On the Reaction of Oxalic Amidines with Carbonic Acid Derived Heterocumulenes to Imidazolidine‐DerivativesA useful synthetic route to nonsymmetric derivatives of imidazolidine (derivatives of parabanic acid) is demonstrated by the reaction of tetraarylated oxalic amidines1with different alkyl‐ and aryl substituted isothiocyanates4, isoselenocyanates31, and carbodiimides30.Acylisothiocyanates9react differently leading to a mixture of three cycloacylation products12,13and14. Silylated heterocumulenes such as25and26react with1to give28and29transsilylation. Based on the13C‐n.m.r. spectroscopic investigation of the reaction between1aand methyl isothiocyanate and phenyl isothiocyanate respectively a mechanism is pro