyl carbodiimide to obtain symmetrical 1,3-diazetidin-2-ones via a novel intramolecular rearrangement. 1,3-diazetidin-2-one - aza-β-lactam - carbodiimide - arylsulfonyl