<jats:p> By using the known structure and stereochemistry of the alkaloid ormosanine, C<jats:sub>20</jats:sub>H<jats:sub>35</jats:sub>N<jats:sub>3</jats:sub> (I), as reference, the structure and stereochemistry of the isomeric compounds piptanthine (II), tetrahydro-ormojanine (XIV), isotetrahydro-ormojanine (XIII), and dasycarpine (XX) have been elucidated by isomerization and reduction experiments. Three additional C<jats:sub>20</jats:sub>H<jats:sub>35</jats:sub>N<jats:sub>3</jats:sub> bases possessing the ormosanine skeleton have been prepared by the reduction of pyridine VII. Thus, of the 15 possible stereoisomers of ormosanine, 7 are now known.The study leads to the stereo-formula XIX for the alkaloid ormojanine, C<jats:sub>20</jats:sub>H<jats:sub>31</jats:sub>N<jats:sub>3</jats:sub>. The alkaloid ormosinine has been found to be a dimer of panamine.Stereochemical aspects of the catalytic and chemical reductions of the double bonds and pyridine rings contained in the described compounds are discussed. A possible biogenetic intermediate is proposed which explains the presence of both antipodal series of Ormosia alkaloids in nature.