Degradation pathway of carnosic acid in methanol solution through isolation and structural identification of its degradation products

Carnosic acid is the primary rosemary-derived phenolic diterpenes with greatest antioxidant activity, but it is rather unstable in methanol solution. Understanding the degradation pathway of carnosic acid and the interaction between these phenolic compounds is important if they are to be utilized to a greatest effect. In this study, the HPLC analysis of an aged methanolic solution of carnosic acid revealed that carnosic acid was degraded, and several compounds were produced in the solution. From the time-course and quantitative analyses of the formation of the products and their structural analysis, a novel degradation pathway of carnosic acid in methanol solution is proposed. Carnosic acid can firstly decompose into carnosic acid quinone in methanol, which was the first time to be confirmed the existence of carnosic acid quinone. Carnosic acid quinone is the quinone intermediate of carnosic acid that can be reduced into carnosic acid or can be further converted into carnosol. Carnosol can further decompose to form epirosmanol, rosmanol, 7-methoxyrosmanol, and 7-methoxy-epirosmanol. The latter three compounds were identified as the final degradation products of carnosic acid in methanol because they appeared rather stable in the study.