First Rational Synthesis of Antiaromatic 5,15‐Dioxaporphyrin and Its β,β‐Linked Dimer Formation upon Oxidation

5,15‐Dioxaporphyrin was synthesized for the first time by a nucleophilic aromatic substitution reaction of a nickel bis(α,α'‐dibromodipyrrin) complex with benzaldoxime, followed by an intramolecular annulation of the α‐hydroxy‐substituted intermediate. This unprecedented molecule is 20π‐electron antiaromatic in terms of Hückel's rule of aromaticity because lone pair electrons of oxygen atoms are incorporated into the 18π‐electron conjugated system of porphyrin. A theoretical analysis based on the gauge including magnetically induced current method confirmed its antiaromaticity and a dominant inner pathway for the ring current. Unique reactivity of 5,15‐dioxaporphyrin forming a β,β‐linked dimer upon oxidation was also revealed.