Structure Elucidation of Secondary Metabolites: Current Frontiers and Lingering Pitfalls.

Analytical methods allow for the structure determination of submilligram quantities of complex secondary metabolites. This has been driven in large part by advances in NMR spectroscopic capabilities, including access to high-field magnets equipped with cryogenic probes. Experimental NMR spectroscopy may now be complemented by remarkably accurate carbon-13 NMR calculations using state-of-the-art DFT software packages. Additionally, microED analysis stands to have a profound effect on structure elucidation by providing X-ray-like images of microcrystalline samples of analytes. Nonetheless, lingering pitfalls in structure elucidation remain, particularly for isolates that are unstable or highly oxidized. In this Account, we discuss three projects from our laboratory that highlight nonoverlapping challenges to the field, with implications for chemical, synthetic, and mechanism of action studies. We first discuss the lomaiviticins, complex unsaturated polyketide natural products disclosed in 2001. The original structures were derived from NMR, HRMS, UV-vis, and IR analysis. Owing to the synthetic challenges presented by their structures and the absence of X-ray crystallographic data, the structure assignments remained untested for nearly two decades. In 2021, the Nelson group at Caltech carried out microED analysis of (-)-lomaiviticin C, leading to the startling discovery that the original structure assignment of the lomaiviticins was incorrect. Acquisition of higher-field (800 MHz