Towards Rational Synthesis of Chlorin Building Blocks

Daniel T. Gryko,  Michał Gałęzowski

Institute of Organic Chemistry, Polish Academy of Sciences, Warsaw, Poland

The chlorin macrocycle is one of Nature’s most important cofactors, providing the basis for photosynthesis in plants (chlorophyll a nad b) and various algae.¹ While photosynthesis is their dominant biological role, chlorines also mediate enzymatic redox processes in bacteria (heme d) and serve as hormones in sea worms (bonellin). Chlorins are members of porphyrinic family but differ from porphyrins in having one pyrrole ring saturated at the β positions. Though porphyrins and chlorines have many similarities, the altered symmetry and path of conjugation resulting from the reduced pyrrole ring give rise to differences of profound importance for photochemical applications. A vast literature exists on synthetic porphyrin-based photosynthetic model systems, but relatively few synthetic chlorin-containing donor-acceptor systems have been prepared. The striking difference in development of porphyrin and chlorin model system stems from the availability of the requisite building blocks. We sought to develop rational yet broadly accessible synthetic route to a family of chlorines building blocks. ¹

Our idea is based on regioselective Diels-Adler reaction of substituted porphyrins with highly reactive dienes. The regioselectivity of Diels-Adler reaction is enforced by introducing electron-withdrawing groups at distinct sites at the perimeter of the macrocycle. The most challenging task is to develop the efficient method for the synthesis of respective porphyrins.  Different approaches will be presented with special emphasis put on the use of Friedel-Crafts acylation as a main step. Acylation of both porhyrins and their precursors will be compared in terms of efficiency and products distribution.

1.        H. Sheer, in Chlorophylls; H. Sheer, Ed.; CRC Press: Boca Raton, FL, 1991; pp 3-30.