Assignment of the ¹H and ¹³C NMR spectra and conformational analysis of the pyrrolizidine alkaloid 13-<i>O</i>-acetyldicrotaline

The 1H and 13C NMR spectra of the pyrrolizidine alkaloid 13-O-acetyldicrotaline were studied at 4.7 T. A full assignment of the 1H and 13C signals was achieved by application of a variety of NMR techniques, including homonuclear 1H NOE difference, 2D δH/δH phase-sensitive COSY and δC/δH direct and long-range correlation experiments. The stereochemistry at C-13 in dicrotaline is confirmed to be S, and the conformation of the eleven-membered macrocycle is compared with that of related pyrrolizidine alkaloids