Regioselective synthesis of prenylisoflavones. Syntheses of allolicoisoflavone A, 2,3-dehydrokievitone, and related compounds

The palladium-catalyzed coupling reaction of 2',4',5,7-tetrakis(benzyloxy)- 5'-iodoisoflavone (12), synthesized from the 5-iodochalcone 9, with 2-methy l-3-butyn-2-ol gave the corresponding 5'-(3-hydroxy-3-methyl-1-butynyl)isof lavone 13. The catalytic hydrogenation of 13 gave 2',4',5,7-tetrahydroxy-5' -(3-hydroxy-3-methylbutyl)isoflavone (2). Dehydration of the benzoate 14 of 2 afforded a mixture of 5'-(3-methyl-2-butenyl)isoflavone 15 and the isome r 5'-(3-methyl-3-butenyl)isoflavone 16. The separation of 15 was accomplish ed by a treatment of the mixture (15 and 16) with mercury(ll) nitrate. Hydr olysis of 15 afforded 2',4',5,7-tetrahydroxy-5/-prenylisoflavone (allolicoi soflavone A) (1). In a similar manner, 2',4',5,7-tetrahydroxy-8-prenylisofl avone (2,3-dehydrokievitone) (3) and 2',4',5,7-tetrahydroxy-8-(3-hydroxy-3- methylbutyl)isoflavone (2,3-dehydrokievitone hydrate) (4) were synthesized from the corresponding 8-iodoisoflavone 22. The tetramethyl ether 5 of 3 wa s also prepared from the 8-iodotetramethoxyisoflavone 32.