Abstract: Daidzein and genistein were derived from the demethylation of formononetin and biochanin A in KBr-H3PO4 or KI-H3PO4 solution, respectively. The antioxidant activities of daidzein and genistein were also evaluated. The optimal conditions for preparing daidzein in KBr-H3PO4 solution were determined as follows: reaction time 5 h, reaction temperature 120 ℃, ratio of solid to solvent 1:45 (g/mL), and degree of saturation 75%, providing maximum reaction yield of 89.42%, while those for preparing genistein were 5 h, 120 ℃, 1:45 (g/mL), and 100%, providing maximum reaction yield of 85.98%; the optimal conditions for preparing daidzein in KI-H3PO4 solution were determined as 4 h, 100 ℃, 1:15 (g/mL) and 75% for reaction time, temperature, ratio of solid to solvent and degree of saturation, respectively, providing maximum reaction yield of 87.03%, while those for preparing genistein were 5 h, 100 ℃, 1:45 (g/mL) and 75%, respectively, providing maximum reaction yield of 88.51%. Compared with the traditional synthesis methods, the proposed method in this study had many advantages such as simpler reaction process, milder reaction conditions, shorter reaction time and higher yield. The daidzein and genistein had 1,1-diphenyl-2-picryl-hydrazyl and hydroxyl radical scavenging ability and a potent inhibitory effect on linoleic acid oxidation suggesting good antioxidant activity.

Key words: demethylation, daidzein, genistein, formononetin, biochanin A