Bartlett pear ester

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. 

Jennings et al (228) fractionated an extract from Bartlett pears into 32 volatile components of which five were found to contribute significantly to the characteristic pear aroma. Later studies indicated that esters of trans-2 cis-4 decadienoic acid and hexyl acetate were significant components of the Bartlett pear aroma (229, 230). More recently, numerous volatiles of Bartlett pears were separated and identified including esters of methyl, ethyl, propyl, butyl, and hexyl alcohols, and Cio to Ci8 fatty acids (231, 232). 

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). 

Unsaturated esters may also be derived from the products of B-oxidation of unsaturated fatty acids. Methyl-and ethyl decadienoates are known as character impact compounds of Bartlett pears and all presumptive intermediates of the pathway were identified by Jennings et al. (JLZ UL) binoleyl-CoA is transformed into (E,Z)-2,4-decadienoyl-CoA by B-oxidation. The further degradation... 

Et ester [3025-30-7]. Pear ester. Constit. of odoriferous principle of Bartlett pears. Bpo.05 84-86 . 4-Bromophenacyl ester Mp 72 . 

Aliphatic esters in various combinations play a major part in many flavors, particularly fruit flavors 441), Hexyl acetate develops the strongest and most typical odor in Cox s Orange Pippin apples (279). Hexyl 2-methylbutyrate contributes highly to the flavor of Golden Delicious apples (265). Methyl and ethyl esters of ( )-3-hexenoic, (Z)-4-decenoic (32) and )- and (Z)-4-octenoic acids have some importance in the flavor of pineapple 408). Methyl (Z)-4-decenoate (32) and methyl thiohexanoate account for about 56% of the odor provided by the components of the oxygenated fraction of hop oil 202). Ethyl (jE, Z)-2,4-decadienoate (35) is considered as the character impact compound in the aroma of Bartlett pears (227) (Table 2). The application of this pear ester in reconstitution work has been facilitated by the development of highly stereospecific syntheses 405, 407, 457). Isobutyl an-gelate is considered as an essential constituent of Roman camomile oil 85). 

The conjugate addition of Grignard reagents to a-phenyl-a/8-unsaturaled esters has been reported. The first example of a conjugate 1,6-addition of a lithium organocuprate to an unsaturated ester has been observed (Scheme 44) in a synthesis of a volatile constituent of Bartlett pears. 

Constituents of Bartlett Pears.— The synthesis of ethyl and methyl E-2,Z-6-dodecadienoates, two constituents of Bartlett pears, has been described these esters have proved essential to the reconstruction of good pear flavour. 

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