Synthesis, Purification, and Acyl Migration Kinetics of 2-Monoricinoleoylglycerol
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- Compton, D.L., Laszlo, J.A., Appell, M. et al. J Am Oil Chem Soc (2014) 91: 271. doi:10.1007/s11746-013-2373-2
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2-Monoricinoleoylglycerol (2-MRG) was synthesized by the Novozym 435 catalyzed alcoholysis of castor oil in excess ethanol (1:70 mol:mol) at ambient temperature. Due to the fatty acid C12-OH group, conventional liquid–liquid extraction methods developed for less polar, non-hydroxylated 2-monoacylglycerols (2-MAG) proved inadequate for 2-MRG purification. Alternatively, 2-MRG was purified by normal-phase flash chromatography (FC) on silica gel using a binary acetone-hexane gradient mobile phase. Gram quantities of 2-MRG were isolated in 63 % yield and contained no residual diacylglycerol (DAG), which fail to separate using liquid–liquid extraction methods. The 2-MRG was obtained at ~90 mol% relative to 1-MRG, proving that the FC method did not appreciably catalyze acyl migration. 1H-NMR spectroscopy was used to monitor the spontaneous acyl migration of isolated 2-MRG from 20 to 80 °C. The relative energy of activation calculated from the Arrhenius relationship of the 2-MRG acyl migration rate constants was 82.9 kJ/mol. This was ~two-fold higher than the theoretical ΔG298.15 calculated from molecular modeling using density functional calculations (B3LYP/6−31 + G*) of 2-MRG, the ketal ring transition state, and 1-monoricinoleoylglycerol (1-MRG). The synthesis and isolation methods reported herein provide a convenient means to access useful intermediates for functionalized structured lipids.