meso-Methylporphyrins and -chlorins

Two efficient synthetic routes to meso-methylporphyrins and -chlorins are described. In the first, meso-formylporphyrins and -chlorins are reduced to the corresponding meso-hydroxymethyl derivatives, and after zinc chelation and treatment with acetic anhydride in pyridine, the resulting meso-acetoxymethylporphyrins are reduced to meso-methyl with sodium borohydride or by catalytic hydrogenation. The second route is more efficient in that copper(II) meso-formylporphyrins and -chlorins can be reduced directly with tetra n-butylammonium borohydride in hot 1,2-dichloroethane to give the copper(II) meso-methyl analog. Using the procedures developed herein, a formal total synthesis of the meso-pheophorbide from Chlorobium chlorophyll 660 band 6 is decribed, and certain photooxidation and electrophilic deuteration problems in the meso-methylchlorin series are clarified. © 1980