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. C-13-C-13 COSY NMR spectroscopy shows direct couplings between C-13 and C-13, 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 C-13-based NMR experiments, due to the low natural abundance of the C-13 nucleus, is problematic when applying these techniques. Our efforts in the C-13 labeling of a microbial natural product, cyclopiazonic acid (1), by feeding C-13-labeled glucose to the fungal culture, enabled us to acquire C-13-C-13 COSY NMR spectra on a milligram scale that clearly show the carbon backbone of the compound. This is the first application of C-13-C-13 COSY NMR experiments for a natural product. The results suggest that C-13-C-13 COSY NMR spectroscopy can be routinely used for the structure determination of microbial natural products by C-13-enrichment of a compound with C-13-glucose.