Lichen metabolites

The main concern of the early work (before about 1955), much of which was due to Gilman, was the examination of the basic chemistry of dibenzofuran with particular reference to electrophilic substitution and metallation. A later development of dibenzofuran chemistry owed its impetus to the discovery of dibenzofuranoid lichen metabolites and the consequent synthetic efforts that this initiated. 

Benzofuran-2-carbaldehydes readily undergo Wittig reactions in tetra-hydrofuran at room temperature with the resonance-stabilized ylide 2-car-boxy-l-methoxycarbonylethyltriphenylphosphorane, affording high yields of ( )-4-(2-benzofuranyl)-3-methoxycarbonylbut-3-enoic acids. This method is preferable to the Stobbe condensation. The Stobbe-type intermediates undergo quantitative cyclization to methyl l-acetoxydibenzofuran-3-carboxylates on exposure to acetic anhydride at 100 C. Examples are shown in Scheme 27. The intermediate 109 has been used in a synthesis of cannabifuran (110), and the intermediate 111 has been used in a synthesis of the lichen metabolite schizopeltic acid (112). ... 

The tundra extends fi om the northern border of the taiga to the Arctic circle. It is a treeless groxmd with fi ozen subsoil that only allows mosses and lichens growing abundantly. Typical lichen metabolites are in the orcein-class of dyes (Chart 6.3). 

Romagni, J.G., Meazza, G, Nanayakkara, N.P.D., Dayan, F.E. The phytotoxic lichen metabolite, usnic acid, is a potent inhibitor of plant p-hydroxyphenylpyruvate dioxygenase. FEBS Letters 2000 480 301-305. 

Ecophysiology and Potential Mode of Action of Lichen Metabolites... 

Antiherbivory roles of metabolites have been well documented.9,25,26 Proksch31 reported that lichens produced secondary metabolites that acted as feeding deterrents which protected them from animal consumption. Several insects appeared to selectively avoid the medullary region, which contained most of the lichen metabolites, grazing primarily on the algal layer.25... 

The potential role of lichen metabolites in allelopathic interactions has recently been reviewed.9,27 The phytotoxic effect of certain lichen metabolites may play a role in the establishment of lichen populations. The depsides, barbatic acid and lecanorin, and the tridepside, gyrophoric acid, have been shown to inhibit... 

Dayan, F. E. and Romagni, J. G., 2001. Structural diversity of lichen metabolites and their potential use in pest management. In Advances in Microbial Toxin Research and Its Biotechnological Exploitation (in press). 

Fernandez, E., Reyes, A., Hidalgo, M. E., and Quilhot, W., 1998. Photoprotector capacity of lichen metabolites assessed through the inhibition of the 8-methoxypsoralen photobinding to protein. J. Photochem. Photobiol. 42, 195-201. 

Romagni, J. G., Meazza, G., Nanayakkara, and D., Dayan, F. E., 2000. The Phytotoxic Lichen Metabolite, Usnic Acid, is a Potent Inhibitor of Plant p-Hydroxyphenylpyruvate Dioxygenase. FEBS Letters 480 (2-3), 301-305. 

Rojas, I. S., Lotina-Hennsen, B., and Mata, R., 2000. Effect of lichen metabolites on thylakoid electron transport and photophosphorylation in isolated spinach chloroplasts. J. Nat. Prod 63, 1396-1399. 

Picroroccellin, 3, 6-dibenzyl-3-hydroxy-6-methoxy-l(or4)-methyl-3, 6-dihydro-2, 5(1/7, 4//)-pyrazinedione (revised structure), a Roccella sp lichen metabolite [87291-18-7].1036,1702... 

Figure 4. Effect of the synthetic diphenyl ether acifluorfen (triangles), the natural diphenyl ether fungal metabolite cyperine (squares) and the dibenzofurandione lichen metabolite usnic acid (circles) on the activity of protoporphyrinogen oxidase. The dotted line represents 50% inhibition of enzyme activity.

A new synthetic approach uses 2-arylazo-2,5-dimethyl-3-oxo-2,3-dihydrofurans. After hydrogenolysis and cyclization, 1,4-dihydropy-ridazines 46 are obtained (79S790). Pulvinic lactone, a lichen metabolite,... 

You are usually advised when making a complex aromatic compound to have any oxygen atoms (directly joined to the benzene ring) already present at the start. In their synthesis of the lichen metabolite dechlorolecideoidin 27, McEwen and Sargent3 chose the fully substituted benzene 29 as precursor for the right hand ring. 

In addition to the numerous fungal and lichen metabolites discussed in the preceding sections, there are several others that do not easily fit into the well-defined structural categories. 

A dehydropeptide has been cyclized to an imidazole derivative to serve as a model for the biosynthesis of luciferin from a crustacean." Unequivocal evidence for the lichen metabolite cyc/o-(R-/3-phenyl-i3-Ala-Pro)2 was provided by total synthesis. The cyclization step (20% yield) was achieved by ONP peptide-bond formation in pyridine at high dilution (6 mmole to 3000 ml). Nine isomeric cyclohexapeptides have been synthesized as models for the study of molecular interactions. ... 

Natural products provide interesting opportunities for measuring opfical activity. An excellent example is the lichen metabolite, usnic acid, which can be easily isolated from its native source,"01d Man s Beard"lichens, as... 

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... 

Elix JA, Yu J (1993) New lichen (3-orcinol depsidones and their congenors. J Hattori Bot Lab 74 317-323 Elix JA, Ferguson BA, Sargent MV (1974) The structure of alectoronic acid and related lichen metabolites. Aust J Chem 27 2403-2411... 

Evans JE (1987) Identification and synthesis of secondary lichen metabolites. Thesis, Australian National Univ., Canberra... 

Ghogomu RT, Bodo B (1982) Structural elucidation of 13-acetoxylichesterinic and 13-acetoxyprotolichesterinic acids, two aliphatic lichen metabolites from Neuropogon trachycarpus. Phytochemistry 21 2355-2358... 

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