Synthesis and characterization of tetraphenyl-21,23-dideazaporphyrin: the best evidence yet that porphyrins really are the [18]annulenes of nature.

McMurry coupling of a pyrrole bisacrylaldehyde afforded a dipyrrolic macrocycle that is structurally midway between 1,10-diaza[18]annulene and the porphyrins. The diprotonated dication of this system retains porphyrin-like properties and provides insights into the nature of porphyrinoid aromaticity.