Northern-Southern Route to Synthetic Bacteriochlorins

A new route to bacteriochlorins via Northern-Southern (N-S) self-condensation of a dihydrodipyrrin-acetal complements a prior Eastern-Western (E-W) route. Each bacteriochlorin was prepared in five steps from an α-halopyrrole and a 2,2-dimethylpent-4-ynoic acid. The first three steps follow Jacobi's synthesis of dihydrodipyrrins: Pd-mediated coupling to form a lactone-pyrrole, Petasis reagent treatment for methenylation, and Paal-Knorr type ring closure to form the 1,2,2-trimethyl-substituted dihydrodipyrrin. Subsequent steps entail conversion of the 1-methyl group to the 1-(dimethoxymethyl) unit and acid-catalyzed self-condensation of the resulting dihydrodipyrrin-acetal. The essential differences between the N-S and E-W routes lie in (1) the location of the gem-dimethyl group (with respect to the 1-acetal unit) at the 2- versus 3-position in the dihydrodipyrrin-acetals, respectively, (2) the method of synthesis of the dihydrodipyrrins, and consequently (3) access to diverse substituted bacteriochlorins including those with substituents at the meso-positions. Ten new bacteriochlorins bearing 0-6 total aryl, alkyl, and carboethoxy substituents at the β-pyrrole and/or meso-positions have been prepared, with yields of macrocycle formation of up to 39%. Four single-crystal X-ray structures (two intermediates, two bacteriochlorins) were determined. The bacteriochlorins exhibit characteristic bacteriochlorophyll-like absorption spectra, including a Q_y band in the region 713-760 nm.