Visible-Light-Driven Germyl Radical Generation via EDA-Catalyzed ET-HAT Process.

We have established a facile and efficient protocol for the generation of germyl radicals by employing photo-excited electron transfer (ET) in an electron donor-acceptor (EDA) complex to drive hydrogen-atom transfer (HAT) from germyl hydride (R3GeH). Using a catalytic amount of EDA complex of commercially available thiol and benzophenone derivatives, the ET-HAT cycle smoothly proceeds simply upon blue-light irradiation without any transition metal or photocatalyst. This protocol also affords silyl radical from silyl hydride.

Total Biosynthesis of Melleolides from Basidiomycota Fungi: Mechanistic Analysis of the Multifunctional GMC Oxidase Mld7.

Mushroom terpenoids are biologically and chemically diverse fungal metabolites. Among them, melleolides are representative sesquiterpenoids with a characteristic protoilludane skeleton. In this study, we applied a recently established hot spot knock-in method to elucidate the biosynthetic pathway leading to 1α-hydroxymelleolide. The biosynthesis of the sesquiterpene core involves the cytochrome P450 catalyzing stepwise hydroxylation of the Δ6 -protoilludene framework and a stereochemical inversion process at the C5 position catalyzed by short-chain dehydrogenase/reductase family proteins. The highlight of the biosynthesis is that the flavoprotein Mld7 catalyzes an oxidation-triggered double-bond shift accompanying dehydration and acyl-group-assisted substitution with two different nucleophiles at the C6 position to afford the Δ7 -protoilludene derivatives, such as melleolide and armillarivin. The complex reaction mechanism was proposed by DFT calculations. Of particular importance is that product distribution is regulated by interaction with the cell membrane.

Revision of the Peniroquesine Biosynthetic Pathway by Retro-Biosynthetic Theoretical Analysis: Ring Strain Controls the Unique Carbocation Rearrangement Cascade.

Peniroquesine, a sesterterpenoid featuring a unique 5/6/5/6/5 fused pentacyclic ring system, has been known for a long time, but its biosynthetic pathway/mechanism remains elusive. Based on isotopic labeling experiments, a plausible biosynthetic pathway to peniroquesines A-C and their derivatives was recently proposed, in which the characteristic peniroquesine-type 5/6/5/6/5 pentacyclic skeleton is synthesized from geranyl-farnesyl pyrophosphate (GFPP) via a complex concerted A/B/C-ring formation, repeated reverse-Wagner-Meerwein alkyl shifts, three successive secondary (2°) carbocation intermediates, and a highly distorted trans-fused bicyclo[4.2.1]nonane intermediate. However, our density functional theory calculations do not support this mechanism. By applying a retro-biosynthetic theoretical analysis strategy, we were able to find a preferred pathway for peniroquesine biosynthesis, involving a multistep carbocation cascade including triple skeletal rearrangements, trans-cis isomerization, and 1,3-H shift. This pathway/mechanism is in good agreement with all of the reported isotope-labeling results.

Elucidation of Late-Stage Biosynthesis of Phomoidride: Proposal of Cyclization Mechanism Affording Characteristic Nine-Membered Ring of Fungal Dimeric Anhydride.

Antihypercholesterolemic agent phomoidride (PMD) B has a highly elaborated bicyclo[4.3.1]deca-1,6-diene core scaffold derived from dimeric anhydride with a nine-membered ring. This report elucidated the late stage transformation from an anhydride monomer to PMD B through the heterologous expression of three enzyme genes, TstC, TstK, and TstE. Additional in vitro studies of TstK and TstE provided evidence on the formation of PMD via dimerization, three-step oxidation, and unusual methylation-triggered bicyclic ketal formation. Elucidation of the function of cyclase TstC prompts us to examine the cyclization mechanism of TstC by using a computational approach. Computational analytical data on PMD and structurally related glaucanic acid indicated that the initial decarboxylation of monomer results in enolate and subsequent double Michael reactions of another monomer, followed by an optional aldol reaction proceeding in an endo-selective manner to give cycloadducts, supporting the fact that the starting orientation of two monomers is directly transferred to the product configurations.

Molecular and Computational Bases for Spirofuranone Formation in Setosusin Biosynthesis.

The 3(2H)-furanone unit is observed in many biologically active natural products, as represented by the antifungal medication griseofulvin. Setosusin (1) is a fungal meroditerpenoid featuring a unique spiro-fused 3(2H)-furanone moiety; however, the biosynthetic basis for spirofuranone formation has not been investigated since its isolation. Therefore, in this study we identified the biosynthetic gene cluster of 1 in the fungus Aspergillus duricaulis CBS 481.65 and elucidated its biosynthetic pathway by heterologous reconstitution of related enzyme activities in Aspergillus oryzae. To understand the reaction mechanism to afford spirofuranone, we subsequently performed a series of in vivo and in vitro isotope-incorporation experiments and theoretical calculations. The results indicated that SetF, the cytochrome P450 enzyme that is critical for spirofuranone synthesis, not only performs the epoxidation of the polyketide portion of the substrate but also facilitates the protonation-initiated structural rearrangement to yield 1. Finally, a mutagenesis experiment using SetF identified Lys303 as one of the potential catalytic residues that are important for spirofuranone synthesis.