Base catalyzed cyclization of a 5,15-di(4-pyridyl)octaalkylporphyrin-5
-acrylate 6 yields smoothly the corresponding purpurin 2. Quaternizati
on with methyl iodide yielded the corresponding cationic N,N'-dimethyl
pyridiniumpurpurin 3; X-ray crystallography of purpurin 2, the first p
urpurin to be studied crystallographically, shows the two p-alkyl grou
ps 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 Angstrom from the least-squares plane calculated for the core a
toms.