Thermodynamic Stability of Flavylium Salts as a Valuable Tool To Design the Synthesis of A-Type Proanthocyanidin Analogues

A convenient method to synthesize A-type proanthocyanidin analogues from flavylium salts and pi-nucleophiles has been developed. It was found that the thermodynamic stability of the starting flavylium salt, assessed by the measurement of the apparent acidity constant (K'(a)), was the key parameter to design effective one-pot reactions between flavylium salts and nucleophiles such as phloroglucinol and (+)-catechin. When flavylium salts have a pK'(a) value of 1.7 or lower, the synthesis of the corresponding 2,8-dioxabyciclo [3.3.1]nonane derivative was properly achieved.