Synthesis and Analysis of Enantiopure Structured ABC-type Triacylglycerols

This thesis describes the synthesis and analysis of a library of highly enantiopure structured ABC-type triacylglycerols (TAGs). In total twenty-seven compounds were synthesized with two different methods; one being specially developed for this project. All synthesised TAGs are common species in dietary fats and oils. The library of TAGs is intended to be used as enantiopure standards in an on-going collaboration with our colleagues at the Food Science Unit at the University of Turku, who are developing new methods in the chiral analysis of TAGs. The purpose of the chiral analysis is to characterise the chromatographic behaviour of these compounds and to gain a better understanding of the relationship between their structure and how it affects their chirality. In the future this knowledge can then be applied to more complex lipid mixtures. Both methods of TAG synthesis are six-step processes using (R)-solketal as starting material. The known method uses benzyl as a protecting group to protect the sn-1 position of solketal. The isopropylidene moiety of solketal is then removed opening up the sn-2 and sn-3 positions. In a chemoenzymatic reaction, Candida antarctica lipase B is then used to regioselectively acylate the terminal position. Then a second fatty acid is introduced via chemical coupling with EDCI as a coupling agent. The benzyl group is then deprotected by catalytic hydrogenolysis and the final fatty acid can be chemically coupled to the glycerol backbone. This method yielded fifteen TAGs in 42-67% total yields from (R)-solketal. However, the method has a major drawback which is the deprotection step. An unsaturated fatty acid cannot survive the hydrogenolysis environment intact. Therefore, a new method was developed for this project to allow for more unsaturation within the target molecules and to complete the library of TAGs. The new method replaces the benzyl protecting group with p-methoxybenzyl (PMB) which can be deprotected by the mild oxidant DDQ. The synthesis had to tailored to the new protecting group PMB which is much more acid sensitive than the previous benzyl group. The new method allowed for unsaturated fatty acid to be incorporated before the deprotection step in the synthesis. The oxidative cleavage of PMB had no effect on monounsaturated fatty acids but the DDQ slowly damaged the skipped polyene system of polyunsaturated fatty acids. However, adjustments could be made to incorporate polyunsaturated fatty acids into the TAGs as well. In total twelve TAG were synthesised with this new method in comparable total yields to the old benzyl method, 48-56%. The library of TAGs was then analysed with various methods. First the purity of the compounds was confirmed with total fatty acid analysis by gas chromatography and regioselective analysis on ammonia NICI tandem mass spectrometry. Then the chiral chromatographic behaviour of the compounds was analysed and characterised by chiral HPLC. Finally, as a part of another project, a small synthesis of four enantiopure diacylglyceryl ethers (DAGE) derived from selachyl alcohol was undertaken. The synthesised DAGEs had a polyunsaturated fatty acid (PUFA), either EPA or DHA, incorporated and a non-steroidal anti-inflammatory drug (NSAID), either (S)-naproxen or (S)-ibuprofen. This synthesis completes a larger library of ether lipids derived from the three most common 1-O-alkyl-sn-glycerols, chimyl, batyl and selachyl alcohols.