Fernene Triterpenoids from the Lichen Pyxine berteriana

Abstract

Two new fernene triterpenoids, fern-9(11)-en-3,19-dione (1) and 3β-acetoxyfern-9(11)-en-19-one (2), together with the known 3β-acetoxyfern-9(11)-en-19β-ol (3) and lichexanthone (4), have been isolated from the acetone extract of the lichen Pyxine berteriana. The structures of the new compounds were established on the basis of IR, extensive 1D and 2D NMR, and MS analyses. Although several fern-9(11)-enes have been isolated from lichens, compounds 1 and 2 are the first examples of naturally occurring fernene triterpenoids with a carbonyl function at C-19.Lichens are symbiotic associations composed of at least a fungal partner, the mycobiont, and a photosynthetic partner, the photobiont. 1 These associations frequently produce characteristic secondary metabolites that are of fungal origin. Most are unique to lichens, and only a small number occur in non-lichenized fungi or higher plants. 2 Many of these lichen secondary compounds exhibit antibiotic, antitumor, antimutagenic, allergenic, antifungal, antiviral, enzyme inhibitory, and plant growth inhibitory properties. 2,3 Triterpenoids are widely distributed in lichens, being commonly present in genera such as Nephroma and Pseudocyphellaria as well as in different genera of the Physciaceae (e.g., Dirinaria, Physcia, and Pyxine) and the Parmeliaceae (e.g., Parmelia and Evernia). 3,4 A previous report on the secondary metabolites of Pyxine berteriana (Physciaceae) from Brazil indicated that it contained atranorin, lichexanthone, methyl pyxinate, and pyxinol, according to TLC analysis. 4 In the course of the search for new metabolites from the lichen P. berteriana (Fée) Imshaug we have isolated two new fernene triterpenoids, fern-9(11)-en-3,19-dione (1) and 3β-acetoxyfern-9(11)-en-19-one (2), together with the known 3β-acetoxyfern-9(11)-en-19β-ol (3) and lichexanthone (4), which is a chemical marker of a group of species in the genus Pyxine. 5,6 The structure elucidation of compounds 1 and 2 is described herein. (Table 1) In accordance with the COSY spectrum, the signal at δ H 2.76 (H-2β) showed cross-peaks with the signals at δ H 2.20 (H-1β), 2.23 (H-2α), and 1.64 (H-1α). On the basis of the HMBC and HSQC spectra, the signals at δ H 2.76 and 2.23 (δ C 35.1, H-2) showed crosspeaks with the signals at δ C 40.4 (C-1), 216.6 (C-3), and 37.6 (C-10), establishing that C-3 corresponded to the carbonyl group at δ C 216.6. Further correlations in the HMBC spectrum of the singlet at δ H 1.30 (δ C 24.2) with the signals at δ C 37.6 (C-10), 40.4 (C-1), 46.4 (C-5), and 148.9 (C-9) allowed us to assign this methyl resonance to C-25. The NOESY correlations between H-5/CH 3 -23 and CH 3 -25/CH 3 -24 in conjunction with HSQC data permitted assignment of the 1 H and 13 C resonances of CH 3 -23 (δ H 1.04, δ C 24.3) and CH 3 -24 (δ H 1.12, δ C 21.7). HMBC correlations of these methyl protons with the signal at δ C 216.6 confirmed the assignment of this carbonyl group to C-3. The broad doublet at δ H 2.09 (δ C 38.8) was assigned to H-8 on the basis of the cross-peaks with the si...

Xanthones from Fungi, Lichens, and Bacteria: The Natural Products and Their Synthesis

“…Lichexanthone ( 62 , Figure ) was initially identified from the lichens Parmelia formosa and Parmelia sp . and has been since identified in many more lichen species, for example, Athocleista djalonensis, Pertusaria sp., P. sulphurata , Pyxine consocians Vainio, and P. berteriana . The synthesis of this molecule was performed as a confirmation of structure from orsellinic acid and phloroglucinol, , and then from everninic acid and phlorglucinol .…”

“…131 and has been since identified in many more lichen species, for example, Athocleista djalonensis, 134 Pertusaria sp., 69 P. sulphurata, 135 Pyxine consocians Vainio, 136 and P. berteriana. 137 The synthesis of this molecule was performed as a confirmation of structure from orsellinic acid and phloroglucinol, 129,130 and then from everninic acid and phlorglucinol. 121 In 1993, Feige and coworkers described the development of a standardized highperformance liquid chromatography (HPLC) assay for the identification of many lichen-derived natural products, with particular mention of 55 xanthones and 1 xanthone-containing compound (secalonic acid B, 449, Figure 74).…”

Natural Enfumafungin Analogues from Hormonema carpetanum and Their Antifungal Activities

“…Fernane-type triterpenoids are a class of metabolites mainly found in ferns and lichens, − and rarely in fungi. , Enfumafungin is a 9(11)-fernane-type triterpene glycoside featured with an acetate group at C-2, a glucose unit O-linked to C-3, a hemiacetal functionality at C-25, and a carboxylic acid group at C-18. The E-ring in the fernane triterpene skeleton is thought to be oxidatively cleaved during the biosynthesis of 1 .…”

Triterpenoids

“…The lichen Pyxine berteriana is the source of fern-9(19)-ene-3,19-dione 478 and the corresponding 3b-acetoxy derivative 479. 274 Two 2,3-secofernanes, alstonic acids A 480 and B 481, have been isolated from Alsonia scholaris. 275 The structure of alstonic acid A 480 was confirmed by X-ray analysis and a biosynthetic pathway to alstonic acid B 481 has been proposed.…”