Functionalization of porphyrins: towards the synthesis of bifunctional chelates for bismuth coordination

We report the condensation of 3-chloromethyl-benzoyl chloride with two atropisomers ααββ and αβαβ of meso-5,10,15,20-tetrakis-(2-amino)phenylporphyrin (TAPP), followed by the reaction of the anion of either cyano-acetic acid ethyl ester or (4-nitro-phenyl)-acetic acid ethyl ester to prepare various pre-organized strapped porphyrins. These two reagents were selected as both allow the easy formation of the anion in the α position of the ester group while their electron-withdrawing group (EWG) can be further transformed in a reactive functional group. In the ααββ series, this reaction leads to three isomeric porphyrins differing only by the location of their ethoxycarbonyl groups, oriented either towards the center of the porphyrin or maintained outside of the cavity. In the αβαβ series, as expected, a single porphyrin is obtained in which both straps bear an ethoxycarbonyl group, precursor of a hanging carboxylic function and a cyano or a 4-nitro-phenyl group, which can be reduced to an amine function, suitable for the coupling on a biomolecule.