Pantothenic biological synthesis

The hydrogenation of ketones with O or N functions in the a- or / -position is accomplished by several rhodium compounds [46 a, b, e, g, i, j, m, 56], Many of these examples have been applied in the synthesis of biologically active chiral products [59]. One of the first examples was the asymmetric synthesis of pantothenic acid, a member of the B complex vitamins and an important constituent of coenzyme A. Ojima et al. first described this synthesis in 1978, the most significant step being the enantioselective reduction of a cyclic a-keto ester, dihydro-4,4-dimethyl-2,3-furandione, to D-(-)-pantoyl lactone. A rhodium complex derived from [RhCl(COD)]2 and the chiral pyrrolidino diphosphine, (2S,4S)-N-tert-butoxy-carbonyl-4-diphenylphosphino-2-diphenylphosphinomethyl-pyrrolidine ((S, S) -... 

The process of racemization has a number of practical application in the laboratory and in industry. Thus, in the synthesis of an optical isomer it is frequently possible to racemize the unwanted isomer and to separate additional quantities of the desired isomer. By repeating this process a number of times it is theoretically possible to approach a 100% yield of Synthetic product consisting of only one optical isomer, An example of the utilization of such a process is found in the production of pantothenic acid and its salts, In this process the mixture of D- and L-2-hydroxy-3,3-butyrolactones are separated. The D-lactone is condensed with the salt of beta-alanine to give the biologically active salt of pantothenic acid, The remaining L-lactone is racemized and recycled. 

Since pantothenic acid contains an asymmetric carbon atom, chemical synthesis yields the racemic mixture. On the other hand, when enzymatic synthesis is used, only the biologically active form of pantothenic acid is produced. 

PUutothenie acid is a widely distributed compound in animals and plants, consisting of 2,4-dihydroxy-33-dimethylbutync acid (pantoic acid) linked to alanine by an amide bond Most organisms have the abili to synthesize pantoic acid from valine, and P-alanine from asparate, but humans lack the enzyme, pantothenate synthetase, which catalyses the condensation of p-alanine and pantoic acid to form pantothenic add Only the n(-i-)-form of pantothenic add is biologically active. It is required for the synthesis of Coenzyme A (see). Non-experimental human defi-dency states have not been observed, so pantothenic add is presumably present in suffident quantity in all diets. 

Some nonstandard amino acids are not found in proteins. Examples include lanthionine, 2-aminoisobutyric acid, dehydroalanine and the neurotransmitter gamma-aminobutyric acid. Nonstandard amino acids often occur as intermediates in the metabolic pathways for standard amino acids — for example ornithine and citrulline occur in the urea cycle, part of amino acid catabolism (see below). A rare exception to the dominance of a-amino acids in biology is the P-amino acid beta alanine (3-aminopropanoic acid), which is used in plants and microorganisms in the synthesis of pantothenic acid (vitamin B5), a component of coenzyme A. 

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