No abstract

Flavone (1) was eas ily re duced by us ing the elec tro chem i cal method to give two hydrodimers of 2,2 -biflavanone(racemate) (5a) and 2,2 -biflavanone(meso) (5b) and one reductive prod uct of flavanone (6). Their yields were de pend ent on the na ture of elec trodes, the kinds of support ing elec tro lytes and the re ac tion tem per a ture. They were found to af ford higher yields of 2,2 -biflavanone(racemate) (5a) and 2,2 -biflavan one (meso) (5b) (32.4% and 24.8%, 35.8% and 13.4%, re spec tively,) in the re ac tion con di tions of Pb(-)/C(+)-H 2 SO 4 -7F/mol and C(-)/C(+)-H 2 SO 4 -5F/mol. IN TRO DUC TIONBiflavonoids are widely dis trib uted in nat u ral plants. Baker-Venkataraman re ar range ment and cyclization, 5 cycliza tion from bichalcone, 6 ox i da tive cou pling 7 and reductive cou pling 8 were de vel oped for prep a ra tions of some bi flavonoids.Re cently, Pd-cathode re duc tion 9 and photoinduced elec tron trans fer re ac tion 10 were car ried out for trans for ma tion of flavones to biflavanones. In our con tin u ous study for syn thetic ap pli ca tion of biflavonoids, we re port herein syn the sis of 2,2 -biflavanones from fla vones via elec tro lytic reductive cou pling. RE SULTS AND DIS CUS SIONo-Hydroxyacetophenone (1) un der went an esterification with benzoyl chlo ride in the pres ence of pyridine to give o-benzoyloxyacetophenone (2) with a yield of 69.6%. oBenzoyloxyacetophenone (2) fol lowed a base-catalyzed Fries re ar range ment to con vert into o-hydroxy dibenzoylmethane (3) with a yield of 72.8%. Finally, flavone (4) was ob tained from cyclization of o-hydroxydibenzoylmethane (3) in the pres ence of acid with a yield of 32.0%, as shown in Scheme I. 11The elec tro chem i cal re duc tion of flavone (4) was carried out by us ing the H-type cell with a glass-filter di a phragm equipped with a se ries of elec trodes and meth a nol with sul furic acid or p-toulenesulfonic acid used as sup port ing elec trolytes. Flavone (4) was found to be eas ily re duced to yield two dimers of 2,2 -biflavanone(racemate) (5a) and 2,2 -bi flavanone(meso) (5b) and one reductive prod uct of flavanone (6), as shown in Scheme II.As shown in Ta ble 1 and Ta ble 2, yields were largely affected by the na ture of elec trodes, the kinds of sup port ing Jour nal of the Chi nese Chem i cal So ci ety,