The processes of humification occur mainly under aerobic conditions. Animals that live in the soil may initially reduce the size of the fresh organic residues. Further transformations are promoted by the activity of the enzymes of bacteria and fungi living in the soil. Cellulose, proteins and fats are readily available carbon sources for the microorganisms, whereas such compounds as lignin and other phenolic plant constituents are decomposed more slowly. These, as well as some of the new substances that are formed through oxidation of phenolic units, are toxic to different degrees. They can therefore serve as carbon sources only for special species of microorganisms. Besides the organic residue derived from higher animals and plants, many substances synthesized by microorganisms can serve as carbon sources.

Chemical Composition and Physical Properties of Humic Substances

The biosynthesis of steroids and triterpenoids

The ready oxidation of phenols to dimeric products has been well known for more than a century. The invaluable survey of the area by Musso1 lists over 20 papers published before 1900 on, for example: the formation of the dilactone (1) from gallic acid2 and its derivatives (1871);3 the production of both parapara and ortho-ortho coupled dimers from 1-naphthol (1873);4 the synthesis of the diphenoquinone (2),5 in 97% yield, from 2,5-dimethoxyphenol (1878); and the preparation of 2,2′-dihydroxybiphenyl from phenol itself (1878).6 Reagents used in such early work include iron(III) salts, potassium ferricyanide, oxygen and the halogens; electrochemical methods were also known, and enzyme-catalyzed reactions were reported in, for example, the oxidative dimerization of eugenol (1896).7 Extensive chemical investigations into this major reaction continued this century and further momentum was gained from the recognition of the role of oxidative coupling in biogenesis, signalled by the important and influential papers of Barton and Cohen,8 and of Erdtman and Wachtmeister.9 Subsequent biosynthetic investigations have confirmed the significance of the title reaction in the in vivo formation of many aromatic natural products including alkaloids (Battersby1 estimated that such coupling was involved in ca. 10% of known alkaloids), lignans, lignin, tannins, and plant and insect pigments. Many biomimetic studies of oxidative phenolic coupling were made, either for synthetic ends or to demonstrate possibilities in biological processes. Low chemical yields were often reported in such work, particularly in certain well-known alkaloid cases, and attention was turned to new reagents, e.g. manganese(III), vanadium(V) and thallium(III), which generally proved more effective. Further, oxidants such as vanadium(V) were found to effect coupling of phenol ethers, and interest was revived in the anodic oxidation of such aromatic substrates. The oxidation of phenol ethers has proved a valuable synthetic process, often more predictable and higher yielding than the corresponding phenol oxidations, and this reaction has been employed in recent years in a number of notable natural product syntheses.

Oxidative Coupling of Phenols and Phenol Ethers

Enantiomerically pure ( R )-2-naphthylmethoxyacetic acid (2-NMA) was synthesized from 2-naphthaldehyde via an integrated chemo-enzymatic procedure. The one-pot, successive use of SnBr 2 –TMSCN and AcBr worked effectively to give a racemic cyanohydrin acetate. Lipase from Burkholderia cepacia then mediated the highly enantioselective hydrolysis of the ( S )-enantiomer of the racemate, leaving the ( R )-acetate with an e.e. of >99.9%. The resulting product of this enzyme-catalyzed hydrolysis, an ( S )-cyanohydrin, spontaneously decomposed into naphthaldehyde, the starting material of this synthetic route, which could be recycled. The hydration of nitrile to amide as well as the hydrolysis of the acetate was performed with a microorganism, Rhodococcus rhodochrous , under very mild conditions without any loss of the enantiomeric purity. The amide group was hydrolyzed with nitrosylsulfuric acid, and the product was isolated as an α-hydroxy ester. The α-hydroxyl group was methylated with diazomethane–silica gel and the final task, hydrolysis of the ester, was accomplished under conditions as mild as neutral pH with an esterase from Krebsiella oxytoca to give enantiomerically pure 2-NMA.

Chemo-enzymatic synthesis of enantiomerically pure (R)-2-naphthylmethoxyacetic acid

Although copper(II)-mediated oxidation of phenols results in oxidative coupling rather than in oxygenation, it was recently reported that naturally occurring 5-alkylresorcinols undergo oxygenation in the presence of copper(II). To explore the generality of this reaction, the copper(II)-mediated autoxidation of 4-tert-butylresorcinol and 4,6-di-tert-butylresorcinol was investigated and was found to result in direct oxygenation at open activated positions and, at the tert-butyl-substituted positions, in oxygenation with competing loss of (as isobutylene) and 1,2-rearrangement of the tert-butyl group. 5-tert-Butyl-2-hydroxy-1,4-benzoquinone is the major product from both starting materials, and the final product mixture reflects, in part, coupling of metastable initially formed electrophilic and nucleophilic side products. Mechanisms that are consistent with the observed products and control reactions are proposed. The key step appears to be equilibration of a copper(II)−resorcinolate with a charge-transfer ...

Copper(II)-mediated autoxidation of tert-butylresorcinols.

Die Bildung von Hydroxyaryl‐chinonen durch Addition von Phenolen an Chinone

Synthesis of novel HMG-CoA reductase inhibitors. I: Naphthalene analogs of mevinolin

Der auffallende Unterschied in den Absorptionsspektren und pK-Werten zwischen Resorcinblau- und Orceinfarbstoffen wird durch sterische Mesomeriebehinderung und Wasserstoffbrucken erklart und an farblosen Tetrahydroxybiphenylderiaten bestatigt.

Über Orceinfarbstoffe, XIX. Der Einfluß von o‐Methylgruppen auf die Elektronenspektren und pK‐Werte von Orceinfarbstoffen und Hydroxy‐biphenylderivaten

Uwe I. Záhorszky

Papers

Die Bildung von Hydroxyaryl‐chinonen durch Addition von Phenolen an Chinone

Synthesis of novel HMG-CoA reductase inhibitors. I: Naphthalene analogs of mevinolin

Über Orceinfarbstoffe, XIX. Der Einfluß von o‐Methylgruppen auf die Elektronenspektren und pK‐Werte von Orceinfarbstoffen und Hydroxy‐biphenylderivaten

Diels-Alder-Reaktionen mit Fulgensäurederivaten und 2,3-Dimethylen-1,4-butandiol