Epoxidation of a substituted cyclohexene affords a product where the R group resides in the pseudo-equatorial position. Nucleophilic ring-opening of this class of epoxides can occur by an attack at either the C1 or C2-position. It is well known that nucleophilic ring-opening reactions of these substrates can proceed with excellent regioselectivity. The Fürst-Plattner rule attributes this regiochemical control to a large preference for the reaction pathway that follows the more stable chair-like transition state (attack at the C1-position) compared to the one proceeding through the unfavored twist boat-like transition state (attack at the C2-position). The attack at the C1-position follows a substantially lower reaction barrier of around 5 kcal mol depending on the specific conditions.  Similarly, the Fürst-Plattner rule applies to nucleophilic additions to imines and halonium ions.