Add time:08/31/2019         Source:sciencedirect.com

A simple and reliable method for synthesizing four isomers of parinaric acid from α-linolenic acid (ALA) in high yields is described. The methylene-interrupted, cis triene system (1,4,7-octatriene) of ALA and common to other naturally occurring polyunsaturated fatty acids was transformed to a conjugated tetraene system (1,3,5,7-octatetraene). The synthesis involves bromination of ALA using 0.l M Br2 in a saturated solution of NaBr in methanol, esterification of the fatty acid dibromides, double dehydrobromination by 1,8-diazabicyclo[5.4.0]undec-7-ene and saponification of the conjugated esters to a mixture of free conjugated acids. Addition of one molecule of bromine to the 12,13-double bond of ALA and subsequent dehydrobromination produces α-parinaric acid (9Z,11E,13E,15Z-octadecatetraenoic acid); addition of Br2 to the 9,10-double bond or 15,16-double bond and then dehydrobromination and rearrangement yields 9E,11E,13E,15Z-octadecatetraenoic or 9E,11E,13E,15Z-octadecatetraenoic acids, respectively. The mixture of parinaric acid isomers is obtained in 65% yield, and the isomers can be purified by preparative HPLC; alternatively, the isomers can be converted by base catalyzed cis–trans isomerization (or by treatment with I2) to exclusively β-parinaric acid (9E,11E,13E,15E-octadecatetraenoic acid). The various parinaric acid isomers were characterized by 1H NMR, 13C NMR, UV, GLC, HPLC and mass spectrometry.

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