Rhombifoline is isomeric with thermopsine and anagyrine, and, like both these alkaloids, contains an α-pyridone ring. It can be dealkylated to cytisine, and, on catalytic hydrogenation, yields octahydrodesoxyrhombifoline, which is identical with hexahydrodesoxy-N-(n-butyl)-cytisine, the hydrogenation product of N-(n-butyl)-cytisine. The results of the hydrogenation thus indicate that the molecule of rhombifoline contains a third double bond, which is shown by its infrared spectrum to be located at a terminal methylene group. Rhombifoline is therefore N-(but-3-enyl)-cytisine, a conclusion which was confirmed by the partial synthesis of the alkaloid from cytisine.