Sunday, July 19, 2015

On Biosynthetic and Biomimetic Steroid Chemistry: The Polyene Cyclization Mechanism of Lanosterol

            The biogenetic mechanism for the formation of the 6-6-6-5 tetracyclic steroid lanosterol (outlined below), an intermediate in the biosynthesis of cholesterol, has been studied by chemists for over half a century. Enzyme crystallographic data, together with other bioorganic and mutational studies, have yielded a biomolecular mechanism for enzymatic triterpene cyclization starting from acyclic 2,3-oxidosqualene. The mechanism involves initiation through protonation by a specific aspartic acid residue. The polycyclization cascade then proceeds away from the catalytic acid, with intermediates shielded by aromatic amino acid residues that line the active site cavity with p-electron-rich faces. However, this biosynthetic conversion is incredibly complicated and many of its precise details have not been fully resolved. One key question that has remained unanswered until recently concerns the extent to which ring formation is concerted with the initial protonation step. Earlier this year, in an effort to sort out uncertainties associated with this complex mechanism, Ruibo Wu’s group in Guangzhou, China, in collaboration with a team at Vanderbilt University led by the theoretical computational chemist Andes Hess, successfully characterized the cyclization of squalene oxide at the highest computational level carried out to date.
            The investigators used quantum mechanics/molecular mechanics molecular dynamics (QM/MM MD) simulations to construct the overall cyclization free-energy profile (reproduced below) for this fascinating biosynthetic transformation. The study shows that the entirety of the process (acyclic substrate à protosterol cation) entails two concerted, asynchonous reactions which proceed through two metastable states (the A and B states) and one stable intermediate, under oxidosqualene cyclase (OSC) enzyme catalysis. The overall cyclization can be considered a carbocation migration and is exergonic by ~4.4 kcal/mol. The rate-limiting step is the concerted formation of the A-ring, with opening of the epoxide ring. The stable intermediate (the C’ state) is a tricyclic 6-6-5 species that is subsequently converted to the tetracyclic protosterol cation (the D state) by a ring expansion with concomitant (concerted) formation of the D-ring. Importantly, all previous experimental mutagenic data is consistent with the identified reaction mechanism.

Hess and Wu. ACIE 2015, 54, 8693.
            The 2015 computational mechanistic study of Hess and Wu will no doubt inspire the synthetic enthusiasts among us to reflect on the myriad of creative ways in which organic chemists have tried to mimic the action of the enzyme OSC in a laboratory flask or reactor. This area of so-called ‘biomimetic’ research was pioneered by W. S. Johnson at Stanford University in the 1960’s and 70’s. Johnson’s efforts were later extended by countless others. In his 1976 Review Article in Angewandte Chemie, Johnson elegantly summarizes the synthetic strategy: “Certain polyenic substances having trans olefinic bonds in a 1,5 relationship can be induced to undergo stereospecific, non-enzymic, cationic cyclization to give polycyclic products with all-trans (“natural”) configuration. These transformations appear to mimic in principle the biogenetic conversion of squalene into polycyclic triterpenoids…” A highlight from Johnson’s impressive body of research is a remarkably short synthesis of racemic progesterone, starting from the cyclopentenol shown below. Cyclization under Brønsted acidic conditions produces a crystalline tetracyclic product in 71% yield. Subsequent zonolysis followed by intramolecular aldol condensation then furnishes synthetic progesterone. A related process (lower panel below) was also conceived by Johnson to generate a valuable 11a-substituted intermediate that intersects with a known commercial route for the production of hydrocortisone acetate. More recently, ‘biomimetic’ polyolefin carbocyclization has been elegantly applied by E. J. Corey’s laboratory to the chemical synthesis of complex limonoid steroidal systems (highlighted here).

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