Stereochemistry of Challenging Natural Products Studied by NMR-based Methods

Stereochemistry is a very important subject in chemistry and life sciences, as most of the biologically active molecules, including proteins, oligonucleotides, sugars, lipids and small molecules exist in one chiral form. In addition, small molecule compounds with biological activity – drugs – with the same constitution yet different configuration can exhibit divergent biological activities. The determination of the relative configuration of stereocenters was first established by Fischer in 1891 via organic synthesis. The question of absolute configuration was not solved until the work of Bijvoet in 1951 with anomalous X-ray diffraction of single crystals. For the compounds which can neither be crystallized nor easily be converted into crystallizable derivatives, it can still be difficult to establish their stereochemistry. Within this thesis, the development and application of Residual Dipolar Coupling (RDC)-enhanced NMR techniques for the stereochemical elucidation of organic molecules – the RDC-based approach – will be presented. Residual dipolar couplings provide valuable information about the orientation of internuclear vectors with respect to the molecular frame, which complements nuclear Overhauser effects (NOEs), scalar J-couplings and chemical shifts that are short-range in nature. RDCs have been used for structure determination and dynamic studies of biomolecules for decades, however the use of this anisotropic parameter for configurational and conformational studies of small molecules has yet to be fully tapped. Herein, the new developments and improvements of this RDC-based approach for the study of small molecules can be categorized as: 1. Conformationally flexible molecules: Both single and multiple alignment tensor approaches were employed and compared. Molecular mechanics (MM) and molecular dynamics (MD) simulations were applied to statistically sample the extensive conformational space. 2. Natural products with limited availability: The preparation of a slim PH-gel for 1.7 mm NMR tubes has been developed for reproducibility and efficiency. RDC measurements in this low volume gel system have been made using a number of molecules. 3. Large numbers of unknown stereocenters: NOE and J-coupling restrained MD simulations with the floating chirality method were investigated. Combined with an RDC analysis in the final step, this enables the determination of the relative stereochemistry of a large number of stereocenters in a complex natural product. 4. Absolute configuration: An integrated approach is herein developed. This includes diastereomer differentiation using both isotropic and anisotropic NMR data. The chiroptical properties calculated using DFT of the NMR derivedensemble from this first step are used for enantiomer differentiation by comparison to experiments. Using these improved methods, the stereochemistry of six challenging naturalproducts – LLG1, naphth 1, naphth 2, comp.540a, vatiparol, and fibrosterol sulfateA – and one synthetic molecule – a Michael addition product – has been successfully established