Dyotropic reaction
In type I rearrangements (Y-A-B-X conversion to X-A-B-Y), the two migrating groups are oriented trans to each other. The reaction leaves both groups on the opposite sides. The first example of a dyotropic rearrangement involving a carbon-carbon bond was reported by Cyril A. Grob and Saul Winstein. They saw the exchange of two bromine atoms in a certain steroid.