Synthesis of diverse acyclic precursors to pyrroles for studies of prebiotic routes to tetrapyrrole macrocycles

A chemical model for the origin of tetrapyrrole macrocycles under prebiotic conditions entails the condensation of acyclic dicarbonyl compounds and α-aminoketones to form pyrroles that are equipped for subsequent self-condensation. Development and exploration of the scope of the chemical model (including combinatorial reactions, studies of the effects of structurally defective substrates, and reactions in aqueous or organic media) have relied on the availability of diverse starting materials prepared by traditional chemical synthesis methods. Here the synthesis of all acyclic dicarbonyl compounds and α-aminoketones used in the prior prebiotic model studies is described. There are five sets of acyclic dicarbonyl compounds including (i) β-ketoesters bearing diverse 4-substituents, (ii) levulinic acid derivatives bearing selected 5-substituents (i.e., analogues of δ-aminolevulinic acid, ALA), (iii) meso-substituted β-ketoesters, (iv) meso-substituted β-diketones that contain one 4-substituent, and (v) hybrid molecules that contain both the β-ketoacyl unit and the levulinic acid skeleton (or homologue thereof). A variety of α-aminoketones (homologues of ALA) also have been prepared. Altogether, the synthesis of 53 compounds is described, encompassing 28 new compounds as well as 25 known compounds that have been more fully characterized or prepared via alternative routes. The ability to convert selected acyclic compounds directly via pyrroles to porphyrinogens in a single-flask process may also prove useful in mainstream syntheses of diverse tetrapyrroles regardless of possible prebiotic relevance.