Spirocyclic iridoid alkaloids from <i>Plumeria rubra</i>

Plumerianoids A-D (1-4) with a new intact spirocyclic iridoid alkaloid skeleton, along with a new degraded alkaloid 8-<i>epi</i>-plumerianine (5), and a known one (6), were isolated and characterized from <i>Plumeria rubra</i>. The structure of 6 was revised as (8<i>R</i>,13<i>S</i>)-plumerianine. These alkaloids consist of three epimeric pairs (1/2, 3/4, and 5/6), exhibiting virtually identical NMR spectra within each pair. Furthermore, the electronic circular dichroism (ECD) curves of each epimeric pair are roughly mirror-like, reminiscent of enantiomers. This study calls for attention on the structural elucidation of special compound classes when the associated chiral centers are far away from the chromophores and/or at the terminal of the molecules. In addition, compound 3 exhibited moderate antiplasmodial activity.