Methyl linoleate reduction

Figure 1.7 Typical zero-order and corresponding second-derivative electronic absorption spectra of ethanol-reconstituted lipid/chloroform extracts of autoxidized model polyunsaturated fatty-acid compounds and inflammatory synovial fluid obtained after (1) reduction with NaBH4 and (2) dehydration with alcoholic H2S04- (a) Methyl linoleate subsequent to autoxidation in air at ambient temperature for a period of 72 h (—), or exposure to a Fenton reaction system containing EDTA (5.75 x 10 mol/dm ), H2O2 (1.14 X 10 mol/dm ) and Fe(ll) (5.75 x IO mol/dm ) as an aqueous suspension (—) (b) as (a) but with methyl linolenate (c) untreated rheumatoid knee-joint synovial fluid.

Phenols are important antioxidants, with vitamin E being the most important endogenous phenolic membrane-bound antioxidant. Membrane levels of vitamin E are maintained through recycling of the vitamin E radical with ascorbate and thiol reductants. Vitamin E is a mixture of four lipid-soluble tocopherols, a-tocopherol being the most efiective radical quencher. The reaction of a-tocopherol with alkyl and alkylperoxyl radicals of methyl linoleate was recently reported. These are facile reactions that result in mixed dimer adducts (Yamauchi etal., 1993). 

This complex reached a maximum of about 10% in the early stages of reduction and then decreased. Initially the relative specific radioactivity was 1.93 for methyl linoleate and zero for conjugated diene. Relative specific radioactivity in the products at 50% reduction was about equal in the monoene (1.0), stearate (0.96), and diene-Fe(CO)3 (1.0), but only minor in the free conjugated diene (0.14). Although the diene-Fe(CO)3 complex is a significant intermediate in the reduction of linoleate, the free conjugated diene is not. 

During the inhibited self-initiated autoxidation of methyl linoleate by a-Toc in solution, Niki and coworkers made the interesting observation that a-Toc acts as an antioxidant at low concentrations, but high concentrations (up to 18.3 mM) actually increased hydroperoxide formation due to a pro-oxidant effect. The pro-oxidant effect of a-Toc was observed earlier by Cillard and coworkers in aqueous micellar systems and they found that the presence of co-antioxidants such as cysteine, BHT, hydroquinone or ascor-byl palmitate inverted the reaction into antioxidant activity, apparently by reduction of a-To" to a-Toc . Liu and coworkers ° found that a mixture of linoleic acid and linoleate hydroperoxides and a-Toc in SDS micelles exhibited oxygen uptake after the addition of a-Toc. The typical ESR spectrum of the a-To" radical was observed from the mixture. They attributed the rapid oxidation to decomposition of linoleate hydroperoxides, resulting in the formation of linoleate oxy radicals which initiated reactions on the lipid in the high concentration of the micellar micro-environment. Niki and coworkers reported pro-oxidant activity of a-Toc when it was added with metal ions, Fe3+25i Qj. jjj (jjg oxidation of phosphatidyl choline liposomes. a-Toc was found... 

Lie and coworkers31 reported the synthesis and NMR properties of all geometrical isomers of conjugated linoleic acids. Pure geometric isomers of conjugated linoleic acid (CLA) were prepared from castor oil as the primary starting material. Methyl octadeca-9Z, 11 /i-dienoate (36) and methyl octadeca-9Z,llZ-dienoate (38) were obtained by zinc reduction of methyl santalbate (35, methyl octadec-11 -en-9-ynoatc) and methyl... 

Hilditch and Pathak (58) studying the catalytic reduction of methyl eleostearate, found that the reaction at 110° and at 170° was extremely selective in the presence of Raney nickel. No methyl stearate was formed until 90% of the linoleate had been transformed into octadeceno-ates. Ehrhart (59) used Raney nickel to hydrogenate compounds of the type RCH(CN)NHCOR The nitrile was reduced to the amine, but the amide was untouched. 

Hydrogenation of methyl p-eleostearate (methyl frani,fran5,fra s-9,ll,13-octadecatrienoate) with [Cr(CO)3(arene)] yields the diene products from 1,4-addition trans-9-cis- 2- and CK-10-fran5-13-oc-tadecadienoates). With a-eleostearate (methyl d5,fran5,fra i-9,ll,13-octadecatrienoate), stereoselective 1,4-reduction of the trans,trans-diene moiety yields linoleate (cis,cis-9,l2) accompanied by cm,Irani-1,4-dienes which are formed from the isomerization of a- to p-eleostearate. ... 

The Fe(CO)4 intermediates of types III and VIII in Scheme IV explain the direct reduction paths evidenced in the hydrogenation of mono- and diunsaturated fatty esters. Competition between monoene and diene hydrogenation can be related to the stability of the Fe(CO)3-and Fe(CO)4-complexes. At a low concentration of Fe(CO)5, the formation of Fe(CO)a complexes is favored because they are more stable. At a high concentration of Fe(CO)s, formation of mono- and di-Fe(CO)4 complexes becomes important, and selectivity for diene hydrogenation is decreased. Although the occurrence of olefin-Fe(CO)4 complexes has precedence in the literature (i9), no such species has yet been identified with either methyl oleate or linoleate. 

The same result (7b) was obtained in a comparable experiment with linolenic acid labeled in position 17 (Table II). Here the question was raised of whether a transition to polyenoic acids of the linoleic or oleic acid type occurs by hydrogenation of the double bonds next to the methyl end. In this case reductive ozonolysis would yield not only radioactive propanal but also hexanal and nonanal. It is evident from Table II that the specific activity of hexanal and nonanal is practically zero that of propanal, however, is 9000-12,000 dpm/mg. 

The desired increase in melting point is achieved by partial hydrogenation. Complete reduction would produce a fat with too high a melting point. The products of partial hydrogenation, however, are more complex than might have been predicted and understanding of this topic has developed from studies with individual esters such as methyl oleate, linoleate and linolenate. Useful information has also come from deuteration studies. 

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