Cyclizations of 1′,8′-dimethyl-a,c-biladiene salts to give porphyrins: a study with various oxidizing agents

Cyclizations of the a,c-biladiene salts (3) and (4) to give the porphyrins (5) and (6), respectively, have been studied in the presence of a variety of chelating agents and oxidants. As a standard, the established procedure [copper(II) chloride–boiling dimethylformamide–6 min] gave compounds (5) and (6), in 37 and 20% yields respectively. Using zinc(II) acetate as a chelating agent for the a,c-biladienes, a large number of oxidants have been studied (in place of the copper chloride); best yields of porphyrin were obtained with potassium chromate [31% of (5); 22% of (6)] and silver iodate [35% of (5); 31% of (6)]. Other oxidants [PbO2, K2Cr2O7, Kl, Hg(OAc)2, Ag2O, and AgOAc] gave moderate yields of porphyrin, and others (Table 1 ) gave either a trace of porphyrin, or none at all. In a useful modification of the copper(II) chloride cyclization, a,c-biladienes have been cyclized during 2 h at room temperature to give porphyrins in yields of 34–65% after removal of chelated copper.