Phosphoric Acid Systems, Part 8. An Improved Procedure for the Phenol-Dienone Rearrangement of Sterically Hindered Phenols

Abstract

A ready entry into the field of polyfunctional cyclohexa-2,5-dienones involves the reaction of sterically hindered phenols with electrophilic reagents.^2–6 Considerable effort has been expended in optimizing the conditions for this reaction^5,6 but no general procedure exists. Furthermore, a difficulty commonly encountered is the displacement of a group sensitive to acid.^5–7 For instance, loss of a t-butyl group during the bromination of t-butyl substituted phenols 1a and 1b to cyclohexadienones 2a and 2b, caused by the liberated hydrogen bromide, has been noted by a number of workers.^5–7 Although methods exist to circumvent this difficulty,^2–6 we have developed a general procedure for the decomposition of the hydrogen halide produced during the halogenation reaction particularly useful with substrates possessing an acid labile group. The method utilizes trimethyl phosphate (TMP) both as the halogenating medium and as the hydrogen halide acceptor and is based on the propensity of TMP to hydrolyze int he presence of hydrogen halide, liberating methly halide.