Jacobus A. Steenkamp

Phenolic metabolites from rooibos tea (Aspalathus linearis)

Abstract The processed leaves and stems of Aspalathus linearis contain hydroxylated benzoic and cinnamic acids, luteolin, chrysoeriol, quercetin, isoquercitrin, the C–C linked β- d -glucopyranosides based on four flavones and the dihydrochalcone aspalathin.

(S)- and (R)-Eriodictyol-6-C-ß-D-glucopyranoside, novel keys to the fermentation of rooibos (Aspalathus linearis)

The processed leaves and stems of Aspalathus linearis contain a new diastereomeric pair of the flavanones, (S)- and (R)-eriodictyol-6-C-beta-D-glucopyranoside, which is also formed via the oxidative cyclization of the dihydrochalcone, aspalathin, under conditions which mimic the fermentation process.

Phlobatannins, a novel class of ring-isomerized condensed tannins

Four members of a novel class of natural ‘phlobaphene’ condensed tannins, representing the products of stereospecific ring-isomerization of those 2,3-trans-3,4-trans-(–)-fisetinidol units present in ‘conventional’[4,8]-bi-and [4,6:4,8]-tri-flavanoid profisetinidins, are defined as functionalized 8,9-trans-9,10-cis-3,4,9,10-tetrahydro-2H,8H-pyrano[2,3-h]chromene (1) and the homologous hexahydrodipyrano[2,3-f,2′,3′-h]chromene (5) by nuclear Overhauser effect difference spectrometry.

Oligomeric flavanoids. Part 3. Structure and synthesis of phlobatannins related to (–)-fisetinidol-(4α,6)- and (4α,8)-(+)-catechin profisetinidins

Several members of the novel class of natural ‘phlobaphene’ condensed tannins, representing the products of c-ring isomerization of 2,3-trans-3,4-trans-(–)-fisetinidol units present in (4,6)- and (4,8)-biflavanoid profisetinidins, have been characterized by 1H n.m.r. n.o.e. difference spectroscopy. These include the functionalized 8,9-trans-9,1 0-cis-tetrahydropyrano[2,3-h]chromenes (9) and (12), and the [2,3-f]- and [2,3-g] regioisomers (14) and (19). Since the (4α,8)-biflavanoid (1) is subject to extensive base-catalysed rearrangement and epimerization, the protected 4-O-methyl ethers (E-ring)(2) and (6) were utilized to confirm the proposed structures of phlobatannins by stereospecific C-ring isomerization of (2) and (6) under basic conditions.

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.

Absolute configurations of flavan-3-ols and 4-arylflavan-3-olsvia a modified mosher's method

Abstract 1 H NMR Analysis of R-(+)-α-methoxy-α-trifluoromethylphenyl acetic acid (MTPA) esters of different sets of enantiomeric flavan-3-ols and 4-arylflavan-3-ols respectively, permits assessment of the absolute configurations at C-3 of these condensed tannin structural units.

Regio- and stereoselective oxidation of flavan-3-ol- 4-arylflavan-3-ol- and biflavanoid derivatives with 2,3-dichloro-56-dicyano-1,4-benzoquinone (DDQ)

Abstract The phenolic methyl ethers of flavan-3-ols, 4/gb-arylflavan-3-ols, and (−)-fisetinidol-(4β,8)-(+)-catechin biflavanoids are susceptible to regio- and stereoselective methoxylation at C-4 in moderate yields with DDQ in CHCl 3 -MeOH solution. The observed asymmetric induction with exclusive formation of 2,4- trans products is compatible with the intermediacy of a diastereogenic donor-acceptor interaction, DDQ acting as the acceptor and oxidant. The 4-funtionalized analogues are of both synthetic and degradative significance in condensed tannin chemistry.

Oligomeric flavanoids. Part 4. Base-catalysed conversions of (–)-fisetinidol-(+)-catechin profisetinidins with 2,3-trans-3,4-cis-flavan-3-ol constituent units

Additional novel members of the class of natural ‘phlobaphene’ condensed tannins, representing the products of C-ring isomerization of 2,3-trans-3,4-cis-(–)-fisetinidol units present in (4β,6)- and (4β,8)-biflavanoid profisetinidins have been characterized. These comprise the functionalized 8,9-trans-9,1 0-trans-3,4,9,10-tetrahydro-2H,8H-pyrano[2,3-h]chromenes (2) and (11), and 8,9-cis-9, 10-trans analogue (8), and a 6,7-cis-7,8-trans-[2,3-f]-regioisomer (27). Analogues (8) and (11) are prototypes of a unique class of phlobatannins in which the resorcinol A- and pyrocatechol B-rings are interchanged relative to their positions in the more common isomers. Their formation represents a novel rearrangement of profisetinidins with 2,3-trans-3,4-cis-flavan-3-ol units, e.g. (1) with concomitant inversion of absolute configuration at 3-C(C), under base catalysis. The proposed structures of the natural products were confirmed by synthesis via base-catalysed conversion of (–)-fisetinidol-(4β,8)- and (4β,6)-(+)-catechin O-methyl ethers (1) and (14).

Epiafzelechin-(4β→8, 2β→0→7)-ent-afzelechin from Cassipourea gerrardii

Abstract A new A-type proanthocyanidin has been isolated from the bark of Cassipourea gerrardii. Its structure has been established from spectroscopic studies as epiafzelechin-(4β→8, 2β→0→7)-ent-afzelechin. 1H NMR analysis of R-(+)- and S-(−)-α-methoxy-α-trifluoromethylphenylacetic acid (MTPA) esters of the title compound permits assessment of the absolute stereochemistry at both C-3(C) and C-3(F).

Tyrosinase catalysed biphenyl construction from flavan-3-ol substrates

Mushroom tyrosinase catalysed oxidation of three flavan-3-ols, viz. catechin, fisetinidol and mesquitol, was conducted to construct biphenyl bonds. Exposure of the flavan-3-ols to tyrosinase and subsequent trapping of the o-quinone intermediates resulted in the formation of novel flavan-3-ol derivatives, the structures of which were elucidated by mono- and two-dimensional 1H-NMR experiments. Application of the methodology resulted in the improved synthesis of the natural flavan-3-ol dimer, mesquitol-[5-->8]-catechin, previously isolated from Prosopis glandulosa.