Isolation of the Pyrrolizidine Alkaloid Intermedine-N-oxide from Cerinthe glabra and ab initio Calculation of its 13C NMR Shifts

The pyrrolizidine alkaloid intermedine-N-oxide was isolated from Cerinthe glabra for the first time. Due to the questionable assignments of NMR signals of this compound in previous literature reports ab initio calculations of the C NMR values based on the GIAO approach, using HF and B3LYP levels of theory and the four basis sets 6-31G(d), 6-311G(d), 6-31G(d,p), and 6-311G(d,p) were performed. The assignments obtained this way are in very good accordance with the experimental values.