Carboxylic acids, unsaturated degradative oxidation

Degradative oxidation of unsaturated carboxylic acids leads either to aldehydes or to dicarboxylic acids. In the presence of sodium carbonate and benzene, o-nitrocinnamic acid is oxidized with potassium permanganate at 10 °C to o-nitrobenzaldehyde in 64% isolated yield [842]. 

Some further monosubstituted furoxans which have been reported are either of very doubtful authenticity or have been discredited. The phenacyl derivatives of Harries and Tietz,462 formed by the nitrosation of /J-aryl-oc,/ -unsaturated oximes, have been shown to be pyrazolone di-JV-oxides 463 The degradation of Holleman s peroxide (29) by hydroxylamine and alkali was reported to form structure 218, or its 3-substituted isomer, by Boyer and Chang.29 It does, however, seem unlikely that a monosubstituted furoxan could withstand such reaction conditions. Decarboxylation of furoxandi-carboxylic acid has been suggested to provide the 4-monocarboxylic acid,464 but Ponzio and De Paolini465 have assigned a nitrile oxide structure to the product. 

Two chemical features are characteristic for the naturally occurring mono- and polyunsaturated fatty acids a) the double bonds possess all-cis configuration, b) the double bonds are isolated by one CHg-group. It is a reasonable assumption that the saturated carboxylic and terminal methyl end of unsaturated acids are degraded by the reaction sequence described in the preceeding chapter. However, oxidation of the cis-/S, y- and cis-od, /S-unsaturated acyl-CoA intermediates requires two additional enzymes a) cis-/S, y-trans-od,/3-enoyl-CoA-isomerase and b) I)(—) hydroxyacyl-CoA epimerase. The isomerase catalyzes the following reaction ... 

During soya processing, volatile degradation compounds (hexanal, etc.) with a bean-like aroma defect are formed because of the enzymatic oxidation of unsaturated fatty acids. These defects can be eliminated by the enzymatic oxidation of the resultant aldehydes to carboxylic acids. Since the flavor threshold values of these acids are high, the acids generated do not interfere with the aroma improvement process. 

Oxidation of Fatty Acids.— The molecule of an unsaturated fatty acid such as oleic, is most likely to undergo oxidative attiack at the point of unsaturation the saturated acids, stearic and palmitic, are degraded by terminal oxidation. Fat oxidation occurs chiefly, if not entirely, in the liver, and under normal conditions the process is complete, and ends in COg and HjO. However, in diabetes and other conditions of carbohydrate inadequacy, fat oxidation in the liver is unable to proceed beyond acetoacetic acid, which suggests that this compound is an intermediate in fat metabolism. The natural fatty acids almost without exception contain an even total number of carbon atoms, and to explain the process of acetoacetic formation, Knoop proposed, in 1904, his theory of /5-oxidation of the fatty acids, according to which, the point of oxidative attack is the carbon atom in the /5-position, or next but one to the terminal carboxyl group. By this means the fatty acids are degraded two carbon atoms at a time. 

Meadowfoam is composed of unique long-ehain fatty aeids with 5-eieosenoie acid (62%) as the major fatty aeid. The other main eomponents are 5,13-doeosa-dienoic acid (19%), 5-docosenoic acid (3%) and 13-docosenoic acid (10%). The A5 unsaturation provides an excellent chemical moiety for synthetic modifications to the oil or fatty acids by providing a site at which a carboxylate stabilized carbocation can form. In addition, the A5 unsaturation has enhanced resistance to oxidative degradation (8) as evidenced by the high OSI of the oil (246.9 h at 110°C) or its methyl ester (69.4 h at 90°C). 

---