Biflavanoid proguibourtinidin carboxylic acids and their biflavanoid homologues from Acacia luederitzii

[4,6]- and [4,8]-Proguibourtinidin carboxylic acids (3,7,4′-trihydroxyl functionality) of 2,3-trans-3,4-trans: 2,3-cis- and 2,3-trans-3,4-trans: 2,3-trans-configuration based on (?)-epicatechin or (+)-catechin as constituent units, and their associated biflavanoid homologues, predominate in the heartwood of Acacia luederitzii. They are accompanied by stereochemical and functional analogues and by their putative flavan-3,4-diol and flavan-3-ol precursors.     

8-O-methyl- and the first 3-O-methylflavan-3,4-diols from Acacia saxatilis

The light purple heartwood of Acacia saxatilis contains (+)-2,3-trans-3,4-trans- and (+)-2,3-trans-3,4-cis-diastereoisomers of 8-methoxy-7,j',4'-trihydroxy- and 7,3′,4′-trihydroxyflavan-3,4-diols as major components. Evidence was also obtained of the first 3-methyl ether of metabolites: of this type, notably of (+)-8-methoxy-7,3′,4′-trihydroxy-2,3-trans-flavan-3,4-cis-diol. Flavonol, dihydroflavonol and flavanone analogies accompany these. The correlation between colour of Acacia heartwoods and structure, phenolic substitution, stereochemistry and composition of their flavonoid components is discussed.     

The first 2,3-trans-3,4-cis procyanidin

Condensation of (+)-leucocyanidin with (+)-catechin under acidic conditions afforded the novel 2,3-trans-3,4-cis: 2,3-trans [4,8]-bi-[(+)-catechin] as the first representative of a 3,4-cis procyanidin unit.     

(+)-2,3-Trans-pubeschin,the first catechin analogue of peltogynoids from Peltogyne pubescens and P. venosa

The heartwoods of Peltogyne pubescens and P. venosa contain the predominant pair (+)-peltogynol and (+)-mopanol, their 4-epimers, (+)-peltogynol B and (+)-mopanol B, together with the first catechin analogue of peltogynol, (+)-2,3- trans-pubeschin. These are accompanied by ±-2,3-cis- and ±-2,3-trans-3-O-methylfustins, and by α, 2′,3,4,4′-pentahydroxychalcone. Other minor metabolises are 4′,7-dihydroxy- and 3′,4′,7-trihydroxy-flavanones and 5,6-dihydroxyphthalide. (+)-2,3-Trans-pubeschin trimethyl ether was synthesized by reduction of the corresponding (+)-2,3-trans-peltogynone analogue with NaBH₄/BF₃ in diglyme, and its absolute configuration shown to be 2R: 3S.     

Configuration of guibourtacacidin, and synthesis of isomeric racemates

1. Diacetates of the four possible racemates of 4′,7-dimethoxyflavan-3,4-diol have been synthesized. 2. Comparison of their nuclear-magnetic-resonance spectra and the ionophoretic mobilities of the diols in borate buffer with those of the corresponding derivatives of guibourtacacidin shows that the natural 4′,7-dihydroxyflavan-3,4-diol has a 2,3-cis–3,4-trans configuration, but is accompanied by 2,3-trans–3,4-trans and 2,3-trans–3,4-cis isomers. These occur in the approximate proportions 5:1:1. 3. The occurrence of guibourtacacidins in Guibourtia coleosperma appears to be of taxonomic significance. Their association with a large excess of related tannins in the heartwood suggests that flavan-3,4-diols with these configurations are suitable precursors in tannin biosynthesis.     

Chemical and enzymatic synthesis of monomeric procyanidins (leucocyanidins or 3′,4′,5,7-tetrahydroxyflavan-3,4-diols) from (2R,3R)-dihydroquercetin

The major product from the reduction of (2R,3R)-dihydroquercetin with sodium borohydride is the 2,3-trans-3,4-trans isomer of leucocyanidin [(2R,3S,4R-3,3′,4,4′,5,7-hexahydroxyflavan] whereas the enzymatic reduction product is the 2,3-trans-3,4-cis isomer [(2R,3S,4S)-3,3′,4,4′,5,7-hexahydroxyflavan]. The 3,4-trans isomer may be partly converted to the 3,4-cis isomer under mild acid conditions. The 3,4-cis isomer is more acid-labile, and more reactive both chemically with thiols and enzymatically with a diol reductase, than the 3,4-trans isomer.     

Natural and synthetic diastereoisomeric (?)-3′,4′,7-trihydroxyflavan-3,4-diols

1. Rhodesian copalwood (Guibourtia coleosperma) contains three diastereo-isomeric leuco-fisetinidins. These consist of the (−)-2,3-cis–3,4-cis (2R,3R,4R) and (−)-2,3-cis–3,4-trans (2R,3R,4S) 3′,4′,7-trihydroxyflavan-3,4-diols, and the third was shown to be a 2,3-trans–3,4-cis isomer by means of paper ionophoresis. 2. There occurrence in similar proportions as tannin precursors also in the tropical hardwoods G. tessmannii and G. demeusii implies a close taxonomic relationship between these, and with G. coleosperma. 3. Epimerization of the natural (−)-3′,4′,7- trihydroxy-2,3-trans-flavan-3,4-trans-diol affords a mixture from which the (−)-2,3-cis–3,4-cis isomer was separated readily, but the (−)-2,3-trans–3,4-cis isomer was obtained with difficulty. These were formed by epimerization of the (−)-2,3-trans–3,4-trans isomer at C-2 and C-4, and at C-4, respectively.     

Isolation and synthesis of trans- and cis-(−)-clovamides and their deoxy analogues from the bark of Dalbergia melanoxylon

N-(3′,4′-Dihydroxy-trans-cinnamoyl)-3-(3,4-dihydroxyphenyl)-L-alanine [(?)-clovamide], the major phenolic metabolite (0.1%) in the bark of Dalbergia melanoxylon, is associated with minor proportions of its cis-isomer, and similar pairs of geometrical isomers of their deoxy analogues N-(4′-hydroxycinnamoyl)-3-(3,4-dihydroxyphenyl)-L-alanine and N-(4′-hydroxycinnamoyl)-3-(4-hydroxyphenyl)-L-alanine. (?)-Trans-clovamide is synthesized by direct condensation of the acid chloride of caffeic acid with L-DOPA. Diagnostic CD spectra of these compounds and ¹³C spectra of (?)-trans- and (?)-cis-clovamides are recorded.     

Enzymic and nonenzymic reduction of (+)-dihydroquercetin to its 3,4,-diol

A NADPH-dependent reductase activity, capable of converting (+)-dihydroquercetin (2,3-trans) to its 3,4-diol (a leucocyanidin), has been demonstrated in crude, soluble protein extracts derived from cell suspension cultures of Douglas fir (Pseudotsuga menziessi). Neither NADH nor ascorbate substituted as the H-donor. Quantitative analyses were based on the production of cyanidin, the formation of an adduct with vanillin, and on absorbance at 280 nanometers. Nonenzymic reduction of (+)-dihydroquercetin with NaBH₄ produced two presumably isomeric flavan-3,4,-diols. One of these was similar to the enzymically produced diol, based on products isolated by chromatography on paper, on thin-layer cellulose and on C₁₈ reversed-phase columns (high performance liquid chromatography), and on the conversion of the diol to the all-trans dimer of (+)-catechin upon the addition of (+)-catechin.     

On the non-existence of eloxanthin

Abandonment of the name eloxanthin is proposed. The principal carotenoids in various species of Elodea were (3R, 3′R, 6′R)-lutein (β,ε-carotene-3, 3′-diol) and β, β-carotene. The minor pigments were neoxanthin-X (5′, 6′-epoxy-6, 7-didehydro-5, 6, 5′, 6′-tetrahydro-β, β-carotene-3, 5, 3′-triol), 9′-cis-neoxanthin- X, 9- and 13-cis-violaxanthin (5, 6, 5′, 6′-diepoxy-5, 6, 5′, 6′-tetrahydro-β, β-carotene-3, 3′-diol), antheraxanthin (5, 6-epoxy-5, 6-dihydro-β, β-carotene-3, 3′-diol), neolutein A (13- or 13′-cis-lutein) and neolutein B (9- or 9′-cis-lutein). All attempts to isolate eloxanthin failed.     

α-Hydroxychalcones as intermediates in flavonoid biogenesis: The significance of recent chemical analogies

α-Hydroxychalcones are shown to be prominent metabolites amongst the flavonoids of certain heartwoods. Their existence mainly in the keto-form confers properties which are presumably responsible for their previous oversight. Indications of the wider distribution of α-hydroxychalcones raises interesting speculation regarding their hitherto unconsidered role in the biogenesis of 3-hydroxyflavonoids. Cyclization involving the β-position of the trans-enolic isomer of α-hydroxychalcones to form both 2,3-cis- and 2,3-trans-dihydroflavonols, infers for the first time biosynthetic paths which lead by way of successive reductions to 2,3-cis- and 2,3-trans-diastereoisomers of flavan-3,4-diols and flavan-3-ols, and hence by condensation via carbonium ion or quinonemethide intermediates to polyflavonoid tannins. Alternative cyclization to the α-position, or more likely to the equivalent carbonyl of the keto-form, represents the only feasible mode of biogenesis of 2-hydroxy-2-benzylcoum aranones. Newly-established associations of the complete range of peltogynoid analogues in heartwoods, often with α-hydroxychalcones or their peltogynoid equivalent, permits similar conjecture.     

Condensed tannins. Optically active diastereoisomers of (+)-mollisacacidin by epimerization

1. (+)-Mollisacacidin [(+)-3′,4′,7-trihydroxy-2,3-trans-flavan-3,4-trans- diol] is converted by autoclaving into the optically active free phenolic 2,3-trans-3-4-cis (12% yield), 2,3-cis-3,4-trans (11%) and 2,3-cis-3,4-cis (2·8%) diastereoisomers through epimerization at C-2 and C-4. 2. The relative configurations of the epimeric forms were determined by nuclear-magnetic-resonance spectrometry and paper ionophoresis in comparison with synthetic reference compounds, and was confirmed by chemical interconversions. 3. From this a scheme of epimerization is inferred and their absolute configurations are assigned as (2R:3S:4S), (2S:3S:4R) and (2S:3S:4S) respectively from the known absolute configuration (2R:3S:4R) of (+)-mollisacacidin.     

Occurrence of 15-cis-violaxanthin in Viola tricolor

A new cis isomer in the violaxanthin series has been isolated from the blossoms of Viola tricolor and identified by MS, IR and UV as the central-monocis form. It was converted to all-trans-violaxanthin by stereomutation. The CD correlation between 15-cis-violaxanthin and natural violaxanthin (5,6,5′,6′-diepoxy-5,6,5′,6′-tetrahydro- β,β-caroten-3,3′-diol) provided the basis for assignment of the absolute configurations 3S, 5R, 6S, 3′S, 5′R, 6′S. Trans—cis isomerization of all-trans-violaxanthin also resulted in 15- cis-violaxanthin. In addition a quantitative determination of the carotenoids was conducted.     

Benzofuranoid neolignans from Aniba simulans

Forteen neolignans, isolated from the benzene extract of Aniba simulans (Lauraceae) trunk wood, included the hitherto undescribed (2S, 3S, 5R)-5-allyl-5,7-dimethoxy-2-(3′,4′,5′-trimethoxyphenyl)-3-methyl-2,3,5,6-tetra-hydro-6-oxobenzofuran, (2R,3S,5R) -5-allyl-5-methoxy-2-(3′-methoxy-4′,5′-methylenedioxyphenyl)-3-methy1-2,3,5, 6-tetrahydro-6-oxobenzofuran, (2S,3S)-6-O-allyl -5-methoxy-2-(3′-methoxy-4′-5′-methylenedioxyphenyl)-3-methyl-2,3-dihydrobenzofuran, (2R,3S)-6-O-allyl-5-methoxy-2- (3′-methoxy-4′,5′-methylenedioxyphenyl)-3-methyl-2,3-dihydrobenzofuran and 7-allyl-6-hydroxy-5-methoxy-2-(3′-methoxy-4,5′ -methylenedioxyphenyl)-3-methylbenzofuran.     

Isolation of 4′-dihydroabscisic acid from immature seeds of Vicia faba

4′-Dihydroabscisic acid [1′, 4′-cis-diol of (+)-ABA] was isolated from immature seeds of Vicia faba. Its identity was proved by TLC and MS.     

Formation of (-⊃-1′,2′-epi-2-cis-xanthoxin acid from a precursor of abscisic acid

Tomato shoots and avocado mesocarp supplied with (±)-[2-¹⁴C]-5-(1,2-epoxy-2,6,6-trimethylcyclohexyl)-3-methylpenta-cis-2-trans-4-dienoic acid metabolize it into (+)-abscisic acid and a more polar material that was isolated and identified as (?)-epi-1′(R),2′(R)-4′(S)-2-cis-xanthoxin acid. The (+)-1′(S),2′(S)-4′(S)-2-cis-xanthoxin acid recently synthesized from natural violaxanthin, has the 1′,2′-epoxy group on the opposite side of the ring to that of the 4′(S)-hydroxyl group and the compound is rapidly converted into (+)-abscisic acid. The 1′,2′-epoxy group of (?)-1′,2′-epi-2-cis-xanthoxin acid is on the same side of the ring as the 4′(S) hydroxyl group: the compound is not metabolized into abscisic acid. The configuration of the 1′,2′-epoxy group probably controls whether or not the 4′(S) hydroxyl group can be oxidized. (+)-2-cis-Xanthoxin acid is probably not a naturally occurring intermediate because a ‘cold trap’, added to avocado fruit forming [¹⁴C]-labelled abscisic acid from [2-¹⁴C]mevalonate, failed to retain [¹⁴C] label.     

Synthesis of metabolic intermediates of diethylstilbestrol

This report details the synthesis of 1) 3,4,4′-trihydroxy-α,α′-diethyl-trans-stilbene; 2) 3,4-bis-(p-hydroxyphenyl)-trans-3-hexenol; 3) 3,4-bis-(p-hydroxyphenyl)-2,4-cis,cis-hexadienol; 4) 3,4-bis-(3′-methoxy-4′-hydroxyphenyl)-trans-3-hexene; 5) 3,4-bis-(3′, 4′-dimethoxyphenyl)-trans-3-hexene. These compounds are suspected metabolites of diethylstilbestrol.     

Chemical constituents from Potentilla fragarioides L

Phytochemical investigation on Potentilla fragarioides L. has led to the identification of twelve compounds including β-sitosterol (1), β-daucosterol (2), ursolic acid (3), pomolic acid (4), swinhoeic acid (5), (1-p-hydroxy-cis-cinnamoyl)cinnamic acid (6), trans-caffeoylisocitric acid (7), trans-caffeic acid (8), quercetin (9), quercetin-3-O-β-D-glucuronide (10), (+)-catechin (11) and 3-O-methylellagic acid-4′-O-ɑ-L-rhamnopyranoside (12). Among them, compounds 4–7 were first identified from the genus Potentilla. And the other compounds except compounds 8 and 11 were found in Potentilla fragarioides for the first time. Chemotaxonomic significance of these compounds was discussed.     

Cyclisation of 1-trans-1′-norsqualene-2,3-epoxide and 1-cis-1′-norsqualene-2,3-epoxide by a cell free system of corn embryos

Squalene-2,3-epoxide-cycloartenol cyclase and cycloeucalenol-obtusifoliol isomerase activities were found in microsomal fractions of corn (Zea mays) embryos. Squalene-2,3-epoxide, 1-trans-1′-norsqualene-2,3-epoxide and 1-cis-1′-norsqualene-2,3-epoxide were incubated. Squalene-2,3-epoxide was cyclized giving only cycloartenol, whereas 1-trans-1′-norsqualene-2,3-epoxide gave 31-norcycloartenol and 31-norlanosterol with a reduced yield, 1-cis-1′-norsqualene-2,3-epoxide was not significantly cyclized.