Elaboration of the 6,7,8 Oxygenation Pattern in Simple Coumarins: Biosynthesis of Puberulin in Agathosma Puberula Fourc.1
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Elaboration of the 6,7,8 Oxygenation Pattern in Simple Coumarins: Biosynthesis of Puberulin in Agathosma puberula Fourc.1 Stewart A. Brown *, D. E. A. Rivett **, and H. Joan Thom pson * * Department of Chemistry, Trent University, Peterborough, Ont., Canada K9J 7B8. ** Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa. Z. Naturforsch. 39 c, 31-37 (1984); received July 11, 1983 Biosynthesis, Puberulin, Coumarins, Agathosma puberula Umbelliferone and scopoletin are well utilized as precursors of puberulin, a 7-O-prenyl ether of isofraxidin elaborated by Agathosma puberula Fourc. (Rutaceae). As ferulic, sinapic, and caffeic acids were all more poorly utilized than 4'-hydroxycinnamic (/?-coumaric) acid, the partial biosynthetic route: 4'-hydroxycinnamic acid -*-> umbelliferone -*• aesculetin -* scopo letin -*-► puberulin is suggested as the major pathway. Prenyl ether formation apparently occurs at or beyond the scopoletin stage. The implication that ferulic acid does not participate in the formation of scopoletin from 4'-hydroxycinnamic acid is at variance with the known role of ferulic acid as a scopoletin precursor in tobacco. Table I. Distribution of Introduction Oxygenation Number of natural coumarins ac pattern coumarins The naturally occurring coumarins, of which well cording to oxygenation pattern, as reported up over 800 have now been described [1], are charac 5.7 287 to 1980. terized by a variety of oxygenation patterns on the 7.8 6 6 6.7 53 benzopyrone nucleus. The vast majority bear an 5.7.8 28 oxygen function at C-7 (c f Structure 1), and the 6.7.8 22 largest single category is oxygenated at this position 5.6.7 17 5.6.7.8 1 only — 335 com pounds in a recent listing [1], As for those coumarins oxygenated at more than one carbon, examination of a tabulation published by Biosynthetically, coumarins in several categories Murray et al. [1] reveals the distribution of types of the Table have been investigated, and from those shown in Table I. In addition to these, other patterns studies distinct differences have emerged. The of oxygenation involving the lactone ring, such as 7-hydroxyl of umbelliferone is, without doubt, the 4,7 pattern of the coumestans, have been re introduced at the phenylpropanoid acid stage, cognized. before lactone ring formation, and the resulting 4'-hydroxycinnamic acid is then 2'-hydroxylated [1], There is very good evidence also that the 6,7-di- oxygenated simple coumarin, scopoletin, derives in at least one species from the analogously oxy OMe genated ferulic acid. This is indicated by a number of tracer studies [1], and an enzyme which hydrox- 1 ylates ferulic acid at position 2' has been reported a R =CH2-ChUCMe2 from Hydrangea macrophylla [2], This 6-hydroxyl- ation (coumarin numbering), therefore, also can b R = H Structure 1 occur before elaboration of the coumarin nucleus. Other oxygenation patterns have also been Abbreviation: TMC, 6,7,8-trimethoxycoumarin. studied, three of which are represented among the 1 The studies reported here were supported by grants from the Natural Sciences and Engineering Research linear furanocoumarins: bergapten (5,7), xantho- Council of Canada and the South African Council for toxin (7,8), and isopimpinellin (5,7,8). In all these Scientific and Industrial Research. instances there is no evidence for introduction of Reprint requests to Prof. S. A. Brown. oxygenation at C-5 or C-8 prior to elaboration of 0341-0382/84/0100-0031 $01.30/0 the lactone ring [1], and, in fact, it would appear Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung This work has been digitalized and published in 2013 by Verlag Zeitschrift in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der für Naturforschung in cooperation with the Max Planck Society for the Wissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht: Advancement of Science under a Creative Commons Attribution Creative Commons Namensnennung 4.0 Lizenz. 4.0 International License. 32 St. A. Brown et al. ■ Biosynthesis of Puberulin in Agathosma puberula that the principal pathway to these coumarins [l4CH3]Scopoletin. The synthesis of Braymer et al. involves hvdroxylation of psoralen, which possesses [10] was adapted to the labelling of the O-methyl a fully formed furanocoumarin nucleus, oxygenated carbon of scopoletin. 7-O-Benzylaesculetin (167 mg, at C-7 only. However, oxygenation of an inter 0.625 mmol) in 3 ml of dry acetone was stirred mediate 4'.5'-dihydrofuranocoumarin. marmesin, at under reflux for 4h with 77 mg of (,4CH3)2S0 4 positons 5 or 8 can also take place in some species (15MBq/mmol) and 250 mg of anhydrous potas [3-5]. Among the angular furanocoumarins. Steck sium carbonate. A further 77 mg of carrier dimethyl and Brown [6] showed that angelicin (C-7 only) is a sulphate was then added, and refluxing was con very efficient precursor of isobergapten (5,7), tinued for another 4 h. Removal of the solvent gave sphondin (6.7), and pimpinellin (5,6,7) in a quantitative yield (based on the coumarin) of Heracleum lanatum. It is. therefore, highly probable benzvloxyscopoletin, which was recrystallized from that there is also no 5- or 6-hydroxylation in furano aqueous methanol to yield two crops of crystals coumarins until the lactone ring has been synthe totalling 170 mg. This intermediate was debenzyl- sized. However, this is in clear contrast to the ated by heating 6 h at 100 °C in 2 ml of glacial above-mentioned case of scopoletin, whose oxy acetic acid + 1 ml of concentrated HC1. The product genation pattern is that of sphondin. was decolorized with charcoal in methanol and Of the 22 coumarins bearing the 6,7,8 pattern, sublimed at 170 °C, < 1 Torr, to provide 115 mg of none has yet been investigated biosynthetically. We scopoletin. A final recrystallization from 50% have chosen for the study reported here to examine ethanol gave a yield of 91 mg. m. p. 205-208 °C. the biosynthesis of puberulin (la) in Agathosma puberula Fourc., from the aerial parts of which this Administration of labelled substrates to A. puberula coumarin, a prenyl ether of isofraxidin (lb) was Plant shoots were collected on the farm “Upper isolated earlier [7], Gletwyn” east of Grahamstown. usually from a single bush, and the cut ends immediately plunged Experimental into water. Immediately before the feeding, the ends Preparation of labelled substrates were recut at an angle under water with sharp secateurs. About 0.02 mmol of the labelled substrate [2-l4C]Hydroxycinnamic Acids - [2-l4C]- was weighed into a small test tube (fusion tube), Labelled 4'-hydroxycinnamic, caffeic, ferulic, and and in the case of acids a slight excess of sodium sinapic acids were prepared from [2-14C]malonic carbonate was added. After the addition of 1 ml of acid (New England Nuclear, Boston, USA) water the mixture was warmed to effect solution. essentially according to the procedure of Vorsatz [8]. The phenols, scopoletin and umbelliferone. were In a typical synthesis, 0.5 mmol of the appropriate dissolved in 1 ml of water and 3 drops of 0.02 m aldehyde (4-hydroxybenzaldehyde, 3,4-dihydroxy- NaOH with warming. The freshly cut ends of the benzaldehyde, 3-methoxy-4-hydroxybenzaldehyde, shoots (15-25 g) were dipped into the substrate or 3,5-dimethoxy-4-hydroxybenzaldehyde, re solutions and the shoots exposed either to sunlight or spectively) was dissolved in 0.3 ml of anhydrous to a 200 W lamp placed 30 cm away in a fume hood pyridine, to which was added 0.5 mmol of the through which a gentle stream of air was flowing. labelled malonic acid and 10|il of aniline. The The ratio of molar dose to shoot rate was kept reaction mixture was heated at 55 °C in an oil bath reasonably constant. The solution was taken up by for ~ 7 h. Carrier malonic acid (0.5 mmol) was then the shoots during 2 h. The shoots were then allowed added, and the heating continued a further 7 h. to absorb two rinses of water, placed in an excess of Dilution with water and acidification with HC1 tap water, and allowed to metabolize for 2 -3 days. yielded in each case crystals of the cinnamic acid derivative, which was recrystallized from hot water. Processing o f treated plants Radiochemical yields in the 70-80% range were recorded. The shoots were transferred to a Soxhlet [2-l4C]Umbelliferone. This was prepared by a apparatus and extracted for two days with cyanoacetate condensation from K14CN as methanol. The solvent was removed under reduced previously described [9], pressure, the residue dissolved in 40 ml of warm St. A. Brown et al. • Biosynthesis of Puberulin in Agathosma puberula 33 chloroform, and shaken with 2 x 20 ml of 4% crystallize. The crude isolate was therefore chroma aqueous NaOH to remove acids and phenols. The tographed on 20 x 20 cm chromatoplates of silica chloroform extract was evaporated and the residue gel G, 1 mm thick, developed in chloroform-ace- refluxed 1 h with methanol containing 2% sulphuric tone, 9:1, repeated if necessary. The bands cor acid (40 ml) to convert puberulin to isofraxidin. Ten responding to isofraxidin were extracted into hot per cent NaOH was added and the pH carefully acetone in Soxhlet extractors and the solvent was adjusted to 9-10 with the aid of Merck test papers. evaporated to give crude, usually crystalline iso At more alkaline pH green pigments are later fraxidin. These residues were converted to TMC as extracted with the isofraxidin and make purifica described above. tion difficult. The methanol was removed under reduced pressure, the mixture made up to about D egradation o f T M C 40 ml with water, and heated on the steambath for a few minutes to ensure complete solution of iso TMC from isofraxidin recovered after feeding fraxidin.