Stereostructure Assignment of Medium-Sized Rings through an NMR-Computational Combined Approach. Application to the New Germacranes Ketopelenolides C and D

The new germacrane derivatives ketopelenolides C (2) and D (3) have been isolated from great mugwort (Artemisia arborescens). Their stereostructure elucidation exemplifies some of the, most common pitfalls facing the configurational assignment of medium-sized polyfunctionalized compounds. It was established through a combined strategy including chemical derivatization, NMR data analysis, molecular modeling, and quantum-mechanical calculations including a comparison between experimental C-13 NMR data and a Boltzmann-weighted average of DFT-calculated C-13 NMR chemical shifts.