Branched-chain of natural occurrence

Principal Branched-chain Sugars and Cyclitols of Natural Occurrence... 

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... 

F. Shafizadeh, Branched-chain sugars of natural occurrence, Adv. Carbohydr. Chem., 11 (1962) 263-283. 

Shafizadeh, F., Branched-chain Sugars of Natural Occurrence, XI, 263-283... 

Occurrence in nature of branched-chain carbohydrates has prompted interest in the syntheses of these complex structures and stimulated the preparation of analogues for biological evaluation. Consequently, new methods for the construction of these particular skeletons have been devised [1]. The use of carbohydrates as a cheap source of chiral starting materials [2-4] for the synthesis of complex, nonsugar molecules has prompted the emergence of new imaginative methods for formation of carbon-carbon bonds adapted to the particular reactivity of sugar moieties. 

I he occurrence of a spontaneous explosion in a chemically reacting system is a complicated process. However, the events that lead to explosion can be characterized as being either of a branching chain or of a thermal nature. Branching-chain explosions occur in systems that react by a chain mechanism, the details of which allow the chain carrier concentration, and hence, the over-all reaction rate to increase without limit, even under isothermal conditions. Such a condition is possible only if one or more of the steps in the reaction chain results in a multiplication of chain carriers—i.c., X + A — Y + Z + , where X, F, and Z arc chain carriers. 

In O Scheme 1, O Scheme 2 and O Scheme 3, some of the important representatives of natural monosaccharides are presented and compiled by the three different classes deoxy-sugars, amino-sugars, and branched-chain sugars. O Table 1 lists their names and natural sources of occurrence. 

Reviews of fungal fatty acid composition (, 7, ) reveal that their primary constituents are 12- to 20-carbon chain length unbranched compounds, with even-numbered chains predominant. Both saturated and unsaturated compounds occur, with palmltolelc (C-16 l), oleic (C-18 l), llnolelc (C-18 2) and llnolenlc (C-18 3) acids the most common unsaturated moieties ( ). As with most naturally occurring fatty acids (9), monounsaturated compounds usually contain a els oleflnlc bond and polyunsaturated acids have methylene-interrupted els double bonds. Although rare In occurrence and subjected to limited study, branched chains, hydroxy, oxo and epoxy acids are also synthesized ( 5). Lists of the structures of unusual fatty acids which can be employed In structure activity studies are presented In detail elsewhere (9-14). 

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