Mechanism and Origins of Enantioselectivity in the Nickel-catalyzed Asymmetric Synthesis of Silicon-Stereogenic Benzosiloles.

As important π-skeletons, benzosiloles often possess unique electronic and optical properties and have been widely used in semiconductor materials. Therefore, great attention has been drawn to the area of developing novel synthetic methods for various benzosiloles. However, the synthesis of enantioenriched silicon-stereogenic benzosiloles is still at an early stage and remains to be explored. Herein, we performed systematic density functional theory studies on the recently reported nickel-catalyzed asymmetric synthesis of silicon-stereogenic benosiloles, which was enabled by an enantioselective desymmetrization of (2-alkenyl)aryl-substituted silacyclobutanes. Our computational study shows that the reaction mechanism involves ligand exchange, oxidative addition, alkene insertion, and hydrogen-transfer coupled reductive-demetalation steps. The proposed transmetalation and ß-hydride elimination mechanism was not found, which might be due to the unfavorable ring strain of the multicyclic intermediates. The novel hydrogen-transfer coupled reductive-demetalation mechanism was shown to be reasonable for the generation of the silicon-stereogenic benzosilole. Noncovalent interactions (including C-H···π and hydrogen bonding) in the rate-determining alkene insertion transition state account for the origins of the enantioselectivity. Our computational study sheds light on the detailed reaction mechanism and also provides insights for the development of novel approaches for synthesis of high-value silicon-stereogenic compounds.

Catalytic Enantioselective Biltz Synthesis.

The Biltz synthesis establishes straightforward access to 5,5-disubstituted (thio)hydantoins by combining a 1,2-diketone and a (thio)urea. Its appealing features include inherent atom and step economy together with the potential to generate structurally diverse products. However, control of the stereochemistry of this reaction has proven to be a daunting challenge. Herein, we describe the first example of enantioselective catalytic Biltz synthesis which affords more than 40 thiohydantoins with high stereo- and regio-control, irrespective of the symmetry of thiourea structure. A one pot synthesis of corresponding hydantoins is also documented. Remarkably, experimental studies and DFT calculations establish the reaction pathway and origin of stereoselectivity.

14-3-3 proteins are involved in growth, hyphal branching, ganoderic acid biosynthesis, and response to abiotic stress in Ganoderma lucidum.

Ganoderma lucidum, which contains many pharmacologically active compounds, is regarded as a traditional medicinal fungus. Nevertheless, the scarcity of basic research limits the commercial value and utilization of G. lucidum. As a class of highly conserved, phosphopeptide-binding proteins present in all eukaryotes, 14-3-3 proteins play vital roles in controlling multiple physiological processes, including signal transduction, primary metabolism, and stress responses. However, knowledge of the roles of 14-3-3 proteins in Basidiomycetes is sparse. In this article, two homologs of 14-3-3 proteins, encoded by the two distinct genes GlBmh1 and GlBmh2, were distinguished in G. lucidum. We found that GlBmh1 and GlBmh2 were expressed at various developmental stages, including in vegetative mycelium cultivated on solid medium and in primordia and fruiting bodies. Moreover, we constructed GlBmh1 single-silenced strains, GlBmh2 single-silenced strains, and 14-3-3 double-silenced mutants for further study. When GlBmh1 and GlBmh2 were inhibited by RNA interference, the growth rate of mycelia was decreased, and the distance between the aerial hyphal branches was reduced; responses to various abiotic stresses such as oxidants and cell wall and osmotic stressors were also changed. Furthermore, the contents of secondary metabolite ganoderic acids (GAs) were increased after GlBmh1 and GlBmh2 were simultaneously silenced. Taken together, we provide evidence that implicates potential roles for the two 14-3-3 proteins in affecting growth and GA biosynthesis, thereby providing new insights into the basic functions of 14-3-3 proteins in G. lucidum.

Alternative oxidase impacts ganoderic acid biosynthesis by regulating intracellular ROS levels in Ganoderma lucidum.

The alternative oxidase (AOX), which forms a branch of the mitochondrial respiratory electron transport pathway, functions to sustain electron flux and alleviate reactive oxygen species (ROS) production. In this article, a homologous AOX gene was identified in Ganoderma lucidum. The coding sequence of the AOX gene in G. lucidum contains 1038 nucleotides and encodes a protein of 39.48 kDa. RNA interference (RNAi) was used to study the function of AOX in G. lucidum, and two silenced strains (AOXi6 and AOXi21) were obtained, showing significant decreases of approximately 60 and 50 %, respectively, in alternative pathway respiratory efficiency compared to WT. The content of ganoderic acid (GA) in the mutant strains AOXi6 and AOXi21 showed significant increases of approximately 42 and 44 %, respectively, compared to WT. Elevated contents of intermediate metabolites in GA biosynthesis and elevated transcription levels of corresponding genes were also observed in the mutant strains AOXi6 and AOXi21. In addition, the intracellular ROS content in strains AOXi6 and AOXi21 was significantly increased, by approximately 1.75- and 1.93-fold, respectively, compared with WT. Furthermore, adding N-acetyl-l-cysteine (NAC), a ROS scavenger, significantly depressed the intracellular ROS content and GA accumulation in AOX-silenced strains. These results indicate that AOX affects GA biosynthesis by regulating intracellular ROS levels. Our research revealed the important role of AOX in the secondary metabolism of G. lucidum.