Total Synthesis of Regioselectively 15N-Labeled Phycocyanobilins

The synthesis of open-chain tetrapyrroles with specified functionalities is important for understanding biological photoreceptors bearing tetrapyrrole chromophores. In this study, we regioselectively synthesized 15N-labeled phycocyanobilins by preparing and then successively coupling their A-, B-, C-, and D-pyrrole rings. 15NH4Cl was selected as the 15N source, which was converted into 15N-labeled formamide. 15N-Labeled tert-butyl formylglycinate and N-(tosylmethyl)formamide were readily prepared from the labeled formamide and then dehydrated to give 15N-labeled tert-butyl isocyanoacetate and p-toluenesulfonylmethyl isocyanide, respectively. The B-, C-, and D-ring 15N-labeled pyrroles were prepared in good chemical yields from the isocyanides using Barton pyrrole-formation chemistry. The total syntheses of the B-, C-, and D-ring 15N-labeled phycocyanobilins were achieved using the 15N-labeled pyrroles. The methine protons at the meso-positions of the synthesized tetrapyrroles were unambiguously assigned by 1H NMR spectroscopy. Furthermore, tautomerization involving electronic conjugation between the B- and C-rings is discussed based on the 15N NMR spectra acquired for the B- and C-ring 15N-labeled phycocyanobilins and their diallyl esters.