Structure and Conformational Analysis of Napelline-Type Diterpenoid Alkaloids

X-ray crystal structure analyses of six compounds (napelline, napelline N-oxide EtOH solvate, 1-O-benzoylnapelline EtOH solvate, songorine hydrochloride monohydrate EtOH solvate, 1-O-benzoylsongorine, and N-deethylsongorine) showed that the diterpenoid alkaloid napelline had the beta-configuration of the C12 hydroxyl. It was found that the N-oxide in napelline N-oxide had the R-configuration. Two types of intramolecular H-bonds could occur in the studied alkaloids and their salts, i.e., between the unshared electron pair on the skeletal N atom and the C1 OH and between the C12 and C15 OH groups. Ring A adopted the boat conformation if the first type of H-bond formed. The second type of H-bond did not cause significant conformational changes.