NMR in oriented medianew methodologies and studies on new alignment media

Resumo

This thesis describes the development and application of new methodologies for the structural determination of small organic molecules based on nuclear magnetic resonance (NMR) in aligned media. Nuclear Magnetic Resonance is the most important technique in the structural study of organic molecules in solution. In the last decade, residual dipolar couplings (RDCs) emerged as a powerful tool for the structural determination of the relative configuration of small- to medium-size organic molecules in solution. In cases where the classical 3JAB and NOE analysis is not enough to provide an unambiguous solution to the structural elucidation problem and crystallization is not possible, RDCs proved to be extremely helpful. The technique is based on the use of an alignment medium, normally a cross-linked polymer or a lyotropic liquid crystal, that imposes a certain degree of anisotropy in a sample. One of the effects of this anisotropy is the appearance of a new NMR observable, the so-called residual dipolar coupling, which encodes non-local angular information between spin pairs. Three main topics will be described in this work: • Application of RDCs to several practical cases and description of an automatic procedure for the structural determination from molecular formula to the 3D absolute configuration of natural products aided by Automatic Structural Elucidation Software and RDCs. • Study of the properties of disodium cromoglicic salt (cromolyn) as alignment media in aqueous media. • Synthesis of organic soluble cromolyn mimetizers. After a description of the theoretical basis of residual dipolar couplings, the experiments normally used to acquire the data, how it is treated afterwards and which are the main alignment phases developed to date, the first topic of the thesis is developed through Chapters 2, 3 and 4. Chapter 2 shows a real case in which RDCs were used as tool to determine the final product of a reaction in which a photochemical ring closure can give rise to two different structures, impossible to discern by classical 3JAB and NOE analysis. A new experimental approach (combined use of JSB-HSQC and JSB-HSQC/HMBC experiments) to the assignment of prochiral methylene protons in natural products is described in Chapter 3. Last, Chapter 4 shows the development of an automatic procedure for the determination of the absolute configuration of relatively rigid small organic molecules in solution combining automatic structural determination software with RDCs and computation of the electronic circular dichroism of several commercially available natural products. The second topic is developed through Chapters 5 and 6. In Chapter 5, the use as alignment media of cromolyn, a compound that gives its name to a family of water-soluble lyotropic liquid crystals is described. The studies comprise the preparation of the different cromolyn/water liquid crystals, temperature and concentration studies to determine the limits of the LC phase depending on those two variables, and 2D-pseudo imaging NMR studies that show the homogeneity of the phases depending on their concentration. 3D Diffusion studies of N-methyl codeinium iodide are also described here. The LC was also doped first with NaCl and then with L-alanine. The addition of NaCl has an effect on the concentration needed to form the LC phase, whereas L-alanine turns the LC from nematic to cholesteric. Chapter 6 shows the full characterization of the cromolyn liquid crystalline phase when it is doped with L-alanine by using polarization optical microscopy (POM), rheology, differential scanning calorimetry (DSC), X-ray diffraction powder and 2H NMR. Also, experiments to test the enantiodiscriminating properties of the cholesteric liquid crystal phase are shown in this chapter. Cromolyn showed outstanding properties as alignment media, but its main disadvantage is its own nature, since it is a water-soluble mesogen. This fact led to the synthesis of organic solvent compatible molecules with planar structures as cromolyn, and with different substitution patterns in order to try to mimic in organic solvents the properties of cromolyn in water. Synthetic procedures, gelation tests and optical micrographs for the different compounds synthesized are summarized in Chapter 7.La resonancia magnética nuclear (RMN) en medios alineados experimentó en la última década grandes avances en su aplicación a la determinación estructural de compuestos orgánicos, permitiendo la determinación simultánea de varios estereocentros. En este proyecto de tesis pretendemos explorar nuevas metodologías y áreas de aplicación de esta técnica. El primer objetivo de esta tesis será la exploración de la posibilidad del uso de las constantes dipolares residuales (RDCs) en la determinación de la estructura de complejos “host-guest” como pueden ser los complejos entre ciclodextrinas y diversos ligandos orgánicos. Investigaremos diferentes medios de alineamiento compatibles con el medio acuoso. Si esta investigación tiene éxito investigaremos las aproximaciones computacionales necesarias para la determinación de la estructura tridimensional de estos complejos usando las RDCs obtenidas. Otro objetivo de esta tesis es la extensión de la técnica de RDCs a la determinación estructural de compuestos fluorados. Investigaremos mediante técnicas computacionales la influencia de la parte anisótropa del tensor de acoplamiento escalar en la medida de las RDCs e implementaremos el software necesario para el análisis del tensor de alineamiento en esos compuestos. Finalmente investigaremos, como objetivo final, la influencia del disolvente sobre el tensor de alineamiento para distintos medios de alineamiento. Para ésto mediremos distintos compuestos orgánicos en distintas combinaciones de medios de alineamiento y disolventes, lo que nos dará información valiosa sobre la naturaleza del proceso de alineamiento. Por otra parte esta investigación será reforzada mediante simulaciones de dinámica molecular en cajas de disolvente anisótropas, como un modelo simple del medio de alineamiento.A resonancia magnética nuclear (RMN) en medios alineados experimentou na última década grandes avances na súa aplicación á determinación estructural de compostos orgánicos, permitindo a determinación simultánea de varios estereocentros. Neste proxecto de tese pretendemos explorar novas metodoloxías e áreas de aplicación desta técnica. O primeiro obxectivo desta tese será a exploración da posibilidade do uso das constantes dipolares residuais (RDCs) na determinación da estructura de complexos “host-guest” como poden ser os complexos entre ciclodextrinas e diversos ligandos orgánicos. Investigaremos diferentes medios de alineamento compatíbeis co medio acuoso. Se esta investigación ten éxito investigaremos as aproximacións computacionais necesarias para a determinación da estructura tridimensional destes complexos usando os RDCs obtidos. Outro obxectivo desta tese e a extensión da técnica de RDCs á determinación estructural de compostos fluorados. Investigaremos mediante técnicas computacionais a influencia da parte anisótropa do tensor de acoplamento escalar na medida dos RDCs e implementaremos o software necesario para a análise do tensor de alineamento neses compostos. Finalmente investigaremos, como obxectivo final, a influencia do disolvente sobre o tensor de alineamento para distintos medios de alineamento. Para isto mediremos distintos compostos orgánicos en distintas combinacións de medios de alineamento e disolventes, o que fornecerá información valiosa sobre a natureza do proceso de alineamento. Por outra parte esta investigación será reforzada mediante simulacións de dinámica molécular en caixas de disolvente anisótropas, como un modelo simple do medio de alineamento.