Lichen structural chemistry

An article by Barreto-Bergter (Rio de Janeiro, Brazil) and Gorin (Saskatoon, Canada) likewise invokes strong emphasis on n.m.r. methods for structure determination, in this instance by use of carbon-13 techniques in delineating the structural chemistry of polysaccharides from fungi and lichens. 

Orcein is the principal colouring component found in orchil q.v.). Treatment of the lichen Rocella tinctoria (litmus, q.v.) with ammonia forms orcin which is then oxidised to form orcein, C28H24N2O7. This compound has been extensively studied by Hans Musso, who has produced a series of papers on the structure, chemistry and properties of orcein and its derivatives see Musso (1956) and Musso (1960) for a review of his work. 

Structural Chemistry of Polysaccharides from Fungi and Lichens, E. Barreto-Bergter and P. A. J. Gorin, Adv. Carbohydr. Chem. Biochem., 41 (1983) 68-104. 

Lobaria pulmonaria (L.) Hoffm. var. meridonalis (Vain) Zahlbr. is a widespread lichen characterized by a lobed structure similar in appearance to alveolar tissue. As Gonzalez et al. (1994) pointed out in their introduction to a discussion of the chemistry of this lichen, its specific epithet reflects its use as a treatment for pulmonary tuberculosis following the Doctrine of Signatures. The varietal name was appended much later to specimens collected in the Philippines. 

Bajt S, Clark SB, Sutton SR, Rivers ML, Smith JV (1993) Synchrotron X-ray microprobe determination of chromate content using X-ray absorption near-edge structure. Anal Chem 65 1800-1804 Balistrieri L, Brewer PG, Murray JW (1981) Scavenging residence times of trace metals and surface chemistry of sinking particles in the deep ocean. Deep-Sea Res 28A 101-121 Banfield JF, Barker WW, Welch SA, Taunton A (1999) Biological impact on mineral dissolution application of the lichen model to understanding mineral weathering in the rhizosphere. Proc Nat Acad Sci USA 96 3404-3411... 

This widespread lichen metabolite is the material isolated in this experiment. Usnic add was first isolated and identified in 1843 by Rochleder, but a molecule of this complexity was beyond the structural knowledge of organic chemistry in those days. The structure was finally determined in 1941 by Schopf, and in 1956 it was synthesized in the laboratory by Sir D. H. R. Barton (Nobel Laureate). Barton s route involved a spectacular one-step dimerization of a simple precursor, a synthesis that very closely rnirnicked the actual biogenetic pathway (see chemistry). The key step was the one-electron (1 e ) oxidation of methylphlo-racetophenone, which leads directly to the dimerization. The mechanism of this... 

Asahina and Shibata published another classic, Chemistry of Lichen Substances, in 1954. Both scientists and their coworkers not only elucidated the structure of numerous compounds, but also described their synthesis and introduced the determination of lichen substances by microcrystallization, a remarkable progress in times without thin layer chromatography and high performance liquid chromatography. Other important work on the chemistry of lichen substances was done by Roller, Pfau,... 

X-Ray analysis is the only physical method which gives directly the complete structure, stereochemistry and absolute configuration and has been applied in lichen chemistry for numerous compounds Table 18. 

Elix JA, Engkanian U, Jones AL, Raston CL, Sargent MV, White AH (1978a) Chemistry and crystal structure of leprolomin, a novel diphenyl ether from the lichen Psora leprolomum. Aust J Chem 31 2057-2068 Elix JA, Musidlak HW, Sala T, Sargent MV (1978b) Structure and synthesis of some lichen xanthones. Aust J Chem 31 145-155... 

Wachtmeister CA (1958) Studies on the chemistry of lichens. XL Structure of picrolichenic acid. Acta Chem Scand 12 147-164... 

Akermark, B. Studies on the Chemistry of Lichens. 14. The Structure of Calycin. Acta Chem. Scand. 15, 1695 (1961). 

Chemical methods As with many areas of natural product chemistry, new impetus in the chemistry of lichen substances is provided by the more rapid and improved methods for detecting, isolating and purifying these compounds and in determining their structure. The techniques of preparative TLC, radial chromatography and preparative HPLC provide rapid and efficient methods for the purification of lichen substances and developments in mass spectrometry, proton and carbon-13 nuclear magnetic resonance (NMR) spectroscopy and X-ray analysis greatly aid structural smdies. 

The development in the chemistry of structurally related groups of lichen metabolites will now be discussed in detail, in approximate biogenetic order. 

Akermark, B., H. Erdtman, and C. A. Wachtmeister Chemistry of Lichens. XIII. Structure of Pannaric Acid. Acta Chem. Scand. 13, 1855 (1959). 

Elix, J. A., U. Engkaninan, A. J. Jones, C. L. Raston, M. V. Sargent, and A. H. White Chemistry and Crystal Structure of Leprolomin, a Novel Diphenyl Ether from the Lichen Psoroma leprolomum. Austral. J. Chem. 31, 2057 (1978). 

Lichen substances are organic compounds synthesized by lichens. They have long attracted the attention of organic chemists, and their chemistry, structure, properties, and distribution have been treated in detail recently in major works by Asahina and Shibata (1954) and Culberson (1969,1970). This field is also reviewed by Huneck (1968, 1971). Further details on the biosynthesis of lichen substances may be found in Chapter 16 by Mosbach. 

The explanation of the chemistry of lichens was begun by Pfaff (1826), when he found eine eigentumliche Saure from Iceland moss (Cetraria islan-dica). Pfaff s finding, evidently fumarprotocetraric acid (Hesse, 1904), was the beginning of the rapidly expanding field of lichen chemistry. Usnic acid was discovered in 1844 by Knop. The largest number of lichen substances has been isolated by Hesse (1861-1905) and Zopf (1907), and as a result of their investigations they totaled nearly 150 in number by 1907. Of this total, the structure of only a few had been studied several of them later proved to be mutually identical or else impure. 

Lichen chemistry continues to be a flourishing branch of natural product chemistry. Since the last major review by Elix et al. in volume 35 of this series 274) many new and interesting lichen substances have been isolated and synthesized. This suggests that an update is desirable. As in the previous articles new methods for structure elucidation and synthesis, biosynthesis, chemotaxonomy and biological activities of lichen substances will be discussed. Some other relatively recent treatments of the subject need to be mentioned A short chapter by... 

H- and C-NMR spectroscopy and the corresponding special techniques are of unique importance in structure elucidation of natural products and are widely used in lichen chemistry. For example Gagnaire et al. (309) and Gagnaire and Vincendon (310) have discussed the NMR... 

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