Application of ¹³C-labeling and ¹³C–¹³C COSY NMR experiments in the structure determination of a microbial natural product

Abstract

The elucidation of the structures of complex natural products bearing many quaternary carbons remains challenging, even in this advanced spectroscopic era. ¹³C–¹³C COSY NMR spectroscopy shows direct couplings between ¹³C and ¹³C, which comprise the backbone of a natural product. Thus, this type of experiment is particularly useful for natural products bearing consecutive quaternary carbons. However, the low sensitivity of ¹³C-based NMR experiments, due to the low natural abundance of the ¹³C nucleus, is problematic when applying these techniques. Our efforts in the ¹³C labeling of a microbial natural product, cyclopiazonic acid (1), by feeding ¹³C-labeled glucose to the fungal culture, enabled us to acquire ¹³C–¹³C COSY NMR spectra on a milligram scale that clearly show the carbon backbone of the compound. This is the first application of ¹³C–¹³C COSY NMR experiments for a natural product. The results suggest that ¹³C–¹³C COSY NMR spectroscopy can be routinely used for the structure determination of microbial natural products by ¹³C-enrichment of a compound with ¹³C-glucose.