Regio‐ and Stereoselective Diels–Alder Additions of Maleic Anhydride to Conjugated Triene Fatty Acid Methyl Esters

Abstract

Keywords: Diels-Alder reaction / Cyclization / Methyl calendulate / Methyl α-eleostearate / Maleic anhydride / Renewable feedstockThe thermal, solvent-free addition of maleic anhydride to methyl calendulate (methyl 8,10-trans,12-cis-octadecatrienoate) (1a) occurs with very high regio-and stereoselectivity at C-8 and C-11 of the fatty compound giving the endo-DielsAlder adduct 4 with retention of the cis-configured double bond. Analogously, the Diels-Alder addition of maleic anhydride to methyl α-eleostearate (methyl 9-cis,11,13-trans-octadecatrienoate) (2a) takes place regio-and stereoselectively

Preparation of conjugated linoleic acid enriched derivatives by conventional and biphasic isomerisation

“…The mechanism of thermal cycloaddition was the combination of concerted and stepwise pathway like in the case of PTAD ( Figure 5, scheme A, B). The high temperature interconversion of endo Diels-Alder adduct to exo product resulted in the observation of two peaks eluted very close to each other (retention time 18.600 and 18.716 min) with similar EI-MS spectra (Young & Vouros 1987;Dobson 1998;Biermann et al 2007). Maleic anhydride and benzoquinone were shown to be powerful dienophiles, which reacted more stereo-and regioselectively with conjugated diene systems and benzoquinone was the only reagent suitable to determine (E,E)-octadecadienoic acid derivatives (conversion ≤ 95%) in the presence of other geometrical isomers.…”

“…Conjugated linoleic acid derivatives undergo the Diels-Alder reaction with normal electron demand. Three powerful dienophiles were selected for [4+2] cycloaddition between the highest occupied molecular orbital (HOMO) of the CLA methyl esters and the lowest unoccupied molecular orbital (LUMO) of electron-poor dienophiles (4-phenyl-1,2,4-triazoline-3,5-dione, maleic anhydride, and 1,4-benzoquinone) (Young & Vouros 1987;Dobson 1998;Biermann et al 2007).…”

“…(9E,11E)-and (10E,12E)-octadecadienoic acid methyl esters, which could adopt the s-cis conformation, reacted via a concerted pathway ( Figure 5, scheme A) according to the WoodwardHoffmann rules. Other geometrical and positional CLA isomers with s-trans conformation were much more hindered and the reaction was not symmetry allowed (Young & Vouros 1987;Dobson 1998;Biermann et al 2007). Therefore, PTAD afforded endo cycloadducts with (E,Z)-, (Z,E) and (Z,Z)-octadecadienoic acid derivatives through the stepwise pathway with diradical intermediate in the transition structure ( Figure 5, scheme B).…”

“…The most intense ions at 312 and 384 m/z indicated double bond positions of rumenic acid methyl ester after the α-cleavage fragmentation to the neighbouring nitrogen heteroatoms. Another possible mechanism was allylic α-cleavage Food Sci., 34, 2016 (6) fragmentation of the substituted cyclohexene derivative (Young & Vouros 1987;Dobson 1998;Biermann et al 2007;Pavia et al 2009). Maleic anhydride and benzoquinone were other strong electron acceptors readily forming cycloadducts with CLA isomers.…”

“…In brief, 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD, 3 mg, 17.0 µmol) was mixed with CLA methyl esters (1 mg, 3.4 µmol) in 3 ml of dichloromethane for 10 s at room temperature (Young & Vouros 1987;Dobson 1998). The maleic acid anhydride and 1,4-benzoquinone adducts were prepared by solvent-free thermal cycloaddition (150°C, 1 h under nitrogen) of CLA methyl esters (3 mg, 10.2 µmol) with 10.2 µmol of maleic acid anhydride (Biermann et al 2007) or 1,4-benzoquinone. Diels-Alder cycloadducts of CLA methyl esters with several dienophiles were separated from impurities by passing them through solid-phase extraction cartridges (silica gel Supelclean TM LC-Si SPE tubes, 6 ml, 1 g).…”