A class of heterocyclic compounds (I) which has contributed a certain amount of interest in connection with structural requirements is probably best known by the special case, the sydnones. The proposed meso-ionic representation (Ia) and the more recently suggested use of a coordinate covalent bond between the two ring nitrogen atoms in sydnones (Ib) 3 have indicated the complex nature of this ring system.
![[(2-methylcyclohexyl)amino]urea Structure](https://image.chemsrc.com/caspic/159/6635-49-0.png)
