Double-bond migration and cyclization

The movement of double bonds in long-chain unsaturated acids has been known since Varrentrapp converted oleic acid to palmitic by fusion with alkali, an observation which led to the invalid conclusion that oleic acid was the A2 or A3 acid. Double bond migration unaccompanied by chain fission occurs under milder conditions. The reaction also occurs more easily with methylene-interrupted polyene acids to give products with conjugated unsaturation which are easily recognized by ultraviolet spectroscopy (Section 9.3). 

Homogeneous organometallic catalysts such as iron and cobalt carbonyls and rhodium complexes effect double-bond migration as do some metals and their oxides, sulphur dioxide and iodine. 

Reactions occurring at allylic sites are generally accompanied by double-bond migration. This is important in oxidation (Sections 10.2 and 10.3). 

Polyene acids undergo cyclization (without dimerization) at elevated temperatures. This reaction requires double bonds in appropriate positions and of correct configuration and reagents such as alkali (to promote double-bond migration) and sulphur or iodine (to promote stereomutation) are frequently used. Cyclization of linoleate has been 

Isomerization has been effected under a range of alkaline conditions including potassium hydroxide in ethanediol at 170-180 °C, potassium r-butoxide in r-butanol at 70 °C or in diethyl ether at 25 °C, the potassium salt of the methyl ether of triethylene glycol at 25 or 60 °C, dimsyl sodium or dimsyl potassium in dimethyl sulphoxide and/or tetra-hydrofuran at 25 °C, the potassium salt of 1,3-diaminopropane at 50 °C and tetramethylammonium hydroxide. 

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Double-bond Migration and Cyclization Reactions.—Linolenic acid (18 3 9cl2d5c) is converted by potassium hydroxide at 180°C or by sodium hydroxide at 220 °C, first to a conjugated trienoic acid, and then to a di-substituted cyclohexadiene (13—21 %), which is identified after aromatization with palladium-charcoal. A series of octadecatrienoic acids, when submitted to this reaction, gave the product(s) indicated in parentheses 9cl2cl5c (28 n = 3), 9/12/15/ (28 n =2,3, and 4), 9cll/13/ (28 n = 3, 4, and 5),... 

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