Annulated 1,3,5-triazine-2,4(1H,3H)-diones and -2,4(1H,3H)-dithiones (8 and 9) are synthesized by a 4pi+2pi cyclodimerization of heterocyclic heterocurnulenes (6 and 7). These heterocumulenes are generated from partially cyclic 1,3-diaza-1,3-butadienes (1a-c) by means of a 2pi+2pi cycloaddition reaction with phenyl isocyanate or phenyl isothiocyanate and followed by dissociation of the resulting cycloadducts (1,3-diazetidinones (3) and 1,3-diazetidithiones (4)).