AbstractPorphobilinogen‐11‐13C was prepared by using benzyl 3‐(β‐methoxycarbonylethyl)‐4‐ (methoxycarbonylmethyl)‐2‐pyrrolecarboxylate as a starting material. A Vilsmeier‐Haak formylation with N,N′‐dimethylform‐13C‐amide gave the 2‐formylpyrrole, which was transformed into its oxime, and the latter was hydrogenated to the hydrochioride of 5‐carboxyporphobilinogen dimethyl ester. The hydrochloride cyclized to 5‐carboxyporphobilinogen lactam at pH 9, and the latter was first decarboxylated and then sapon