Although the gross structure of haem d1 has been established, the absolute stereochemistry at C-2 and C-7 is unknown. An unambiguous stereoselective synthesis of the ester of the metal-free macrocycle corresponding to haem d1 has been completed which establishes the absolute configuration of the natural cofactor as 2R,7R. Haem d1 is thus shown to match stereochemically other biologically important macrocycles, e.g. those involved in the biosynthesis of vitamin B12, which are related to isobacteriochlorins and also display 2R,7R configurations. The synthetic sequence used is based on a new procedure for assembly of the western and eastern building blocks and it serves as an efficient general route for construction of isobacteriochlorins.
Department of Chemistry
University of Cambridge