DFT-NMR-Validated Full Structure Elucidation of Theionbrunonine C, An Unstable N-Oxide Theionbrunonine from Mostuea brunonis

The structure elucidation of theionbrunonine C, a thioether-bridged dimeric monoterpene indole alkaloid (MIA), and more generally, one of the very few Sulfur-containing MIA, is reported after its isolation from Mostuea brunonis (Gelsemiaceae). This unstable structure had already been targeted for isolation in our former, molecular network-guided, investigation of this plant but this compound had degraded before sufficient spectroscopic data could have been acquired for a complete structure assignment. With this constraint in mind, the rapid acquisition of NMR data enabled retrieving sufficient spectroscopic information for full structure elucidation, although from a partial set of spectroscopic information (1H and 13C NMR; COSY, HSQC, and HMBC). In conjunction with biosynthetic considerations, the cursory examination of 13C NMR data unambiguously defined the complete stereostructure of 1, as further supported by DFT-NMR calculations and subsequent DP4 probability score.