The application of nuclear magnetic resonance in the structural elucidation of natural products

Abstract

Multipulse Nuclear Magnetic Resonance (NMR) techniques were investigated via structural assignment of various natural products. The natural products chosen for investigation of 2D NMR spectroscopy as a structural tool were the opium alkaloids codeine and papaverine, the diterpenoid alkaloids delphinine and aconitine and the triterpenoid derivatives of cucurbitaceae. One dimensional NMR spectroscopy viz 1H, 13C and Distortionless Enhancement by Polarisation Transfer (DEPT) spectroscopy was initially applied to obtain a level of indication of the complexity of the structural problem solving. Various multipulse techniques were chosen depending on the sample quantity available, the sensitivity of the technique, the information provided and its applicabilty in the field of natural products. Both homonuclear scalar and dipolar correlation as well as short and long range heteronuclear correlation was investigated. The homonuclear Correlation Spectroscopy (COSY) experiment which revealed scalar 1H-1H scalar correlations was adequate for the structural assignment. The Heteronuclear Multiple Quantum Coherence (HMQC) experiment offered greater sensitivity on small sample quantities whilst the long range Heteronuclear Correlation (HETCOR) and Heteronuclear Multiple Bond Coherence (HMBC) experiments were essential for complete assignment of the quaternary groups. The stereochemical designation of some of the compounds was confirmed using the Rotating Frame Overhauser Enhancement Spectroscopy (ROESY) experiment which proved to be a more applicable pulse sequence for the natural products under investigation. The 1H and 13C assignments of these derivatives and the experimental approach used to obtain this data was compared to that present in the literature. The results indicate that apart from offering tremendous versatility depending on the special attributes of the compound, two dimensional NMR techniques are imperative for the unambiguous structural assignment of natural products.