The structure of gauche-butadiene. The gauche refers to the conformation about the central C-C single bond. In pericyclic reactions it is customary to write the butadiene in a way that suggests that the two double bonds are on the same side of the central double bond. We already saw that the best structure of butadiene is the trans-structure. Now, observe that the best cis-like structure, is the gauche-structure shown.