Design of a specific oxidant for phenols.
Selective ortho-substitution of phenols can be secured, in principle, by attachment of the substituting reagent to the phenolic hydroxy group and subsequent rearrangement of this derivative by a cyclic mechanism into the orthoposition. So far, only one example of this principle (the Claisen rearrangement) is well established. For ortho-hydroxylation, a phenolic ester of selenium(IV) should have the desired properties. Diphenylseleninic anhydride, PhSE(=0)0.Sec(=0)Ph, has proved has proved to be the new reagent for the application of this mechanism. Several phenols, including models of the phenolic ring A of tetracycline, gave o-hydroxydienones, with quinones and phenylselenated species as by-products, when treated with diphenylseleninic anhydride. When the phenols were converted into their corresponding anions before treatment with the anhydridide, the o-hydroxydienones were obtained in good yield free from other products arising from reaction at the para position. When phenols were added to a warm solution of diphenylseleninic anhydride, they were oxidized to the o-quinones even when the phenols were not substituted in para position.