Electronic and Steric Effects in the Dienone-Phenol Rearrangement of 2-Hydroxy- and 2-Alkoxycyclohexa-2,5-dien-1-ones

A series of 4,4,6-trisubstituted-2-hydroxy- and -2-alkoxycyclohexa-2,5-dien-1-ones (7 and 8) were prepared, where the substituent at C-6 was H, CH₃, Ph, tert-butyl, or OEt. In the acid-catalyzed dienone-phenol rearrangement of 7 and 8, the C-4 substituent migrates regioselectively to C-5, completely shunning the enol double bond, even though the substituents at C-6 are substantially larger than the OH or OMe groups situated at C-2. The C-5 regioselectivity in hydroxy dienones 7a-f and 8a,b, as well as the decreased rate of reaction in the case of dienone 7g, can be simply rationalized by considering the relative electron density at C-3 vs C-5 in the protonated form of 7 or 8. Our results clearly indicate that the regioselectivity of the dienone-phenol rearrangement in these enolic systems is completely controlled by the electronic factors, which far outweigh any steric considerations.