Biflavanoids

The direct preparation of arylboronic esters from aryl halides or triflates now allows a one-pot, two-step procedure for the synthesis of unsymmetrical biaryls (Scheme 1-41) [147]. The synthesis of biaryls is readily carried out in the same flask when the first coupling of the triflate with diboron 82 is followed by the next reaction with another triflate. The synthesis of naturally occurring biflavanoids and the couphng of N-(phenylfluorenyl)amino carbonyl compounds to polymeric supports are reported [154]. 

Biflavanoids in vegetables and fruits play a vital role in the storage of ascorbic acid in leukocytes, in the core of adrenal glands and in other organs. It also causes more effective expenses under ascorbic acid s deficiency in the organism. Phenolic compounds present in the organism, activate detoxication processes in the liver. The flavanoids exceed the tocopherols and carotenoids in terms of antioxidant effect. 

Kumar, V., Brecht, V., and Frahm, A.W., Conformational analysis of the biflavanoid GB2 and a polyhydroxylated flavanone-chromone of Cratoxylum neriifolium, Planta Med., 70, 646, 2004. 

Ullman reaction conditions have also been used in the synthesis of biflavanoids. Heating of 3-haloflavones in the presence of copper powder provided biflavones in good yield <1993SC1075>. Isoflavone and derivatives, for example 425, can be prepared efficiently by the Suzuki cross-coupling of 3-bromo and 3-iodochromones 424 with arylboronic acids (Scheme 71) <1988BCJ3008, 1989CPB529>. 

D-ring to bulky flavanyl electrophiles is demonstrated (IS) by substitution of the phlobatannin (12) by the flavan-3,4-diol (+)-mollisacacidin under mildly acid conditions to give the [4,6]-(-)-fisetinidol derivatives (17) and (18). Differentiation between the entire range of biflavanoid and triflavanoid homologues was possible only via n.O.e. difference spectroscopy of their respective methyl ether acetates. 

F re 2. The structures of kolaflavanone, GB-1, GB-Ia and GB-2 biflavanoids from Garcinia kola. 

Rahman, M. Riaz, M. Desai, U. R., Synthesis of biologically relevant biflavanoids - A review, Chem Biodivers, 2007, 4, 2495-2527. 

Several [4,6]- and [4,8]-biflavanoids have been synthesized from flavan-... 

The natural occurrence and synthesis of a novel class of C-ring isomerized oligomeric flavanoids, termed phlobatannins, was demonstrated some twelve year ago (111, 112). These 3,4,9,10-tetrahydro-2if,8//-pyrano[2,3-/]chromenes, e.g. (104), are characterized by the liberated resorcinol moieties from the A/C-ring arrangement of the parent biflavanoid, e.g. (24), and by the conspicuous absence of the effects of dynamic rotational isomerism in the NMR spectra of their permethylaryl ether diacetates at ambient temperatures. 

Botha, J.J., D. Ferreira, and D.G. Roux Condensed Tannins Direct Synthesis, Structure, and Absolute Configuration of Four Biflavanoids from Black Wattle Bark ( Mimosa ) Extract. J. Chem. Soc., Chem. Commun., 700 (1978) Synthesis of Condensed Tannins. Part 4. A Direct Biomimetic Approach to [4,6]- and [4,8]-Biflavanoids. J. Chem. Soc., Perkin Trans. 1, 1235 (1981). 

Drewes, S.E., and A.H. Ilsley Dioxan-linked Biflavanoid from the Heartwood of Acacia mearnsii. J. Chem. Soc. (C), 1302 (1969). 

Foo, L.Y. Isolation of [4-0-4]linked Biflavanoids form Acacia melanoxylon. First Examples of a New Class of Single Ether Linked Proanthocyanidin Dimers. J. Chem. Soc., Chem. Commun., 1505 (1989). 

Some pigments of wood have also structural relationships to flavonoid compounds. Dracorubin (51) is a constituent of Dragon s blood , which is a commercially available resin from a palm tree, Daemonorops draco (Palmae), from Southeast Asia (82, 94). This resin also contains flavans, biflavanoids, antho-cyaninidins, triflavonoids, and chalcones. An unusual constituent is a seco-biflavonoid (52) (82). 

Owen P J, Scheinmann F 1974 Extractives from Guttiferae. Part XXVI. Isolation and structure of six xanthones, a biflavanoid, and triterpenes from the heartwood of Pentaphalangium solomonse Warb. J Chem Soc Perkin Trans I 1018-1021... 

Botha J J, Ferreira D, Roux D G 1978 Condensed tannins Direct synthesis, structure and absolute configuration of four biflavanoids from black wattle (mimosa) extract. J Chem Soc Chem Commun 700-702... 

Botha J J, Viviers P M, Ferreira D, Roux D G 1981 Synthesis of condensed tannins. Part 4. A direct biomimetic approach to (4,6) and (4,8) biflavanoids. J Chem Soc Perkin Trans I 1235-1244... 

Delcour J A, Tuytens G M 1983 A mass spectrometric criterion for determining the B- and E-ring hydroxylation pattern in dimeric biflavanoids. J Chem Soc Chem Commun 1195... 

Drewes S E, Ilsley A H 1969 Dioxane-linked biflavanoid from the heartwood of Acacia mearnsii. J Chem Soc (C) 897-900... 

Ilyas M, Seligmann O, Wagner H 1977 Biflavones from the leaves of Araucaria rulei F. Muell. and a survey on biflavanoids of the Araucaria genus. Z Naturforsch 32C 206-209... 

Lin K-M, Chen F-C 1974 On the dehydrogenation and rearrangement of biflavanoids. Chem J Chin Chem Soc (Taiwan) 3 67-73... 

Locksley H D 1973 The chemistry of biflavanoid compounds. Fortschr Chem Org Naturst 30 207-312... 

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