Regioselective Synthesis of Prenylisoflavones. Syntheses of 2,3- Dehydrokievitone and Related Compounds.

The palladium-catalyzed coupling reaction of 2′,4′,5,7-tetrakis(benzyloxy)-8-iodoisoflavone with 2-methyl-3-butyn-2-ol gave the corresponding 8-(3-hydroxy-3-methylbutynyl)isoflavone 8. Dehydration of the benzoate obtained from 8 in two steps gave a mixture of 8-prenylisoflavone 10 and its regioisomer [8-(3-methyl-3-butenyl)isoflavone]. Treatment of the mixture with aq. Hg(NO3)2 allowed the isolation of 10, which was hydrolyzed to give 2,3-dehydrokievitone. Similarly, 2,3-dehydrokievitone tetramethyl ether was also synthesized from 8-iodo-2′,4′,5,7-tetramethoxyisoflavone.