A Computational Study of a Prebiotic Synthesis of the Steroid Progesterone (C and D Rings)

The magnesium ion metalloporphyrin complex is shown to bind the ligands propyne (p) and ethyne (e) on the metal or nitrogen pyrrole sites as a two site catalyst in their copolymerization. The order of addition of the monomers is (pepee). The steroid ring D (pep) is formed first from the propyne adduct bound to the metal site and the but-diene adduct bound to the N-site. The optimal orientation of these adducts determines the β-orientation of the 17-substituent. Further reaction with hydroxyl radicals allows this to be a 17 β- acetyl substituent. Further addition of two ethyne monomers forms an N-diene cyclopentene derivative able to cyclise to form the steroid ring C (pee) with a trans conformation and a 13-β methyl substituent. The reactions have been shown to be feasible from the overall enthalpy changes in the ZKE approximation at the HF and MP2 /6-31G* level, and with acceptable activation energies.