New synthesis of tetraazachlorins and tetraazabacteriochlorins by Diels–Alder reaction of tetraazaporphine

The use of the Diels–Alder reaction of unsubstituted tetraazaporphine as dienophile with some dienes of the anthracene series for the synthesis of novel substituted tetraazachlorins, tetraazabacteriochlorins and tetraazaisobacteriochlorins is reported. The condensation of tetraazaporphine with anthracene, 9,10-dimethyl- or 2,6-di(tert-butyl)anthracene in boiling chlorobenzene gives novel dibenzobarreleno-substituted tetraazachlorins 3–5 in yields of up to 71%. When AlCl3 was used as catalyst in the reaction of tetraazaporphine with 2,6-di(tert-butyl) anthracene, the corresponding cis- and trans-dibenzobarreleno-substituted tetraazabacteriochlorins 6a and 6b were obtained. The reaction of tetraazaporphine with the more active diene 2,3-benzanthracene at 130°C gives mainly benzonaphtho[2,3-b]barreleno-substituted tetraazachlorin 7, while the same reaction conducted at 220°C led to a mixture of cis- and trans isomers of tetraazabacteriochlorins 8a and 8b and tetraazaisobacteriochlorins 9a and 9b. The products were characterized using elemental analysis data and mass, 1H NMR and visible spectra. The effect of hydrogenation of one or two double bonds of the tetraazaporphine macrocycle on the electronic absorption spectra is discussed.