The Advanced Floating Chirality Distance Geometry Approach―How Anisotropic NMR Parameters Can Support the Determination of the Relative Configuration of Natural Products

• Main Authors: Matthias Köck, Michael Reggelin, Stefan Immel
• Format: Article
• Language: English
• Published: MDPI AG 2020-06-01
• Series: Marine Drugs
• Subjects: chirality; configurational analysis; distance geometry; NMR spectroscopy; NOE data; residual dipolar couplings
• Online Access: https://www.mdpi.com/1660-3397/18/6/330

The configurational analysis of complex natural products by NMR spectroscopy is still a challenging task. The assignment of the relative configuration is usually carried out by analysis of interproton distances from NOESY or ROESY spectra (qualitative or quantitative) and scalar (<i>J</i>) couplings. About 15 years ago, residual dipolar couplings (RDCs) were introduced as a tool for the configurational determination of small organic molecules. In contrast to NOEs/ROEs which are local parameters (distances up to 400 pm can be detected for small organic molecules), RDCs are global parameters which allow to obtain structural information also from long-range relationships. RDCs have the disadvantage that the sample needs a setup in an alignment medium in order to obtain the required anisotropic environment. Here, we will discuss the configurational analysis of five complex natural products: axinellamine A (<b>1</b>), tetrabromostyloguanidine (<b>2</b>), 3,7-<i>epi</i>-massadine chloride (<b>3</b>), tubocurarine (<b>4</b>), and vincristine (<b>5</b>). Compounds <b>1</b>–<b>3</b> are marine natural products whereas <b>4</b> and <b>5</b> are from terrestrial sources. The chosen examples will carefully work out the limitations of NOEs/ROEs in the configurational analysis of natural products and will also provide an outlook on the information obtained from RDCs.