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Chemical and biodegradation studies on usnic acid Sanchez Flores, Ignacio Humberto

Abstract

This thesis describes investigations on the biodegradation of the lichen substance (+)-usnic acid (35), a natural antibiotic, by several soil fungi and bacteria. Part A of this work is concerned with the biotransformation of 35 into (+)-6-desacetylusnic acid (107) by a Pseudoroorias species. Its structure elucidation, laboratory preparation and subsequent conversion into 35 are described. Thorough investigations on the condensation of 35 with a series of aliphatic and aromatic amines allowed the determination of the characteristic g-diketoenamine structure present in these compounds, as exemplified by (-)-Δ²‧¹¹-enaminousnic acid (132), and concluded with the preparation of the useful (+)-isoxazolo[4,5-b]usnic acid (158) and (+)-2H- [l,2]oxazocinousnic acid (159), a novel eight-membered heterocycle. However, the removal of the aromatic acetyl grouping under strong alkaline conditions resulted in solvent addition to the C₄-C[sub 4a] double bond with concomitant irreversible rearrangement to the isousnic acid series, an important process that had not been recognized by previous workers. All the attempts to control or avoid this isomerization were unsuccessful, resulting in multiple synthesis of the isomeric (+)-8-desacetylisousnic acid (178), which was easily converted via its diacetate 179 into (+)-isousnic acid (113), thus constituting the first total synthesis of this naturally occurring material. The original synthetic goal was finally accomplished by methylation of the isoxazole 158 with methyl iodide-silver oxide in chloroform and Beckmann rearrangement of the corresponding ring A oxime to produce (+)-N-acetyl-6-amino-7,9-di-0-methylisoxazolo[4,5-b]usnic acid (228), one of the various previously unknown of this study. Substitution of the amide functionality by hydrogen resulted from the use of isoamyl nitrite in dioxane. Boron tribromide demethylation and subsequent regeneration of the protected β-triketone system furnished in good yield the metabolite (+)-6-desacetylusnic acid (107), which was converted back to 35 by a one-step acetylation-Fries rearrangement. Simultaneous investigations produced the first high yielding preparations of mono-, di-, and mixed diether derivatives of usnic acid, compounds whose preparation was not achieved by earlier workers. Part B discusses the isolation and structure determination of (+)-2-desacetylusnic acid (108), a product resulting from the biodegradation of 35 by Mucor globosus, Its synthesis was accomplished by means of the Baeyer-Villiger oxidation of (+)-usnic acid (35) with 30% hydrogen peroxide in pyridine, at room temperature to produce the a-acetoxy derivative 251 followed by its chromium (II) chloride reduction in acetone solution. In Part C is described the isolation and identification of three metabolites obtained during the biodegradation of 35 by Mortierella isabellina. Thorough analysis of their spectroscopic characteristics, complemented by chemical degradations, showed that one of them was identical with (+)-2-desacetylusnic acid (108), a product isolated from the Mucor globosus fermentation (Part B). The main component, (+)-2-acetoxyusnic acid (251), has a novel structure resulting from the net insertion of oxygen at the 2-position, and proved identical with the product obtained from the Baeyer-Villiger oxidation of (+)-usnic acid (Part B). The final component was shown to be (+)-1α-hydroxy-2-desacetylusnic acid (252), a compound arising from both deacylation and reduction of the carbonyl group at the one-position. Part D provides the synthesis of (+)-6-¹⁴COCH₃-usnic acid (255), the first radioactive precursor specifically labelled at the aromatic acetyl grouping. This product will find immediate application in the determination of the biodegradation pathways involved during the described transformations.

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