Regenerated nitrogen sources

The AA reaction is closely related to the asymmetric dihydroxylation (AD). Alkenes are enantioselectively converted to protected 3-aminoalcohols (Scheme 1) by syn-addition of osmium salts under the influence of the chirr 1 bis-Cinchona ligands known from the AD process (see Chap. 20.1). As for the AD reaction, a cooxidant is needed to regenerate the active osmium species. But in the AA process the cooxidant also functions as the nitrogen source. Since two different heteroatoms are transferred to the double bond, regioselectivity becomes an important selectivity issue in addition to enantioselectivity. Moreover, chemoselectivity has to be addressed due to the possible formation of the... 

Sources of regenerated nitrogen associated with sediments... 

Cyano-derivatives can be readily obtained by a ruthenium-catalyzed addition of various hydrazines to terminal alkynes [89] in which the cyano carbon atom arises from the terminal alkyne carbon atom. The tris(pyrazolyl)borate (Tp) complex RuCl(Tp)(PPh3)2 (1 mol%) was found to be the most active catalyst, and N,N-dimethylhydrazine (5 equiv.) the best nitrogen source. The proposed mechanism involves the nucleophilic attack of the nitrogen nucleophile on the a-carbon of a vinylidene intermediate (Scheme 8.27). Proton migration in the resulting a-hydrazi-nocarbene, followed by deamination, would give the nitrile derivative and regenerate the catalytic species. 

Whole-cell biocatalysts are basically one-pot cascade reactions, with various enzymatic reactions being carried out concurrently within individual cells. Compared to purified enzymes, whole-cell biocatalysts are inexpensive and easily scalable and can be stably stored indefinitely. Certain enzymes are unstable and lose activity when purified from cells. Living cells also contain and regenerate otherwise expensive redox cofactors and, with metabolic engineering, can produce desired chemicals from inexpensive carbon and nitrogen sources [66, 67]. On the other hand, a major downside of using whole cells for biotransformations is the increased cost of product extrachon and purification from fermentahon broths. One also has to consider the... 

In the urea cycle ammonia is first combined with C02 to form carbamoyl phosphate. This then combines with ornithine to form citrulline. Citrulline then condenses with aspartate, the source of the second nitrogen atom in urea, to form argininosuccinate. This compound is in turn split to arginine and fumarate, and the arginine then splits to form urea and regenerate ornithine The first two reactions take place in the mitochondria of liver cells, the remaining three in the cytosol. 

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