Abstract
The complete NMR elucidation of four pentacycloundecane (PCU) derived ligands is reported. 2D NMR techniques are used to overcome the problem of major overlapping of methine signals on the cage skeleton. One of the cage ligands is chiral and the 13C NMR signals of the leucinol side “arms” to the cage appear to be split into two or more peaks indicating either impurities or conformational differences. Impurities were ruled out and the only logical explanation for this unusual observation appears to be conformational effects due to different positions of the two relative bulky side chains or “arms”. The rigid cage skeleton is known for through space deshielding of signals in close proximity to oxygen atoms attached to the cage skeleton. The leucinol side chains in closer proximity to the cage ether bridge would experience a larger shielding effect causing those carbon atoms to be shifted upfield with respect to the corresponding atoms in other conformations. The intrinsic chiral nature of the cage could also play a role in this case to perhaps enhance the observed effect.
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Acknowledgements
The authors thank Mr Dilip Jagjivan for carrying out all the NMR acquisitions. This work was supported by grants from the National Research Foundation, Gun 2046819 (South Africa) and the University of KwaZulu-Natal. Dr Louis Fourie, North-West University, Potchefstroom, is acknowledged for the MS analysis.
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Boyle, G.A., Kruger, H.G., Maguire, G.E.M. et al. NMR elucidation of some pentacycloundecane derived ligands. Struct Chem 18, 633–639 (2007). https://doi.org/10.1007/s11224-007-9190-x
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DOI: https://doi.org/10.1007/s11224-007-9190-x