Syn elimination is less favorable than anti elimination because the molecule must be in a higher-energy eclipsed conformation for syn elimination to occur. However, syn elimination does occur in rigid molecules where the leaving group and the hydrogen are held syn-coplanar.
An example is provided by the bicyclic compound shown below. This compound is very rigid and it is eclipsed about the bond between the carbons bonded to the red and blue hydrogens and the leaving bromine (green). In this case, 94% of the E2 product results from the loss of the blue H. Only 6% of the E2 product results from the loss of the red H, which has a dihedral angle of 120 degrees with the leaving group. This was established by replacing the blue H with deuterium, as described in the Elaboration on page 319. (Note that the large bromine causes the geometry of the compound to distort somewhat.)