Structures of (−)-rhazinilam, a racemic rhazinilam analogue, and several DPP derivatives, including NSC 613241. IC50’s for microtubule assembly for NSC 362449 and NSC 602746 were 6.2 ± 0.38 and 6.2 ±0.44 μM, respectively, obtained at the same time as the IC50 of 5.8 ± 0.23 μM was obtained for NSC 362455, as described in the text. These were the most active DPP derivatives described previously [9].