Geissoschizine revisited - definite proof of its stereostructure.

By re-assignment and detailed analysis of the NMR spectra of geissoschizine (1) and its derivatives, the hitherto accepted conformer 1a was revised to another possibility 1c that favored the trans-quinolizidine form and the D-ring adopting twist boat form, in which the intramolecular hydrogen bonding between the enol hydroxyl and Nb function was present.