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Abstract (English)

Lipases [E.C.3.1.1.3] are ubiquitous in nature where their role in vivo is the enzymatic hydrolysis of triglycerides. In these transformations lipases display a high degree of regio- and enantioselectivity, properties which are of great interest for the use of these enzymes in vitro. The experimental determination of these selectivities is somewhat problematic due to the fact that the primary products of hydrolysis are notoriously instable due to rapid acyl group migrations especially under protic conditions and at elevated temperatures. These are also substrates of lipases and therefore their optical purity can be strongly conversion dependent. Thus, one prerequisite for the successful determination of the enantioselectivities are extremly small conversions (<2-6%) and a rapid derivatisation of the primary reaction products prior to analysis. Moreover, the method is only of analytical value, the resulting products are not suitable for further synthetic use such as for the preparation of phospholipids. In view of this situation we felt that isosteric mimics of natural lipids would be an attractive alternative for a system and medium independent determination of these important enzyme parameters, as well as for the synthesis of novel carba-analogues of lipids and phospholipids. The systematic replacement of sp3-oxygens by sp3-carbon atoms in triglycerides leads to such mimics with minimal deviation of bond angles and bond distances. Their behaviour should be identically - with the exception of hydrolytic cleavage in the modified position - towards biological systems. In order to test this hypothesis synthetic routes for the synthesis of carba- analogues of triglycerides were developed. Carba-analogues of tricaprylin 11, trilaurin 25 and tripalmitin 26 with modification of the sn-2 position were prepared starting from the corresponding fatty acid chlorides as well as racemic carba-analogues of triacetin 39 and trilaurin 40 with modification of the sn-1(3) position. The synthesis of triglycerides modified in the sn-1,2(2,3) positions - like the triacetin analogues (?)-56 and (S)-56 - were realised via Diels-Alder reaction of 2,3-dimethylbutadiene with the acrylic acid esters of D-pantolactone 42 as the key reaction step. This cycloaddition with 42 in presence of catalytic amounts of titaniumtetrachloride gives the cyclohexene derivative 50 with very high diastereoselectivity. Reduction with LiAlH4 to the alcohol (S)-53 (? 99% ee) proceeds without racemisation and acetylation followed by oxidative cleavage of the C=C-double bound gives the triacetin-analogue (S)-56. One triglyceride-analogue modified in the sn-1,3-positions 64 was synthesized starting from levulinic acid ethyl ester. This general approach also allows the synthesis of novel carba-analogous phospholipids of high enantiomeric purity with a) modification of the sn-2 position such as (S)-79 and b) modification of the sn-1,2(2,3) positions like (S)-82. Such analogues of phospholipids could have interesting biological activities, e.g. as inhibitors of phospholipase A2. Using the above carba-analogues of triglycerides their activities and enantioselectivities towards 44 commercially available lipases was studied in comparison with the corresponding triglycerides. The results - obtained with tricaprylin and its carba-analogue 11 (modification in the sn-2 position) - showed considerable differences between these two substrates towards the studied lipases both regarding their specific activities and enantioselectivities.

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