Barend C. B. Bezuidenhoudt

Flavonoid analogues from Pterocarpus species

Abstract Two novel pterocarpans, 8-hydroxy-3,9-dimethoxy- and 3,8-dihydroxy-9-methoxypterocarpan and a new santal analogue were isolated from the heartwood of P. soyauxii. These are accompanied by several known pterocarpans, isoflavans, isoflavones and trans-pterostilbene. From the heartwood of P. marsupium were obtained 8-C-β- d -glucopyranosyl-3,7,4′-trihydroxy- and -3,7,3′,4′-tetrahydroxyflavone, representative of the first 5-deoxy C-C-coupled flavonol glucosides, and the rare 3′-C-β- d -glucopyranosyl-α-hydroxydihydrochalcone, their structures being determined by means of high resolution NMR techniques.

The first enantioselective synthesis of poly-oxygenated α-hydroxydihydrochalcones and circular dichroic assessment of their absolute configuration

Epoxidation of 4-methoxy-2′,4′-dimethoxymethyl-(E)-chalcone with H2O2 in the presence of poly-α-aminoacid catalysts afforded chiral aromatic oxygenated chalcone epoxides. These were transformed into the corresponding α-hydroxydihydrochalcones and their absolute configurations determined by c.d. spectroscopy. One of these was proved to be identical to a novel αR-analogue from Pericopsis elata.

Conjugate addition of benzyl copper reagents to α,α-enoates and -enones.

Abstract Several benzylic copper reagents, BnCu(CN)MgCl, Bn 2 CuMgCl, BnCu-TMSCl-HMPA, and BnCu-TMSCl-TMEDA, facilitate the conjugate addition of the benzyl ligand to α,β-enones, but only BnCu-TMSCl-TMEDA gave high yields with α,β-unsaturated esters.

The first natural condensed tannins with (−)-catechin ‘terminal’ units

Abstract The functionalized 2,3- trans -8,9- trans -9,10- cis -3,4,9,10-tetrahydro-2 H ,8 H -pyrano[2,3- h ]chromenes 4 and 6 represent the first naturally occurring condensed tannins with (2 S ,3 R )-2,3- trans -(−)-catechin ‘terminal’ moieties; the structure of 4 is unequivocally confirmed by synthesis.

α-hydroxydihydrochalcones and related 1,3-diarylpropan-2-ones from Xanthocercis zambesiaca

Abstract In addition to some previously described 7,8-dioxy-isoflavonoids and 2-benzylbenzo[ b ]furan-3 (2H)-ones the heartwood of Xanthocercis zambesiaca has been shown to contain 7,8,3′-trihydroxy-4′-methoxyisoflavone. These are accompanied by the novel α,3,4,4′-tetrahydroxy-2′-methoxydihydrochalcone and its 3-deoxy analogue, the latter of which presumably served as biogenetic precursor for the unique 1-hydroxy-3-(4-hydroxyphenyl)-1-(4-hydroxy-2-methoxyphenyl) propan-2-one and its 1-deoxy analogue. Such possible biosynthetic relationship was demonstrated by the facile in vitro transformations of the appropriate α-hydroxydihydrochalcone into the different 1,3-diarylpropan-2-ones.

Dispiroketals in synthesis (Part 13): Functionalised dispiroketals as new chiral auxiliaries; highly stereoselective diels-alder reactions using a bifunctional, C2- symmetrical chiral auxiliary☆

Abstract Highly selective, asymmetric, Lewis acid catalysed Diels-Alder reactions are reported with a bifunctional, C 2 -symmetrical diacrylate derivative obtained from a chiral auxiliary based on dihydroxylated dispiroketals.

Regio- and stereoselective oxygenation of flavan-s-ol-, 4-arylflavan- 3-ol-, and biflavanoid-derivatives with potassium persulphate

Abstract The phenolic methyl ethers of flavan-3-ols, 4-arylflavan-3-ols, and (-)-fisetinidol-(4,8)-( + )-catechin biflavanoids are susceptible to regio- and stere- oselective hydroxylation at C-4 in moderate to high yields with potassium persul- phate/cupric sulphate in aqueous acetonitrile. The resultant 4-functionalized analogues are of both synthetic and degradative significance in condensed tannin chemistry.

Dispiroketals in synthesis (Part 12): Functionalised dispiroketals as new chiral auxiliaries; the synthesis of dihydroxylated dispiroketals in optically pure form☆

Abstract A range of rigid, architecturally complex diols derived from dispiroketals have been synthesised. A bifunctional, C2-symmetrical chiral auxiliary derived from these diols can be used in asymmetric synthesis.

Oxidation and Rearrangement Reactions of Condensed Tannins

The chemistry of condensed tannins at alkaline pH is regulated by the formation of A- and/or B-ring quinone-methides as highly reactive intermediates. The former group of electrophilic species is of special significance in analogues with 5,7-dihydroxy A-ring functionality and may be successfully trapped by external nucleophiles such as phenolic nuclei and thiols. B-ring quinone-methides may similarly be trapped but are preferentially susceptible to intramolecular trapping to give analogues with rearranged pyran heterocycles. A mechanistic rationale to explain the base-catalyzed rearrangement reactions and the oxidative conversion of B- to A-type proanthocyanidins is proposed. These reactions are of relevance to the industrial application of condensed tannins.

Oligomeric isoflavonoids. Part 1. Structure and synthesis of the first (2,3′)-isoflavone-isoflavan dimer

The structure and stereochemistry of a novel [2,3′]-isoflavone-isoflavan oligomer, 2′,7-dihydroxy-2-[(3S)-6′,7-dihydroxy-4′-methoxyisoflavan-3′-yl]-4′-methoxyisoflavone (1; R = H), from Dalbergia nitidula are established by synthesis. The strategy of condensation of a C16 to C14 moiety, i.e. C-5′ formylated isoflavan (21) to a 2-hydroxydeoxybenzoin (23) in a modified Baker-Venkataraman approach, should provide general access to this novel class of C-2 coupled oligomeric Isoflavonoids.