Intramolecular alkylation of phenols. Part 5. A regiospecific anionic ring closure of phenols via quinone methides

Abstract

The bis-magnesium salts of the triols (6a) and (6h) cyclise when heated in benzene with high ortho-regiospecificity to the corresponding bis-phenols (7) and (8). The triols (6k) and (6o) under the same conditions cyclise with high para-regiospecificity to the corresponding bis-phenols (7) and (8). Both (6a) and (6h) cyclise via o-quinone methides, and (6k) and (6o)via p-quinone methides. Results from the use of, inter alia, 18-crown-6, indicate that the high ortho-regiospecificity of the cyclisation of (6a) and (6h) is due to intramolecular Mg(II) bridging of the intermediate o-quinone methides. The high para-regiospecificity of cyclisation of (6k) and (6o) is due to steric hindrance towards o-cyclisation of the intermediate p-quinone methides presented by the Mg(II) cation. The unexpected facility with which (6a) and (6k) undergo ring closure is discussed.