Molecular and crystal structures of (+)-homochelidonine, (+)-chelamine, and (−)-norchelidonine

The molecular and crystal structures of three hexahydrobenzo[c]phenanthridine alkaloids (+)-homochelidonine (1), (+)-chelamine (2), and (−)-norchelidonine (3) have been examined by X-ray single crystal diffraction and by CD spectroscopy. All alkaloids 1-3 possess the cis-junction of B/C rings and the C11-hydroxyl in axial position on the half-chair-conformated B ring. The dihedral angles between the aromatic planes A, D are 51.9, 55.0/44.8, and 22.0° in 1, 2, and 3, respectively. In all compounds 1-3, there is intramolecular H-bonding between C11-OH group and nitrogen. Using a combined approach of X-ray diffraction and CD spectra correlations the absolute configurations of the title alkaloids have been determined as (+)-11S,13R,14S-homochelidonine (1), (+)-11R,12R,13R,14S-chelamine (2), and (−)-11R,13S,14R-norchelidonine (3).