Direct oxidation of alkanoic acids and their amides to γ-lactones by peroxydisulphate-containing systems

Abstract

The reaction of one-electron oxidation of alkanoic acids (I) and their amides (VI) on treatment with Na₂S₂O₈-containing systems has been studied. As a result of the direct one-pot reaction acids (I) and amides (VI) are converted into γ- and δ-lactones, the reaction being regioselective and leading mainly to γ-lactones in up to 35% yield. The regioselectivity of the oxidative lactonisation depends greatly on the nature of alkyl substituents. The results obtained are presented in terms of a mechanism suggesting the generation of acyloxyl and amidyl radicals from (I) and (VI), respectively, followed by a rearrangement via a 1,5- or 1,6-H shift into the corresponding 3- and 4-carboxy- or -carboxamido-alkyl radicals. The latter undergo oxidative cyclisation to produce γ- and δ-lactones. The system Na₂S₂O₈–NaCl–NaOH converts carboxamides (VI) into amines with loss of one carbon atom as a result of a Hoffmann type rearrangement.