Formation and Hydrolysis of Lactones of Phenolic Acids

Article abstract of DOI:10.1021/ja00534a041

The lactonization of 2'-hydroxybiphenyl-2-carboxylic acid to 3,4-benzocoumarin has been studied in aqueous solution, 30 deg C, in the range of pH 0-6.5.As with simpler coumarinic acids, the rate-determining step changes from the breakdown of tetrahedral intermediates in the neutral or weakly acidic pH range to formation of intermediates below pH 2.In contrast, the lactonization of methyl-substituted dihydrocoumarinic acids shows no eviednce for a change in rate-limiting step over a wide pH range.The hydrolysis of 8-hydroxy-1-naphthoic acid lactone in the range of pH 0-10 also does not undergo a transition in rate-limiting step.These results suggest that conjugation of the phenolic oxygen with the carbonyl carbon atom through an extended double-bond system may be necessary for the expulsion of water from a cationic tetrahedral intermediate and hence for the change in the rate-determining step.Revision of the rate constants for the acid-catalyzed lactonization of dihydrocoumarinic acid and of 4,4,5,7-tetramethyldihydrocoumarinic acid indicates that the rate-enhancing effect of the "trimethyl lock" is significantly smaller than previously believed.