the two cis double bonds. A trans double bond in the ring would be too strained. The reaction occurs under thermal conditions. The driving force of the reaction is the loss of strain from the cyclobutane ring. Cope rearrangement in biology There are a number of enzymes that catalyze the Cope rearrangement, although its occurrence is rare in nature. Oxy-Cope rearrangement and its anionic variant In the oxy-Cope rearrangement, a hydroxyl group is added at C3 forming an enal or enone after keto-enol tautomerism of the intermediate enol. In its original implementation, the oxy-Cope reaction required high temperatures. Subsequent work showed that the corresponding potassium alkoxides rearranged faster by 1010 to 1017. By virtue of this innovation, reaction proceed well at room temperature or even 0 °C. Typically potassium hydride and 18-crown-6 are employed to generate the dissociated potassium alkoxide: The diastereomer of the starting material shown above with an equatorial vinyl group does not react, providing evidence of the concerted nature of this reaction. Nevertheless, the transition state of the reaction is believed to have a high degree of diradical character. Consequently, the anion-accelerated oxy-Cope reaction can proceed with high efficiency even in systems that do not permit efficient orbital overlap, as illustrated by the synthesis periplanone B: The corresponding neutral oxy-Cope and siloxy-Cope rearrangements failed, giving only elimination products at 200 °C. Organocatalysis in the Cope Rearrangement The organocatalytic [3,3] sigmatropic rearrangement is based on a hydrazide catalyst. Variations Another variation of the Cope rearrangement is the heteroatom Cope reactions such as the aza-Cope rearrangements. Another widely studied [3,3] sigmatropic rearrangement is the Claisen rearrangement. Also see the divinylcyclopropane-cycloheptadiene rearrangement. References Rearrangement reactions Name reactions