Linoleic acid Diels-Alder reaction

In the area of moleculady designed hot-melt adhesives, the most widely used resins are the polyamides (qv), formed upon reaction of a diamine and a dimer acid. Dimer acids (qv) are obtained from the Diels-Alder reaction of unsaturated fatty acids. Linoleic acid is an example. Judicious selection of diamine and diacid leads to a wide range of adhesive properties. Typical shear characteristics are in the range of thousands of kilopascals and are dependent upon temperature. Although hot-melt adhesives normally become quite brittle below the glass-transition temperature, these materials can often attain physical properties that approach those of a stmctural adhesive. These properties severely degrade as the material becomes Hquid above the melt temperature. 

Conjugation as well as geometric and positional isomerization occur when an alkadienoic acid such as linoleic acid is treated with a strong base at an elevated temperature. CycHc fatty acids result from isomerization of linolenic acid ia strong base at about 250°C (58). Conjugated fatty acids undergo the Diels-Alder reaction with many dienophiles including ethylene, propylene, acryUc acid, and maleic anhydride. 

Structure and Mechanism of Formation. Thermal dimerization of unsaturated fatty acids has been explaiaed both by a Diels-Alder mechanism and by a free-radical route involving hydrogen transfer. The Diels-Alder reaction appears to apply to starting materials high ia linoleic acid content satisfactorily, but oleic acid oligomerization seems better rationalized by a free-radical reaction (8—10). 

Antioxidant capacities of common individual curcuminoids were determined in vitro by phosphomolybdenum and linoleic acid peroxidation methods. Antioxidant capacities expressed as ascorbic acid equivalents (pmol/g) were 3099 for curcumin, 2833 for demethoxycurcumin, and 2677 for bisdemethoxycurcumin at concentrations of 50 ppm. The same order of antioxidant activity (curcumin > demethoxycurcumin > bisdemethoxycurcumin) was observed when compared with BHT (buty-lated hydroxyl toluene) in linoleic peroxidation tests. The antioxidant activity of curcumin in the presence of ethyl linoleate was demonstrated and six reaction products were identified and structurally characterized. The mechanism proposed for this activity consisted of an oxidative coupling reaction at the 3 position of the curcumin with the lipid and a subsequent intramolecular Diels-Alder reaction. ... 

Preparation by Chemical Multistep Synthesis. During the preparation of hexadeuter-ated linoleic acid reported by Viala and Labaudiniere (13), (9Z,11 )-[14,14,15,15, 17,18- H]-octadecadienoic acid was obtained as a by-product in 3-20% yields. The formation of this conjugated linoleic acid was attributed to partial isomerization of the key intermediate (Z)-[5,5,6,6,8,9- H]-non-3-enal under alkaline conditions as depicted in Figure 6.2. This labeled CLA isomer was not isolated but was eliminated by chemical purification involving in situ Diels-Alder reaction with maleic anhydride. 

Polyamides. Polyamides are one of the largest volume epoxy curing agents used. They are prepared by the reaction of dimerized and trimerized vegetable-oil fatty acids with polyamines. Dimer acid is made by a Diels-Alder reaction between 9,12- and 9,11-linoleic acids. Subsequent reaction with diethylenetriamine or other suitable multifunctional amines 5uelds the amine-terminated polyamides. They are available in a range of molecular weights and compositions. 

In the presence of a catalytic amount of Sc(OTf)3, conjugated linoleic acid ethyl ester could be used as a diene in Diels-Alder reactions to react with quinines, a,P-unsaturated aldehydes, and ketones in good yields [26]. Sc(OTf)3 catalyst could be recovered by extraction of organic layers with water. After removal of water, the recovered Sc(OTf)3 could be reused in the Diels-Alder reaction without appreciable loss of catalytic activity. 

Naturally occurring linoleic acid contains a 1,4-diene. One might expect that the primary product from an ene reaction would lead to the conjugated diene 19 which could in turn undergo Diels-Alder reaction to give a product such as 20. 

C-21 dicarboxyhc acids are produced by Westvaco Corporation in Charleston, South Carolina in multimillion kg quantities. The process involves reaction of tall oil fatty acids (TOFA) (containing about 50% oleic acid and 50% hnoleic acid) with acryhc acid [79-10-7] and iodine at 220—250°C for about 2 hours (90). A yield of C-21 as high as 42% was reported. The function of the iodine is apparendy to conjugate the double bond in linoleic acid, after which the acryhc acid adds via a Diels-Alder type reaction to form the cycHc reaction product. Other catalysts have been described and include clay (91), palladium, and sulfur dioxide (92). After the reaction is complete, the unreacted oleic acid is removed by distillation, and the cmde C-21 diacid can be further purified by thin film distillation or molecular distillation. 

In recent years some work has been done to link oleochemicals with petrochemicals via oligomerization. One possibility is the Dids-Alder reaction of linoleic acid esters with dienophiles, for instance with quinones or ,/Tun saturated aldehydes and ketones [80]. Using scandium or copper triflates as catalysts the reaction can be carried out at very mild temperature conditions (25-40°C) with good yields (< 94%). For the first time in oleochemistry it was possible to carry out Diels-Alder cycloadditions with low catalyst concentrations instead of stoichiometric amounts of Lewis acids. The most successful way to recycle the catalyst was the successive extraction of the triflates with water. After removing the water and drying in vacuum the catalyst was used three times without any loss of yield. 

So, maleic anhydride Diels-Alder addition products to (conjugated) linoleic acid (CLA, Figure 3B.16, a.) may be prepared both with and without catalytical amounts of iodine, clay, or silica at a reaction temperature generally from about 100°C to about 230 °C. Similarly, the Diels-Alder addition of acrylic acid yields a C21-diacid (Figure 3B.16, b.). Under somewhat more vigorous conditions, the monounsaturated oleic/elaidic acids react with maleic anhydride at 215 °C to about 250 °C to form an ene-adduct (Figure 3B.16, c.). Diels-Alder and ene reaction products ofTOFA form polyfunctional carboxylic acids that can further be sulfonated by reaction with sulfuric acid and/or can undergo esterification or amidation reactions. - ... 

Diels-Alder derivatives can be made from TOFA. For example, an interesting group of chemicals is based on the product of the reaction of TOFA-derived conjugated linoleic acids with acrylic acid. The C21 dibasic acid product and its derivatives have interesting properties that lead to diverse uses such as in detergents, textile lubricants, fabric softeners, corrosion inhibitors, inks, and floor polishes. 

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