Chemical Constituents of the Lichen Stereocaulon azoreum Antonio G. González, Elsa M. Rodrí guez Pérez, Consuelo E. Hernández Padrón*, and Jaime Bermejo Barrera Centro de Productos Naturales Orgänicos Antonio Gonzalez, I.P.N.A. Consejo Superior de Investigaciones Cientificas Carretera La Esperanza 2, La Laguna, 38206 Tenerife, Canary Islands, Spain * Departamento de Botanica, Facultad de Farmacia, Universidad de La Laguna, Tenerife, Canary Islands, Spain Z. Naturforsch. 47c, 503-507 (1992); received April 24, 1992 Stereocaulon azoreum, Lichens, Triterpenes, Sterols, Depsides Column chromatography of the acetone extract of the lichen Stereocaulon azoreum afforded several substances identified by chemical and spectral means; a-amyrin, lupeol, taraxerol, ursolic acid, ergosterol peroxide, brassicasterol, cerevisterol, stictic acid and an acetone-con- densation derivative, lobaric acid. Furthermore atranorin, methyl ß-orsellinate, atranol and, for the first time from the genus Stereocaulon, cryptostictic acid and the dibenzofuran, strepsi- lin were isolated. Introduction guing species was analyzed and afforded various types of substances. Four triterpenes, a-amyrin, Stereocaulon azoreum (Schaer.) Nyl. (Stereocau- laceae, Lecanorales) is a silver-grey lichen with fru- lupeol, taraxerol and ursolic acid, three sterols, er­ ticose thallus, very ramified, graceful pseudopode- gosterol peroxide (1), brassicasterol (2), and cere­ tia covered with squamulose or coralloidsquamu- visterol (3) and various lichen compounds were lose phyllocladia, no soralia, frequent subterminal obtained. apothecia, dark-brown-blackish flat-to-convex The major lichen substances were the depsi- disk. It is saxicolous and its preferred habitat in dones stictic acid (4) and lobaric acid (5) and the the Canaries is on the lava flow found on the depside atranorin, all three of which are of particu­ lar interest in the chemotaxonomic diagnosis of “green mountainside” at 800 to1 0 0 0 m altitude where it may be very abundant locally. It has been the species under study. An acetone-condensation reported as endemic to Madeira, the Azores and artifact of stictic acid, 4 a, two mono-arylic phe­ the Canary Islands (Lanzarote, Gran Canaria, nolic compounds, methyl ß-orsellinate (7) and Tenerife, Gomera, Hierro, and Palma). atranol (8 ) were also separated, as were the depsi- done cryptostictic acid (4 b) and the dibenzofuran The only chemical analysis of S. azoreum to date has been by thin-layer chromatography for che- strepsilin (9), found for the first time in a species of motaxonomic purposes. The most recent general Stereocaulon. study of Stereocaulon, by I. M. Lamb [1] charac­ terized S. azoreum chemotaxonomically as the Results and Discussion only member of the genus in which lobaric and Isolation and structural determination of the stictic acid are definitely known to co-exist and, terpenoids for the first time, indicated that atranorin, norstic- tic acid and, possibly, constictic and consalazinic A mixture of triterpenes was isolated from the acids were also present. From 1858 until recently, fractions 8 - 1 1 of the column chromatography S. azoreum was classified as Stereocaulon sphaero- and later separated by silver nitrate-impregnated TLC into a-amyrin, lupeol and taraxerol, all origi­ phoroides Tuck [2]. In an intensive study of the numerous and var­ nally isolated as lichen substances from Cladina ied Canary lichens, an Me2CO extract of this intri- macaronesica [3]. Fractions 17-19 yielded crystals with m.p. 146-147 °C, [a]D -60° and M + atm /z 398 for the molecular formula C 28H 460. The ’H N M R spec­ Verlag der Zeitschrift für Naturforschung, D-W-7400 Tübingen trum of this compound was very similar to that of 0939-5075/92/0700-0503 $ 01.30/0 ergosterol peroxide (1 ) with signals for four sec- 504 A. G. Gonzalezet al. ■ Chemical Constituents of the Lichen Stereocaulon azoreum 3 R= H 3a R= Ac 4 R= CHO 4a R= C = C / \ H COMe 4b R= CH^OH Me Me T» Me H O<> - (' '>- COOMe HO CHO 0H Me Me oCHO OH 7 9 R= H 10 9a R= Ac A. G. Gonzalezet al. • Chemical Constituents of the Lichen Stereocaulon azoreum 505 ondary and three tertiary methyl groups, a signal traction. In 4 b, the same signal was replaced by for the geminal hydroxy proton as a multiplet at those at 8 4.80 and 4.90 (each 1 H, br s) for a 8 3.51, which underwent a paramagnetic shift of (-CH 2OH) radical. The physical and spectro­ approximately A 5 = 1 ppm by acetylation, a two- scopic data of 4, 4 a and 4 b proved to be identical proton multiplet for a double bond A22-23, and a to those of stictic acid (4), its acetone-condensa- broad doublet at 5 5.38 assigned to another vinyl tion product, 4 a, and cryptostictic acid (4 b), ear­ proton in the molecule. The above data exactly lier obtained from the lichen Lobaria pulmonaria match brassicasterol (2) [4]. [10]. Cryptostictic acid (4b), a substance biogeneti- A triterpene acid and a sterol with the physical cally related to 4, was isolated for the first time and spectroscopic data of ursolic acid [5] and er- from the genus Stereocaulon. Another depsidone gosterol peroxide (1 ) [6 ], respectively, were isolated from the fractions 25-28 crystallized as colourless from the fractions 20-30. needles, m.p. 196-197 °C. Its MS showed the M + Lastly, a sterol was obtained from the 51-58 at m /z 456 in accordance with the empirical formu­ fractions, and, when purified by chromatography la, C 2 5 H 280 8. Both the fragmentation pattern and on Sephadex LH-20 (hexane: chloroform: meth­ the proton signals in MS and ’H N M R coincided anol, 2 : 1 : 1 ), formed colourless needles, m.p. with those given in the literature for lobaric acid 250-253 °C (from CHCl3 :MeOH), M + at m /z (5) [8 , 9], 430, C 28H 460 3. Its 'H N M R spectrum greatly re­ A dibenzofuran with colourless needles, m.p. sembled that of ergosterol peroxide (1 ) with an ex­ 318-320 °C, was isolated from the fractions tra geminal proton at 5 3.57 (1H, d, .7=5.0 Hz) 31-38: its IR spectrum had absorption bands at coupled with a vinyl hydrogen at 8 5.29 ppm (1 H, 3463 and 3395 cm' 1 (hydroxy groups) and d, / = 5.0 Hz) sited at C-7. Treatment with Ac20 1736 cm “ 1 (carbonyl) and a typical aromatic band and Py afforded a diacetylated derivative, 3 a, with at 1626 c m '1. MS showed the M + at m /z 270 for signals for a tertiary hydroxyl in its IR spectrum. the empirical formula. C 15H 10O 5 and 'H N M R These data agree with those of cerevisterol (3), ear­ gave signals for an aromatic methyl at 8 2.89, two lier obtained as a lichen product from Ramalina meta aromatic protons at 8 6.94 and 7.12 (each hierrensis [7]. 1 H, br s), an aromatic proton at 8 7.34 (1 H, s), and a two-proton singlet at 8 6 . 0 1 typical of a (- 0 - C H 2 -Ar) benzylic methylene group. A di­ Isolation and structural determination of the lichen acetyl derivative, 9 a (see Experimental for its MS substances and 'H N M R spectra) was formed when the diben­ The only depside isolated was obtained from the zofuran was treated with Ac20-Py. All these data fractions 8 - 1 1 of the general column chroma­ match strepsilin (9) [11], isolated here for the first tography and its physical and spectral data agree time from the genus Stereocaulon. This is of espe­ with those reported for atranorin (6 ) [8 , 9], a corti­ cial interest as the substance usually isolated from cal substance present in most of the species of the this species is porphyrilic acid (10), a dibenzofuran genus Stereocaulon. produced by a different coupling of the two phenol A mixture of depsidones was obtained from the units [1 2 ]. fractions 39-58 and separated by chromatogra­ One mono-arylic compound was isolated from phy on Sephadex LH-20 (hexane: chloroform; the fractions 12-16 and another from the frac­ methanol, 2:1:1). A mixture of the similar com­ tions 16-19 of the general chromatography and pounds 4, 4 a and 4 b was obtained from the frac­ they were purified by chromatography on Sepha­ tions 18-24 and separated by preparative chroma­ dex LH-20 with 2:1:1 hexane: chloroform: meth­ tography on silica gel (Be: dioxan: HO Ac). The 'H anol as eluant. Their physical and spectral data N M R spectra of these three substances were alike were those of methyl ß-orsellinate (7) [13] and except that a signal for an aldehyde group at atranol (8 ) [14], respectively. These two sub­ 8 10.57 in 4 was replaced in 4a by those at 8 2.43 stances, together with atranorin (6 ) had previously (3H, s) and 7.05 and 7.94 (each 1 H, d ,/ = 16.7 Hz) been isolated from various species of Stereocaulon. for a radical (-CH = CH-COMe) as the result of a Caccamese et al. [14] have argued that 7 and8 are condensation reaction with acetone during the ex­ found in Stereocaulon vesuvianum as true natural 506 A. G. Gonzalezet al. ■ Chemical Constituents of the Lichen Stereocaulon azoreum products, dissenting from the earlier view that (2 g). The rest of the extract was passed through a atranorin degradation during the extraction proc­ column packed with 1 kg silica gel and eluted with ess probably accounted for these compounds [13]. mixtures of hexane-ethyl acetate of increasing Atranorin, which is probably involved in light har­ polarity. Fractions of 500 ml were collected and vesting and radiation protection, may fill another monitored by TLC, giving the following: the frac­ biological role in this context, being a depside with tions 1-7 (wax), the fractions 8-11 (a-amyrin + no intrinsic antibiotic activity, but able perhaps to lupeol + taraxerol (50 mg), 6 (500 mg)]; the frac­ serve as a storage compound which the plant uti­ tions 12-16 [7 (600 mg)]; the fractions 17-19 [8 lizes in order to defend itself against attack by (350 mg), 2 (30 mg)]; the fractions 20-30 [ursolic pathogen microorganisms, by slowly manufactur­ acid (500 mg), 1 (30 mg)]; the fractions 31-38 [9 ing the two bioactive compounds 7, a powerful (10 mg)]; the fractions 39-50 [4 (100 mg), 4 a fungicide, and 8 , a strong antimicrobial agent.