Edward V. Brandt

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.

Synthesis of condensed tannins. Part 17. Oligomeric (2R,3S)-3,3′,4′,7,8-pentahydroxyflavans: atropisomerism and conformation of biphenyl and m-terphenyl analogues from Prosopis glandulosa(‘mesquite’)

(2R,3S)-2,3-trans-3′,4′,7,8-Tetrahydroxyflavan-3-ol [(+)-mesquitol], the predominant metabolite in the heartwood of Prosopis glandulosa, represents a putative precursor of a variety of oligomers, including conventional [4,6]- and [4,8]-biflavan-3-ols, a [1,6]-1,3-diarylpropylflavan-3-ol, [5,6]- and atropisomeric [5,8]-biphenyl-type biflavan-3-ols, and [5,6:5,8]-m-terphenyl-type triflavan-3-ols. Other participants in these condensations are mainly (+)-catechin, and also the flavan-3,4-diol analogue of (+)-mesquitol. Oligomeric structures were confirmed by biomimetic oxidative and acid-induced couplings, and by nuclear Overhauser effect difference spectroscopy. These applications enabled correction of previous structural assignments for atropisomeric [5,8]-(+)-mesquitol-(+)-catechins and [5,6:5,8]-bis-[(+)mesquitol]-(+)-catechins, and determination of their conformations.

O-(dihydrobenzofuranyl)-dibenzo-α-pyrones from Umtiza Listerana

Abstract The novel metabolites (2′S,3′R)-3,10-dihydroxy-9-O-(6′-hydroxy-2′-hydroxymethyldihydrofuran-3-yl)-dibenz-[b,d]-pyran-6-one and its 5′,6′-dihydroxy analogue are accompanied in the heartwood of Umtiza listerana by several known flavonoids and the parent 3,9,10-trihydroxydibenz-[b,d]-pyran-6-one. The latter, used for structural elucidation of the complex dibenzo-α-pyrones, was synthesized via a Hurtley condensation.

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).

Novel cyanomaclurin analogue from Peltophorum africanum

Abstract Several unusual condensed flavanoids and an affluence of familiar pyrano [3, 2- c ] [2] benzopyran-6-(2H)-ones are accompanied in the heartwood of Peltophorum africanum Sond. by a novel 6,12-metano-6H,12H-dibenzo [b, f] [1, 5] dioxocin related to cyanomaclurin.

Profisetinidin-type 4-arylflavan-3-ols and related δ-lactones.

Abstract The natural occurrence of the first profisetinidin-type 4-arylflavan-3-ols, 4α- and 4β-(2,4-dihydroxy-3-methoxyphenyl)-(−)-fisetinidols, and a related δ-lactone, 7,8,9,13-tetrahydroxy-2-(3,4-dihydroxyphenyl)-2,3- trans -3,4- cis -2,3-10-trihydrobenzopyrano[3,4- c ]-2-benzopyran-1-one in the heartwood of Peltophorum africanum is demonstrated. In the heartwood of Burkea africana the δ-lactone is accompanied by its 2-(3,4,5-trihydroxyphenyl)-analogue and the 3- O -galloyl ester of (−)-robinetinidol, the first gallate involving a 5-deoxy flavan-3-ol. The biogenetic implications of the coexistence of these novel metabolites are briefly discussed.

Novel base-catalysed rearrangements of (–)-fisetinidol-(+)-catechin profisetinidins with 2,3-trans-3,4-cis-flavan-3-ol constituent units

The (–)-fisetinidol-(4β, 8)-(+)-catechin-O-methyl ether (1) is subject to facile C-ring isomerizations in NaHCO3–Na2CO3 buffer solution to form a range of novel 8,9-cis-9,10-trans- and 8,9-trans-9,10-trans-3,4,9,10-tetrahydro-2H,8H-pyrano-[2,3-h]chromenes (2)–(5); analogues (4) and (5) are representative of an unique class of phlobatannins in which the resorcinol A- and pyrocatechol B-rings are interchanged.

Synthesis of isoflavanoid oligomers using a pterocarpan as inceptive electrophile

(3S)-2′,7-Dihydroxy-4′-methoxyisoflavan serves as bifunctional nucleophile at C-5′ and C-6, when condensed with the carbocation generated at C-11a of its (6aS, 11aS)-3-hydroxy-9-methoxypterocarpan analogue under mild acid conditions or by photolysis, to form isomeric (3S,4S)-[4,3′]- and -[4,6]-3,4-trans-bi-isoflavanoids, and hence the [4,3′ : 4,6]-tri-isoflavanoid homologue. Structural assignments are supported by an analogous series of condensations. Self-condensation of the pterocarpan generates both isoflavan-pterocarpan and ispflavan-isoflavene analogues in low yields. The [4,3′]-bi-isoflavanoid is identical with the first natural oligomer from Dalbergia nitidula.

Synthesis of condensed tannins. Part 20. Cycloconformations and conformational stability among derivatives of ‘angular’ tetraflavanoid profisetinidins

Conformational analysis of tridecamethyl ether tetra-acetates of two natural and three synthetic ‘tetra-meric’ profisetinidin condensed tannins based on nuclear Overhauser effect difference spectroscopy reveals overall ‘cyclic’ arrangements of flavanyl units in each despite mutual stereochemical and structural differences. The unique thermodynamic stability of their dominant (85–90% abundance) conformers is attributed to the combined effects of the relative configurations of constituent flavanyl units, to steric repulsion by functional groups ortho to interflavanyl bonds, and to steric inhibition of mobility about interflavanyl bonds due to partial overlap of terminal units.