Polyene acids

Various unsaturated thiophene compounds were investigated viz., polyenes, polyenic acids and aldehydes and various acetylenic derivatives (59), (60), (76-81). 

Desaturation to Monoene and Polyene Acids in Plant Systems... 

There is an improvement in stability toward atmospheric oxidation resulting from reduced levels of the methylene-interrupted polyene acids that are so easily oxidized. 

Oils containing Cjg polyene acids Calendula officinalis seed oil. Calendula oil (from marigold) is of interest because it contains about 58% of calendic acid (8fl0fl2c-18 3). This unusual acid is an isomer of a-eleostearic (9cllfl3f-18 3) present in tung oil, and calendula oil should also be a good drying oil. The presence of linoleic acid (30%) will add to the unsaturated nature of this oil (131-133). 

Raney alloy in an alkaline medium reduces aromatic halides to hydrocarbons in good yielil. It has been used for reduction of the chlorinated polyene acid (4) to X phcnyl-/i-valcric acid (5). ... 

Although subsequent studies of Deuteromycotina (Table X) and Ascomycotina (Table XI), performed by Block [57] for the triglyceride fraction and by Cooney [58] for total fatty acids, confirmed the preponderance of unsaturated fatty acids (33 to 79%, but generally 58 to 79%) [58] and co3 and co6 acids, polyenic acids in C20 were only found in small quantities, and no acids were detected in C22.6 [58,59]. 

Fish oils contain a wide range of fatty acids from C14 to C26 in chain length with 0-6 double bonds. The major acids include saturated (14 0, 16 0, and 18 0), monounsaturated (16 1, 18 1, 20 1, and 22 1) and n-3 polyene members (18 4,20 5,22 5, and 22 6). Fish oils are easily oxidized and are commonly used in fat spreads only after partial hydrogenation. However, they are the most readily available sources of n-3 polyene acids, especially, EPA and DHA, and with appropriate refining procedures and antioxidant addition these acids can be conserved and made available for use in food. The long-chain polyene acids are valuable dietary materials and there is a growing demand for high quality oil rich in EPA and DHA. °... 

It was considered that cyclopentane systems might be formed from linoleate and related polyene acids either by the polar reaction sequence shown in Scheme 5 or its radical equivalent. When linoleate was reacted with acetic anhydride in a radical addition process promoted by ditertiarybutyl peroxide, 1 1 and 1 2 products were obtained. The former, after reaction with acidic methanol, were mainly unsaturated diesters such as [1], but some saturated compounds were also present and these may have been the cyclopentane derivatives ([2] and [3]) (9). 

Methylene-interrupted and other non-conj ugated polyene acids... 

Table 1.3 Natural polyene acids (methylene-interrupted) arranged by families ...

Scheme 1.1 The n—3 family of polyene acids based on linolenic acid. The heavy arrows show the relationship between the most important n—3 acids through desaturation (vertical arrows) and chain elongation (horizontal arrows). Other structures result by elongation or shortening. The arrangement is such that acids with the same number of double bonds appear in horizontal arrays and those with the same number of carbon atoms appear in vertical arrays. [ ] is used for as yet unidentified acids.

Also identified are about thirty polyene acids in which unsaturation is not completely methylene-interrupted. These acids are probably derived from the more common monoene and polyene acids by insertion of an additional double bond, most often at A5 (though sometimes A2 or 3). Subsequent chain extension may shift these double bonds to the A7 or A9 positions. Such acids occur in some seed oils, in some micro-organisms and in some marine lipids (especially sponges). This last source, for example, has furnished 26 3 (5,9,19), 28 3 (5,9,19) and 30 3 (5,9,23) which may be produced from hexadec-9-enoic, octadec-9-enoic and hexadec-9-enoic acids respectively (Litchfield et al., 1980). 

Table 1.4 Natural non-methylene-interrupted polyene acids (the double bonds are known to have or expected to have cis configuration unless otherwise indicated)...

Three acetylenic acids present in the lipids of mosses (18 3 6a9cl2c, 18 4 6a9cl2cl5c and 20 3 8allcl4c) are clearly related to the corresponding polyene acids. 

Crystallization is a mild procedure especially suitable for the polyene acids which are so easily oxidized or otherwise modified at elevated temperatures. Separation of saturated and monoene acids from one another by crystallization is good separation of polyene acids from one to another is less satisfactory. 

The procedure is employed for two purposes. It separates straight-chain acids from branched-chain or cyclic compounds with the former normally concentrating in the adduct and the latter normally in the mother liquor. It has thus been used to concentrate branched-chain and cyclic acids in natural mixtures particularly when these are present only as trace constituents and to isolate cyclic acids prepared from polyene acids. One report, for example, describes the isolation of cyclic acids produced from linseed oil by high-temperature reaction with alkali in 90-95% yield and 95% purity. Urea fractionation is also used to separate adds or esters of differing unsaturation and is an important step in the isolation of pure oleic, linoleic, and linolenic acids from natural sources (Section 4.7). For example, the mixed fatty acids of cod liver oil containing 21% of n — 3 adds (mainly 18 4, 20 5 and 22 6) form an 85% concentrate of these adds (in 25% yield) after a single treatment with urea. 

Also at this time the merits of the conjugated oils were recognized. These contain conjugated polyene acids (Section 1.5). Foremost of these was tung oil, originally from China. Compared to linseed oil, tung oil could be heat-bodied in minutes rather than hours. 

Linoleic acid was first synthesized via acetylenic intermediates in the early 1950s but it was not until 1961 that a general procedure for obtaining the natural methylene-interrupted polyenes was outlined (Os-bond et ai, 1961). This has been modified in useful ways by Kunau (1971) and, in the original or modified form, it has been used to prepare a large number of polyene acids (Sprecher, 1979). The necessary poly-ynoic acid is prepared first. This is a crystalline solid which can be thoroughly purified by crystallization, prior to partial hydrogenation with Lindlar catalyst. 

Non-catalytic reduction of alkenes is conveniently carried out with hydrazine. Oxygen is a necessary reagent and the reaction probably involves di-imine (N2H2). The reduction is stereospecific (cw) and occurs without double-bond migration or stereomutation. Partial reduction of a polyene acid or ester thus gives a simpler product than catalytic processes. A typical experiment with linolenic acid is summarized below and shows the range of products resulting from partial reduction. 

The enzymic oxidation of fatty acids in animal systems, involving mainly C20 polyene acids and leading to prostaglandins, thromboxanes and leukotrienes, has already been discussed (Section 1.4) and attention is now directed to oxidation in plant systems under the influence of lipoxygenase (Section 11.1.6). 

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). 

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... 

Interesterification procedures are used industrially to improve the physical properties of lard, to produce cocoa butter substitutes from cheaper oils (usually combined with hydrogenation and fractionation), to produce fats containing acetic acid, and to produce margarine of appropriate melting behaviour with a minimum content of trans acids and maximum content of polyene acids. This has been achieved, for example, by interesterification of soybean oil (80%) and fully hydrogenated soybean oil (20%). 

Occurs in small proportions in ruminant fats (e.g., butter) via biohydrogenation of dietary polyene acids. Mp 43.5-44.1 . 

New acids identified among the polyene acids in Norwegian spruce include three unsaturated anteiso acids (5,9 9,12 5,9,12)... 

From a study of polyene acids in the major food sources of an Arctic diet it is estimated that this typically provides about 7 g... 

Syntheses of polyene acids In the chain-length range S 20 have been reported. 6-Ethynyl-6-propiolactone (27) is used for five-carbon homologation, giving first 3,4-alkadienoic acids and... 

Autoxidation of phospholipids containing polyene acids gives the expected products though the reaction may be slower than anticipated in bilayers where the initiation process is rather inefficient and oxidisability seems to be reduced, possibly by expulsion of peroxy radicals from the autoxidisable part of the bilayer. 

Oxidation of polyene acids with hydrogen peroxide and copper(ii) salts... 

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