Synthesis of the altertoxin III framework

Crystal-Structure Control of Molecular Semiconductors by Methylthiolation: Toward Ultrahigh Mobility

ConspectusThe crystal structure of organic semiconductors has been regarded as one of the crucial factors for realizing high-performance electronic devices, such as organic field-effect transistors. However, although the control of crystal structures of organic semiconductors has been examined in the last two decades of intensive efforts of the development of organic semiconductors, active measures to control crystal structures enabling high carrier mobility are still limited. In 2016, our research group noticed that regioselective methylthiolation could provide a selective crystal structure change from an ordinary herringbone structure to a pitched π-stacking structure, similar to the crystal structure of rubrene, in the benzo[1,2-b:4,5-b']dithiophene (BDT) system. Following this serendipitous finding, our group systematically investigated the relationship between the molecular and crystal structures of a range of methylthiolated aromatic and heteroaromatic compounds.This Account provides a comprehensive overview of our research efforts and advancements in the development of methylthiolated small-molecule-based organic semiconductors (molecular semiconductors). We first describe the outline of the past development of molecular semiconductors, focusing on the types of crystal structures of high-performance molecular semiconductors. Then, we describe our findings on the drastic crystal structure change in the BDT system upon methylthiolation, detailing the causes of the change in terms of the intermolecular contacts and intermolecular interaction energies. This is followed by the confirmation of the generality of the crystal-structure change by methylthiolation of a series of acene and heteroacenes, where the herringbone structure in the parent system is unexceptionally transformed into the pitched π-stacking structure, a promising crystal structure for high-mobility molecular semiconductors well exemplified by the prototypical molecular semiconductor, rubrene. In fact, the methylthiolated anthradithiophene afforded comparable high mobility to rubrene in single-crystal field-effect transistors. Then, we demonstrate that the sandwich herringbone structures of peri-condensed polycyclic aromatic hydrocarbons, including pyrene, perylene, and peropyrene, change into brickwork crystal structures upon methylthiolation and that, among these compounds, very promising molecular semiconductors, methylthiolated pyrene and peropyrene, showing ultrahigh mobility of 30 cm2 V s-1, are realized.Through the studies, by gaining insights into the underlying mechanisms driving the crystal structure changes, we lay a strong foundation for tackling challenges related to controlling the crystal structures and developing high-performance molecular semiconductors. This will be a distinct approach from the past activities in the development of molecular semiconductors that mainly focused on molecules themselves, including their synthesis, properties, and characterization. We thus anticipate that our findings and the present Account will open the door to a new era of the development of molecular semiconductors.

Structure elucidation and biological activities of perylenequinones from an Alternaria species

The KEAP1-Nrf2/ARE pathway is a pivotal cytoprotective regulator against oxidative stress which plays an important role in the development of many inflammatory diseases and cancer. Activation of the Nrf2 transcription factor by oxidative stress or electrophiles regulates antioxidant response element (ARE)-dependent transcription of antioxidative, detoxifying, and anti-inflammatory proteins. Therefore, modulators of the KEAP1-Nrf2/ARE pathway have received considerable interest as therapeutics to protect against diseases where oxidative stress constitutes the underlying pathophysiology. In a search for fungal secondary metabolites affecting the Nrf2/ARE-dependent expression of a luciferase reporter gene in BEAS-2B cells, three new perylenequinones, compounds 1, 2, and 3, together with altertoxin-I (ATX-I), were isolated from fermentations of an Alternaria species. The structures of the compounds were elucidated by a combination of one- and two-dimensional NMR spectroscopy and mass spectrometry. Compound 1 and ATX-I exhibited strong cytotoxic effects with LC₅₀-values of 3.8 µM and 6.43 µM, respectively, whereas compound 3 showed no cytotoxic effects up to 100 µM on BEAS-2B cells. ATX-I induced ARE-dependent luciferase expression approximately fivefold and compound 1 approximately 2.6-fold at a concentration of 3 µM in transiently transfected BEAS-2B cells. In addition, compound 1 and ATX-I exhibited strong oxidative effects, whereas compound 3 did not show significant oxidative properties. For compound 1 and ATX-I, a strong upregulation of heme oxygenase-1 could be observed on mRNA and protein level in treated BEAS-2B cells. Moreover, compound 3 significantly decreased sod3 mRNA levels after induction of oxidative stress with benzoquinone.

High-speed C–H chlorination of ethylene carbonate using a new photoflow setup

We report the high-speed C–H chlorination of ethylene carbonate, which gives chloroethylene carbonate, a precursor to vinylene carbonate. A novel photoflow setup designed for a gas–liquid biphasic reaction turned out to be useful for the direct use of chlorine gas. The setup employed sloped channels so as to make the liquid phase thinner, ensuring a high surface-to-volume ratio. When ethylene carbonate was introduced to the reactor, the residence time was measured to be 15 or 30 s, depending on the slope of the reactor set at 15 or 5°, respectively. Such short time of exposition sufficed the photo C–H chlorination. The partial irradiation of the flow channels also sufficed for the C–H chlorination, which is consistent with the requirement of photoirradiation for the purpose of radical initiation. Near-complete selectivity for single chlorination required the low conversion of ethylene carbonate such as 9%, which was controlled by limited introduction of chlorine gas. At a higher conversion of ethylene carbonate such as 61%, the selectivity for monochlorinated ethylene carbonate over dichlorinated ethylene carbonate was 86%. We found that the substrate contamination with water negatively influenced the performance of the C–H chlorination.

Determination of the absolute configuration of perylene quinone-derived mycotoxins by measurement and calculation of electronic circular dichroism spectra and specific rotations

Altertoxins I-III, alterlosins I and II, alteichin (alterperylenol), stemphyltoxins I-IV, stemphyperylenol, stemphytriol, 7-epi-8-hydroxyaltertoxin I, and 6-epi-stemphytriol are mycotoxins derived from perylene quinone, for which the absolute configuration was not known. Electronic circular dichroism (ECD) spectra were calculated for these compounds and compared with measured spectra of altertoxins I-III, alteichin, and stemphyltoxin III and with reported Cotton effects. Specific rotations were calculated and compared with reported specific rotations. The absolute configuration of all the toxins, except for stemphyltoxin IV, could thus be determined. The validity of the assignment was high whenever reported ECD data were available for comparison, and the validity was lower when the assignment was based only on the comparison of calculated and reported specific rotations. ECD spectra are intrinsically different for toxins with a biphenyl substructure and for toxins derived from dihydroanthracene.

Metabolites from Alternaria fungi and their bioactivities

Alternaria is a cosmopolitan fungal genus widely distributing in soil and organic matter. It includes saprophytic, endophytic and pathogenic species. At least 268 metabolites from Alternaria fungi have been reported in the past few decades. They mainly include nitrogen-containing metabolites, steroids, terpenoids, pyranones, quinones, and phenolics. This review aims to briefly summarize the structurally different metabolites produced by Alternaria fungi, as well as their occurrences, biological activities and functions. Some considerations related to synthesis, biosynthesis, production and applications of the metabolites from Alternaria fungi are also discussed.