Attempted syntheses of N-confused hydroporphyrins through modified Lindsey routes

Abstract

Unlike N-confused porphyrins which are well-known and extensively studied tetrapyrroles, N-confused hydroporphyrins are almost unknown, largely because so far they have resisted attempts at rational synthesis. Here, we report our efforts towards the total synthesis of N-confused hydroporphyrins. We have prepared N-confused building blocks analogous to the non-N-confused substrates in the Lindsey synthesis of sparsely substituted chlorins. We have systematically flipped the A, B and C pyrrole rings in the dipyrrolic precursors of the target N-confused macrocycles, preparing in total an N-confused “Western half” (tetrahydrodipyrrin) and two N-confused “Eastern halves” (brominated formyldipyrromethanes). These were subjected to a range of cyclization conditions. While we successfully isolated and identified three macrocyclic products, none of these proved to be the desired N-confused hydroporphyrin.