3-Nitropropanoic acid and isoxazolin-5-one derived glucosides in chrysomelina larvae : synthesis, biosynthesis, occurrence and toxicity
The dissertation of Tobias Becker describes important aspects of the chemical defense of leaf beetles belonging to the subtribe Chrysomelina. Leaf beetle species of the subtribe Chrysomelina produce glucosides that derive from isoxazolin-5-one and 3-nitropropanoic acid. These glucosides have been synthesized using a cascade reaction as well as an enzyme-catalyzed transesterification. The synthesis of the natural isoxazolin-5-one glucoside was further simplified by the use of glucose as starting material. This novel synthetic protocol was transferred to other sugars to study the scope of the method, which yielded four different isoxazolin-5-one glycosides that derive from glucose, xylose, maltose and fructose. Stable-isotope-labeled [13C6]-glucose was used to synthesize corresponding labeled authentic standards of natural occurring isoxazolin-5-one glucosides. These labeled compounds were used to analyze the concentrations and the occurrence of the natural derivatives in different life stages of Chrysomelina leaf beetles, especially in the larval stage. Then, a number of different stable-isotope-labeled compounds were applied to leaf beetle larvae either by injection or by feeding experiments in order to study the biosynthetic pathways resulting in the de novo production of the natural occurring isoxazolin-5-one and 3-nitropropanoic acid derivatives. Furthermore, larval tissue from the fat body was used as a catalyst to unravel further biosynthetic steps. These experiments resulted in the discovery of an unexpected pathway of the natural isoxazolin-5-one derivatives in Chrysomelina larvae that starts with essential amino acids, as shown in case of valine. In addition, the fundamental relationship in the biosynthesis of the isoxazolin-5-one heterocycle and the 3-nitropropanoic acid moiety was discovered, showing that both structural elements derive from the same precursors.