Biosynthesis of porphyrins and related macrocycles. Part III. Rational syntheses of [β-13C]-, [γ-13C]-, and [δ-13C]-protoporphyrin-IX: assignment of the 13C nuclear magnetic resonance signals from the meso-carbon atoms of protoporphyrin-IX dimethyl ester
The 13C signals arising from the meso-carbon atoms ot protoporphyrin-IX are of key importance for biosynthetic studies. Rigorous assignment is made of the signals from the β- and δ-carbon atoms by synthesis of [β-13C]- and [δ-13C]-protoporphyrin-IX dimethyl ester from unsymmetrical pyrromethenes in 42% yield. The formation of porphyrins from a,c-biladienes and formaldehyde is studied and leads to synthesis of [γ-13C]protoporphyrin-IX dimethyl ester. The spectrum of this product allows assignment of the two remaining signals, from the γ- and α-carbon atoms. The three 13C-labelled samples of protoporphyrin-IX dimethyl ester are converted into 2,4-diacetyldeuteroporphyrin-IX dimethyl ester; assignment is thereby made of the well-spread 13C signals from the meso-carbon atoms of this porphyrin. 13C Chemical shifts are reported for a variety of pyrrole derivatives and porphyrins.