The oxidation of meso-tetrahydrotetraalkylporphyrinogen, which is the chemical and biochemical precursor of porphyrins, has been reexamined using meso-octaalkyl-porphyrinogen as a model compound. This investigation has led to the discovery of oxidized forms of porphyrinogen other than porphyrins, which we call artifical porphyrins. They contain cyclopropane moieties which function as two-electron shuttles via the formation and cleavage of a C–C bond. The metal-assisted modifications of the porphyrinogen skeleton, namely the homologation of a pyrrole to a pyridine ring using carbon monoxide and the functionalization of the aliphatic periphery, are the consequences of the bifunctional acid–base carrier properties of these metal–porphyrinogen complexes. This observation allowed us to establish general synthetic methodologies in the fields of metal-assisted C–H bond activation and C–C bond formation.