Specific Deuteration in Patuletin and Related Flavonoids via Keto-Enol Tautomerism: Solvent- and Temperature-Dependent H-1-NMR Studies

An H/D exchange process in patuletin (1) and its derivatives in D-donor solvents (e.g., CH3COOD), which occurs regioselectively at C(8) was observed for the first time during NMR studies. The effect of substituents and temperature on the deuteration of various flavonoids (see Fig. l) which include apigenin, chrysin, galangin, kaempferol. luteolin, morin, myricetin, patuletin, patulitrin, and quercetin, as well as derivatives of patuletin was examined extensively under MAR conditions. The rate constant of deuteration at C(8) of patuletin (1) and two flavones, luteolin (3) and apigenin (12), was also determined in CF3COOD. The D-atom was introduced into the flavonoids via a keto-enol tautomerism (Scheme 1). During these studies, monodeuterated patuletin was also obtained as a new compound. The examined flavonoids have been reported to possess significant pharmacological activities, and their deuterated derivatives would be of importance for the identification and quantification of these compounds in biological matrices.