The hydrolysis of coumarin diethyl acetal and the lactonization of coumarinic acid ethyl ester. The partitioning of tetrahedral intermediates generated from independent sources

Abstract

The hydrolysis of coumarin diethyl acetal to coumarin proceeds via two detectable intermediates. A short-lived transient is observed in strongly acidic solutions (pH < 2.5); this is the oxocarbonium ion intermediate of the hydrolysis. That this cation can be detected suggests unusual stability, a fact which can be explained in terms of its pyrilium ion nature. A long-lived intermediate is also observed; kinetic and spectral evidence suggest that this is coumarinic acid ethyl ester. The lactonization of this ester shows a change in rate-determining step as the pH is varied. A corresponding change in products is found in the acetal hydrolysis, the coumarinic acid ester being the major product at high pH, with coumarin the major product at low pH. Both observations can be explained in terms of different modes of partitioning of cationic and neutral tetrahedral intermediates. Analysis in quantitative terms shows that the same tetrahedral intermediate is generated in the two different cases.