Conformational control of selectivity in the dienone–phenol rearrangement

We have explored the dienone–phenol rearrangement of substrates where: only the p-cresol pathway is possible and relative migratory aptitudes should play no role in determining the regiochemistry of the reaction. For these substrates the selectivity of the rearrangement was found to depend on the stereochemistry of the spirocyclic intermediate formed during the course of the rearrangement. Rearrangement of one of these substrates gave—surprisingly—a single regioisomeric product. Selectivity in this case can be correlated with the relative stability of cationic intermediates, which lie on the pathway between spirocycle and final product.