Fatty Acid Methyl Esters with Two Vicinal Alkylthio Side Chains and Their NMR Characterization

Abstract

IntroductionNumerous useful procedures have been developed for determining the location of double bonds in unsaturated fatty acid methyl esters (FAME) or alkenes using mass spectrometry (MS) as the analytical method [1][2][3][4][5][6][7]. Besides various methods involving different ionization procedures, several derivatization reactions have been developed which can largely be categorized as belonging to one of two types. One reaction type is derivatization directly at the double bond by addition of suitable molecules while the other type is substitution of the acid or methyl (alkyl) ester moiety at C1 with a different functionality to induce remote fragmentation. The objective of both types of reactions is to synthesize derivatives that produce fragments in mass spectrometry that are more facile to interpret and, therefore, provide unambiguous information regarding the position of the double bond.Among the first type of the aforementioned derivatization reactions is methylthiolation, usually catalyzed by iodine. The application of this reaction to yield compounds for facile mass spectrometric analysis was, likely, first extensively reported in 1981 [8,9], although an earlier report noted the usefulness of gas chromatography-mass spectrometry for the analysis of the methylthio adducts of 1-dodecene [10]. As its name implies, this reaction consists of the addition of methylthio moieties to the double bond using dimethyl disulfide (DMDS; CH 3 S-SCH 3 ) in the Abstract The addition reaction of dimethyl disulfide (DMDS) to double bonds in alkenes and monounsaturated fatty acid esters in the presence of iodine or other catalysts to give bis(methylthio) derivatives has largely served analytical purposes in mass spectrometry with scattered reports on the addition of other disulfides to alkenes also existing. In this work, this iodine-catalyzed reaction was expanded to include the addition of other dialkyl disulfides (RSSR; R=ethyl, propyl, butyl, iso-propyl) besides DMDS to the double bonds in monounsaturated fatty acid methyl esters with 16, 18, 20, and 22 carbon atoms in the fatty acid chain to give the corresponding methyl 1,2-bis(alkylthio) alkanoates. The products are obtained in high to moderate yield after a facile purification procedure and are analytically characterized not only by mass spectrometry but also 1 H and 13 C NMR. The threo and erythro diastereomers obtained from (Z) and (E) fatty acid methyl esters, respectively, can be easily distinguished by the NMR shifts of the protons and carbons in and close to the 1,2-bis(alkylthio) moiety. Various other effects in the NMR spectra are discussed. The 1,2-bis(alkylthio) derivatives of a symmetrical alkene, 7(E)-tetradecene, serve to confirm the NMR assignments besides NMR techniques such as 2D correlations Disclaimer Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity prov...

Structural Analysis of Oxidized Cerebrosides from the Extract of Deep-Sea Sponge Aulosaccus sp.: Occurrence of Amide-Linked Allylically Oxygenated Fatty Acids

“…The threo-diastereomers and erythro-diastereomers can be easily distinguished by NMR shifts of protons and carbons in and close to the 1,2-bis(alkylthio) moiety [ 35 ]. In addition to the data presented in the NMR study of Knothe and Steidley [ 35 ], we used δ Н values of the DMDS adducts of two standards, methyl palmitoleate ( Figure 6 a) and its trans -isomer ( Figure 6 b), to confirm the configurations of double bonds in the monoenoic sphingoid base moieties of the starting oxidized cerebrosides. The δ H values for CH 2 , α to the –CH(SMe)–CH(SMe)– moiety, were obtained through correlations in 1 Н, 1 Н-COSY diagrams.…”

“…Two spingoid base derivatives were the DMDS adducts of acetylated isomeric monoenoic Methylthiolation of cis-monoenes and trans-monoenes with DMDS, as an anti-addition, leads to the threo-adducts and erythro-adducts, respectively [33,34]. The threo-diastereomers and erythro-diastereomers can be easily distinguished by NMR shifts of protons and carbons in and close to the 1,2-bis(alkylthio) moiety [35]. In addition to the data presented in the NMR study of Knothe and Steidley [35], we used δ Н values of the DMDS adducts of two standards, methyl palmitoleate (Figure 6a) and its trans-isomer (Figure 6b), to confirm the configurations of double bonds in the monoenoic sphingoid base moieties of the starting oxidized cerebrosides.…”