Assignment of 13C-signals from the meso-carbons by syntheses of 13C-protoporphyrin-IX dimethyl esters

[β-13C]-, [γ-13C]-, and [δ-13C]-Protoporphyrin-IX dimethyl esters have been synthesised by unambiguous routes from [13C]formaldehyde to allow assignment of the four n.m.r. 13C-signals from the meso-carbons; the dimethyl ester of diacetyldeuteroporphyrin-IX has been similarly studied.