The chemistry of pyrrolic compounds. XL. A new synthesis of protoporphyrins III and XIII and a 1H N.M.R. study of the preferred pathway of electron delocalization in the porphyrin macrocycle

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Abstract
The synthesis of protoporphyrins III and XIII as the dimethyl esters has been completed by an oxidative cyclization of biladiene-ac intermediates. Protoporphyrin III has been converted into deuteroporphyrin III and measurements of the proton coupling constant in the allylic unit, CH3-C=C-H, of this latter porphyrin point to the double bond of this system having a significantly diminished bond order. This finding is in accord with the view that the π-electron delocalization pathway in the porphyrin macrocycle involves the periphery of the molecule.