An efficient synthesis of the potent phytoestrogens 8-prenylnaringenin and 6-(1,1-dimethylallyl)naringenin by europium(III)-catalyzed Claisen rearrangement

Starting from commercially available naringenin (3), the flavanoids 8-prenylnaringenin (1) and 6-(1,1-dimethylallyl)naringenin (2) have been prepared in racemic form using prenyl ether 5 as a general intermediate. While a domino Claisen–Cope rearrangement of 5 was the key step in the synthesis of 1, the cytotoxic compound 2 was additionally secured via a europium(III)-catalyzed Claisen rearrangement of 5 at low temperature. Both 1 and 2 display strong estrogenic activities.