Esters 2,4-diunsaturated

The flavour of distillates from apple and pear is characterised by typical aroma compounds from these fruits formed by enzymatic degradation of fatty acids to C6-fragments like hexanol, trans-2-hexenol, as well as ethyl esters and acetates of hexanoic acid. In distillates of pears, especially of the variety Bartlett pear, the characteristic pear flavour is mainly dominated by the ethyl and methyl esters of frans-2-czs-4-decadienoic acid and trans-2-trans-A-decadienoic acid [27-29], The biogenesis of these monounsaturated, diunsaturated, and triunsaturated esters may be explained by -oxidation of unsaturated linoleic and linolenic acid in the fruits. The sesquiterpene compound a-farnesene, which is formed during postharvest ripening and storage of Bartlett pears [28], shows that quality and intensity of distilled pear spirits is mainly influenced by the quality and degree of ripeness of the fruits. 

Ethyl E7E -2,4-dienoutes.1 These diunsaturaled esters can be prepared by Knoevenagel condensation (piperidine catalyzed) of ethyl 2-phenylsulfinylacetate with aldehydes and thermolysis of the product in the presence of potassium carbonate (equation I). 

Methyl (2E,4E)Senoates. The dianion (2) of 1 can be prepared by treatment with I.DA and then sec-butyllithium in THF at —78°. The dianion is alkylated selectively at the a-allylic position to give 3 as the major product. Treatment of 3 with LDA induces a [2,3]sigmatropic rearrangement to 4. Remaining steps to the diunsaturated ester (6) are methylation, oxidation, and dchydrosulfcnylation.1... 

Reduction of conjugated diunsaturated acids.3 a,/ y, 5-Diunsaturated acids are reduced by sodium dithionite in an alkaline medium (NaOH, NaHC03) to a mixture of (Z)- and (E)-/J,y-unsaturated acids (40 75% yield). A similar reduction of the diunsaturated esters is possible under phase-transfer conditions (Adogen 464, C6H6-HzO). 

Pentadienoates.1 Half esters of malonic acid react with a,(3-enals in pyridine containing dimethylaminopyridine to form these diunsaturated esters, with generally high (2E)-selectivity. 

Methyl2,4-pentadienoates. These diunsaturated esters are obtained in >85% yield by reaction of a, 3-enals with 1 in pyridine catalyzed by dimethylaminopyri-dine. 

For the study of the physical and chemical properties of triply unsaturated fatty acids and for the comparison with y-linolenoic acid [6Z,9Z.12Z)-6,9,l2-octadeca-trienoic acid] coworkers of the Unilever Research Centre in Vlaardingen (Netherlands) synthesized some methyl esters of ( ,Z,Z)-trisunsaturated fatty acids 73). For the preparation of the esters of (2E,9Z, 12Z)-2,9,12-octadecatrienoic acid and of (2 ,11Z,14Z)-2,11,14-eicosatrienoic acid (68 a and b) they used the Wittig reaction to introduce the ( )-2-double bond into the starting diunsaturated aldehyde. Reduction of the acid chlorides of (7Z,10Z)-7,10-hexadecadienoie acid and of linolenoic acid (65a and b) with lithium tri-tert-butoxyaluminium hydride affords the corresponding aldehydes 66 a and b which can be olefinated with the stable ylide 67 to methyl ( ,Z,Z)-alkatrienoates 68a, b with a (Z)-2-isomer content of 4.6%73) (Scheme 13). 

Polyunsaturated aliphatic alkohols, aldehydes, ketones, and esters occur as fragrance components in fats, oils, fruits and plants 158). As an example the synthesis of ethyl (2 ,4Z)-2,4-decadienoate (232, pear ester ), which is responsible for the aroma of bartlett pears 161) is given. 2,4-Diunsaturated ester 232 may be obtained by a number of highly stereoselective syntheses, a lot of them making use of the Wittig reaction. Ohloff and Pawlak condensed 4,5-epoxy-( )-2-pentenal 228 with the ylide generated from 229 (butyllithium/ether) to the alkadiene epoxide 230 which was oxidized with periodic acid to the 2,4-decadienal 231. 231 is subsequently converted with MnOj/NaCN in ethanol to the pear ester 232162) [75 % (Z)-amount of the C-4 double bond] (Scheme 44). 

Monocarboxylic esters were further separated by argentation TLC [silica GF254 impregnated with 5% (w/v) silver nitrate] into saturated esters (Rf = 0.51), monounsaturated esters (R = 0.42), diunsaturated esters (Rf = 0.29), and poly unsaturated esters (Rf < 0.29) by development in n-heptane-diethyl ether-methanol (90 10 1, v/v/v) (19). 

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. 

Two routes for synthesizing macrolides via olefin metathesis (WCl6/Me4Sn catalyst) have been described by Villemin one route involves the preparation of co-hydroxyacid by metathesis followed by cyclization, the second involves macrocyclic ring closure by metathesis of co,co -diunsaturated ester. 

Pettit et al.,ie dehydrogenated the -y,5-unsaturated ester (1) to give the oefiryja-diunsaturated ester (2,3/3-acetoxyisobufalin methyl ester). 

A novel group of compounds characterized by attachment of saturated, mono- and diunsaturated alkyl side chains to position 3 of pyrrol-2-aldehyde (258, 259, 260 and 261) has been isolated from the sponge Oscarella lobularis 196), which has also yielded the corresponding carboxylic acids and their methyl esters. 

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