Oxygen natural occurrence

Much of the synthetic chemistry of xanthones has arisen out of their natural occurrence, a topic which has been reviewed (61CRV591), and also as a result of their incorporation into dyes. There are only two routes of any significance to xanthones and these basically involve ring closure on to either the heterocyclic oxygen atom or the carbonyl carbon atom (Scheme 185). 

The natural occurrence of oxygen heterocycles is well documented (B-63MI22400) and almost every category of the compounds discussed in the earlier sections provides examples of secondary metabolites. A discussion of their biosynthesis is therefore appropriate. 

Shafizadeh, F., Branched-chain Sugars of Natural Occurrence, 11, 263-283 Shafizadeh, F., Formation and Cleavage of the Oxygen Ring in Sugars, 13,9-61 Smith, F., Analogs of Ascorbic Acid, 2, 79-106... 

The replacement of the oxygen atom of the secologanoside moiety (20) of strictosidine by a nitrogen atom in angustine bases 15 and in compounds 7 to 13 raised the question of their natural occurrence in plants. The usual extraction... 

High concentrations of nickel may occur in groundwater in areas with mafic or ultra-mafic rocks. Concentrations of nickel in water from natural occurrences are only likely to be of health concern in environments where pH is less than 4.5 or where groundwater pumping has introduced oxygen into an anaerobic aquifer. 

Sulfonic acids of type 1, chiral by virtue of isotopic substitution, seem not to have been prepared. There is no reason why they could not be synthesized with high isotopic content since both 170 and lsO labeled oxygen and water of high isotopic purity are available. For example, hydrolysis of an ester of type 2, where R is aryl, using 170-labeled water might be a source of 1 of known configuration. The natural occurrence of 1 as a racemic modification is very small and is calculated to be 753 parts per billion2. 

Natural occurrence. Oxygen is the most abundant chemical element in the Earth s crust with 46.1 wt.% mostly combined as silicates (e.g., quartz and silica, feldspars and feld-spathoids, zircon), carbonates (e.g., calcite, dolomite, siderite) and oxides (e.g., hematite, rutile, zincite, cuprite), while it forms 20.65 vol.% of the air and 89 wL% of the ocean water. 

Natural occurrence. Carbon monoxide is produced during the incomplete combustion of carbon and carbon-containing compounds, hence it occurs in the exhaust of internal-combustion engines, in coal stoves, furnaces, and gas appliances functioning with an oxygen deficiency. Carbon monoxide is also naturally present in the atmosphere, chiefly as a product of volcanic activity. It occurs dissolved in molten volcanic rock at high pressures in the Earth s mantle. Carbon monoxide contents of volcanic gases vary from less than 1000 ppm vol. to as much as 2 vol.%. It also occurs naturally in bushfires. 

The term terpene is used to describe a compound, which is a constituent of an essential oil containing carbon and hydrogen or carbon atoms, hydrogen, and oxygen atoms, and is not aromatic in character [24, 25]. This definition is usually extended to include other compounds called terpenoids, which are not of natural occurrence but are very closely related to the natural terpenes. Most terpenes, which include terpenoids, are invariably hydrocarbons, alcohols, aldehydes, ketones, or oxides, and they may be solids or liquids. Terpene hydrocarbons are usually liquids, while terpenes of higher molecular weights, mostly obtained from the natural gums and resins of plants and trees, are not steam volatile. 

The iboga alkaloids distinguish themselves from most of Type I compounds by the ease with which they autodxidize to yield hydroperoxy- and hydroxyindolenines (Chart 9.1) whose further degradation products are 4-hydroxyquinolines, pseudoindoxyls and oxindoles (cf. Chapter III). Therefore, the isolation of these products from the plant cannot by itself be taken as proof of their natural occurrence. Why some of these bases are so susceptible to oxygen is not known and is certainly not so obvious as the... 

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