Synthesis and structure of a 5,15-bis(4-pyridyl)purpurin

Base catalyzed cyclization of a 5,15-di(4-pyridyl)octaalkylporphyrin-5-acrylate 6 yields smoothly the corresponding purpurin 2. Quaternization with methyl iodide yielded the corresponding cationic N,N′-dimethylpyridiniumpurpurin 3; X-ray crystallography of purpurin 2, the first purpurin to be studied crystallographically, shows the two β-alkyl groups on the saturated ring of the purpurin have a syn configuration, and the macrocycle to have a slight saddle shape with a mean deviation of 0.230 Å from the least-squares plane calculated for the core atoms.