Title: Selective heterogeneous acid catalyzed esterification of N-terminal sulfyhdryl fatty acids Authors
Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: May 4, 2011
Publication Date: May 4, 2011
Citation: Compton, D.L., Jackson, M.A. 2011. Selective heterogeneous acid catalyzed esterification of N-terminal sulfyhdryl fatty acids [abstract]. American Oil Chemists' Society. p. 77. Technical Abstract: Our interest in thiol fatty acids lies in their antioxidative, free radical scavenging, and metal ion scavenging capabilities as applied to cosmeceutical and skin care formulations. The retail market is filled with products containing the disulfide-containing free fatty acid, lipoic acid. These products are promoted for the disulfide-containing fatty acid’s beneficial effects towards wrinkle prevention, acne, and rosacea, all claims which are empirical and not clinically proven. Regardless, the skin care industry is starved for additional, naturally derived thiol fatty acids and derivatives. Mercapto fatty acid esters are useful intermediates en route to thiol containing structured lipids and are more efficient substrates than their corresponding acids when used in nonaqueous, enzymatic transesterification processes. Most N-terminal mercapto fatty acid esters are not commercially available and literature methods to the mercapto acid esters describe the catalysis with strong acids, which catalyze reactions at the N-terminal sulfydryl group, resulting in low yields and unwanted side products. Two mesoporous silicas functionalized with propylsulfonic (SBA-15-PSA) and arenesulfonic (SBA-15-ASA) acid groups, and a highly acidic, functionalized styrene divinylbenzene copolymer ion exchange resin (Amberlyst-15) were examined for their ability to catalyze the ethanolic esterification of the N-terminal sulfydryl fatty acid, 11-mercaptoundecanoic acid (MUA), without catalyzing unwanted side reactions at the sulfhydryl group. All three solid acid catalysts catalyzed the MUA esterification in excess ethanol. The activation energy for the catalytic esterifications were determined from 50 to 75°C, resulting in apparent Ea of 54, 71, and 59 kJ/mol for SBA-15-PSA, SBA-15-ASA, and Amberlyst-15, respectively. GC-MS analysis determined that all three catalysts produced near quantitative conversion of MUA to its ethyl ester with very little reactivity towards the sulfhydryl group. This was a marked improvement over the esterifications catalyzed by sulfuric and p-toluenesulfonic acids which produced thiothers and disulfide side products. The MUA ethyl ester synthesis was demonstrated on a gram scale at 70°C catalyzed by Amberlyst-15, and the desired product was isolated in 80% yield at > 95% purity with a minimum of purification.