Absolute Configuration of 12S-Deoxynortryptoquivaline from Ascidian-Derived Fungus Aspergillus clavatus Determined by Anisotropic NMR and Chiroptical Spectroscopy

Tryptoquivalines are highly toxic metabolites initially isolated from the fungus Aspergillus clavatus. The relative and absolute configuration of tryptoquivaline derivates was primarily established by comparison of the chemical shifts, NOE data, and ECD calculations. A de novo determination of the complete relative configuration using NMR spectroscopy was challenging due to multiple spatially separated stereocenters, including one nonprotonated carbon. In this study, we isolated a new tryptoquivaline derivative, 12S-deoxynortryptoquivaline (1), from the marine ascidian-derived fungus Aspergillus clavatus AS-107. The correct assignment of the relative configuration of 1 was accomplished using anisotropic NMR spectroscopy, while the absolute configuration was determined by comparing calculated and experimental ECD spectra. This case study highlights the effectiveness of anisotropic NMR parameters over isotropic NMR parameters in determining the relative configuration of complex natural products without the need for crystallization.