A revised mechanism for the cycioaddition of mesoionic dithioles with heterocumulenes

The reaction of a mesoionic 1,3-dithiolium-4-olate with phenyl isocyanate is not a 1,3-dipolar cycloaddition as the primary cycloadduct is an azetidinedione which probably results from the addition of phenyl isocyanate with the opened ketene form of the mesoionic compound.