Porphyrin Homologues: [22]Porphyrin(2.2.2.2), a “Stretched Porphycene”

The close structural relationship of porphyrin system and porphycene system is strikingly demonstrated with the [22]porphyrin(2.2.2.2) 1/2: on grounds of strain [22]prophyrin(2.2.2.2) does not exist as the isomer with porphyrin symmetry (D2h) 1, but as stretched porphycene 2.