Pyrroles and related compounds. Part XXVII. Syntheses of porphyrins with a nuclear carboxy-group

Abstract

The synthesis of 2,6,7-triethyl-4-methoxycarbonyl-1,3,5,8-tetramethylporphin (2) by the a-oxobilane route is reported. The b-oxobilane approach is used in the synthesis of rhodoporphyrin-XV dimethyl ester (1a) and in the first total synthesis of 2-vinylrhodoporphyrin-XV dimethyl ester (1b); both of these compounds are important degradation products from chlorophyll-a. The vinyl group in the latter porphyrin was generated by elimination of hydrogen chloride from the zinc complex of the corresponding (2-chloroethyl)porphyrin. In order to minimise deactivation of the terminal rings during closure of the macrocycle, it was found to be advantageous to site the electron-withdrawing methoxycarbonyl functions on the internal rings of the open-chain tetrapyrrolic intermediates.