Recent Advances in Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds.

This review surveys recent progress in the chemistry of polycyclic heteroaromatic molecules with a focus on structural diversity and synthetic methodology. The article covers literature published during the period of 2016-2020, providing an update to our first review of this topic (Chem. Rev. 2017, 117 (4), 3479-3716).

Scholl-Type Cycloheptatriene Ring Closure of 1,4,9,12-Tetraarylfenestrindanes: Reactivity and Selectivity in the Construction of Fenestrane-Based Polyaromatic Saddles.

The Scholl-type cyclodehydrogenation, generating up to four cycloheptatriene rings around a fenestrindane core, leads to novel, saddle-shaped polyaromatic hydrocarbon derivatives. In this article, we present the results of in-depth experimental and computational work on the oxidative cyclization of various 1,4,9,12-tetraarylfenestrindanes. In particular, the kinetic control of the four-step cyclization of the electronically activated tetrakis(3,4-dimethoxyphenyl) derivative is elucidated. The reasons for the exclusive emergence of one single among the three possible doubly cyclized intermediates and for the nonappearance of the singly and triply cyclized intermediates are clarified. In addition, the origin of the concomitant bridgehead hydroxylation is studied. The reactivity of a set of fifteen symmetrically and unsymmetrically substituted 1,4,9,12-tetraarylfenestrindanes towards Scholl-type cyclodehydrogenation is presented, pinpointing the structural factors that underlie this reaction and demonstrating the potential and limitations of this synthetic approach. A particularly surprising finding of this study is that the electronically nonactivated 1,4,9,12-tetraphenylfenestrindane can also undergo the fourfold Scholl-type cyclization process and can be transformed into the parent saddle hydrocarbon.

Enantiopure Aromatic Saddles Bearing the Fenestrindane Core.

The synthesis of enantiomerically pure, configurationally stable fenestrindane-based polyaromatic compounds with saddle-like structures is reported. Seven racemic fenestrane synthetic precursors were first screened by chiral HPLC for resolvability into enantiomers. Among the three resolvable precursors, a tribenzofenestrene derivative was resolved on a semipreparative scale, and the absolute configuration of the more slowly eluting enantiomer was established by X-ray crystallography. The enantiopure tribenzofenestrenes were then separately converted, in six steps, to the saddle-shaped fenestrindane derivatives in optically pure form. The two enantiomeric pairs of saddles were characterized by 1H and 13C NMR spectroscopy, mass spectrometry, and circular dichroism spectroscopy. All new compounds reported herein represent the first enantiopure non-natural carbocyclic fenestranes isolated to date.

From Fenestrindane towards Saddle-Shaped Nanographenes Bearing a Tetracoordinate Carbon Atom.

Two saddle-shaped polycyclic aromatic compounds (8 a and 8 b) bearing an all-cis-[5.5.5.5]fenestrane core surrounded by an o,p,o,p,o,p,o,p-cyclooctaphenylene belt were synthesized and characterized by NMR spectroscopy and mass spectrometry. The key step of this synthesis involves the formation of four cycloheptatriene rings from the corresponding electron-rich 1,4,9,12-tetraarylfenestrindane derivatives 7 a and 7 b in Scholl-type cyclizations. The structural details of the D2d -symmetric saddle compound 8 a were determined by X-ray crystallography, and the properties of 8 a and 8 b were studied by UV/Vis and fluorescence spectroscopy and cyclic voltammetry.

Quantitative and Functional Phosphoproteomic Analysis Reveals that Ethylene Regulates Water Transport via the C-Terminal Phosphorylation of Aquaporin PIP2;1 in Arabidopsis.

Ethylene participates in the regulation of numerous cellular events and biological processes, including water loss, during leaf and flower petal wilting. The diverse ethylene responses may be regulated via dynamic interplays between protein phosphorylation/dephosphorylation and ubiquitin/26S proteasome-mediated protein degradation and protease cleavage. To address how ethylene alters protein phosphorylation through multi-furcated signaling pathways, we performed a (15)N stable isotope labelling-based, differential, and quantitative phosphoproteomics study on air- and ethylene-treated ethylene-insensitive Arabidopsis double loss-of-function mutant ein3-1/eil1-1. Among 535 non-redundant phosphopeptides identified, two and four phosphopeptides were up- and downregulated by ethylene, respectively. Ethylene-regulated phosphorylation of aquaporin PIP2;1 is positively correlated with the water flux rate and water loss in leaf. Genetic studies in combination with quantitative proteomics, immunoblot analysis, protoplast swelling/shrinking experiments, and leaf water loss assays on the transgenic plants expressing both the wild-type and S280A/S283A-mutated PIP2;1 in the both Col-0 and ein3eil1 genetic backgrounds suggest that ethylene increases water transport rate in Arabidopsis cells by enhancing S280/S283 phosphorylation at the C terminus of PIP2;1. Unknown kinase and/or phosphatase activities may participate in the initial up-regulation independent of the cellular functions of EIN3/EIL1. This finding contributes to our understanding of ethylene-regulated leaf wilting that is commonly observed during post-harvest storage of plant organs.

Cis-cinnamic acid-enhanced 1 gene plays a role in regulation of Arabidopsis bolting.

Cis-cinnamic acid (CA) is one of many cis-phenylpropanoids found in both monocots and dicots. It is produced in planta via sunlight-mediated isomerization of trans-cinnamic acid. This pair of isomers plays a differential role in regulation of plant growth. A functional proteomics approach has been adopted to identify genes of cis/trans-CA mixture-enhanced expression. Out of 1,241 proteins identified by mass spectrometry, 32 were CA-enhanced and 13 repressed. Further analysis with the molecular biology approach revealed 2 cis-CA (Z usammen-CA)-E nhanced genes, named ZCE1 and ZCE2, which encode members of the major latex protein-like (MLPL) gene family. The transcript accumulation of both genes is positively correlated with the amount of cis-CA applied externally, ranging from 1 to 100 µM. ZCE1 transcript accumulation is enhanced largely by cis-CA and slightly by other cis-phenylpropanoids. Treatment of several well-characterized plant growth regulator perception-deficient mutants with cis-CA is able to promote ZCE1 transcript accumulation, suggestive of distinct signaling pathways regulating cis-CA response. The zce1 loss-of-function mutant produced via the RNA-interference technique produces an earlier bolting phenotype in Arabidopsis, suggesting that ZCE1 plays a role in promoting vegetative growth and delay flowering.

Phosphoproteomic analysis of ethylene-regulated protein phosphorylation in etiolated seedlings of Arabidopsis mutant ein2 using two-dimensional separations coupled with a hybrid quadrupole time-of-flight mass spectrometer.

Ethylene regulates a variety of stress responses and developmental adaptation in plants. In the present study, the phosphoproteomics is adopted to investigate the differential protein phosphorylation by ethylene in Arabidopsis ethylene-insensitive 2 (ein2) mutant. A total of 224 phosphopeptides were identified, of which 64 phosphopeptides were detected three or more times. Ethylene induces a general reduction in phosphorylated proteins in ein2. Totally, three ethylene-enhanced and three ethylene-repressible unique phosphopeptides were identified, respectively. Classification of the cellular functions of these phosphoproteins revealed that 55.5% of them are related to signaling and gene expression. Peptide sequence alignment reveals two highly conserved phosphorylation motifs, PRVD/GSx and SPDYxx. Alignment of these phosphopeptides with Arabidopsis proteins reveals five phosphorylation motifs. Both ethylene-enhanced and -repressible phosphopeptides present in these motifs. EIL-1, ERF110 transcription factors and Hua enhancer 4 (HEN4) are predicted to contain one of the phosphorylation motifs. The phosphorylation of the motif-containing peptides has been validated by the in vitro kinase assays coupled with MS analysis. The differential regulation of phosphorylation by ethylene is substantiated by Western dot blot analysis. Taken together, these results suggest that ethylene signals may be transduced by a phosphor-relay from receptors to transcriptional events via both ein2-dependent and -independent pathways.

Study of cis-cinnamic acid in Arabidopsis thaliana.

Trans-cinnamic acid (CA) can be isomerized to cis-CA in Arabidopsis thaliana extract under sunlight. Piperonylic acid treatment of Arabidopsis under ultraviolet (UV) light increased the level of cis-CA in these treated tissues. Similarly, cis-CA was also detected from Oryza sativa seedlings grown under sunlight. These results suggest that cis-CA may occur in planta. Application of cis-CA to seedlings of both wild type Arabidopsis and auxin-insensitive mutants, aux1 and axr2, resulted in nearly identical dose response curves in root growth, indicating that the mode of action by which cis-CA affects plant growth is different from that of auxins. According to root growth inhibition assay, cis-CA is nearly 10 times more active than trans-CA. These results suggest that cis-CA is a unique plant growth regulator but its in vivo function remains to be elucidated.

Application of 3' untranslated region (UTR) sequence-based amplified polymorphism analysis in the rapid authentication of Radix astragali.

Radix astragali (root of Astragalus membranaceus) is an important traditional Chinese medicine. It has been used as a tonic herb for thousands of years in China. The water extract of the roots has a wide range of immunopotentiating effects and has been proven to be efficacious as an adjunct cancer therapy. Authentication of the herbal plant is routinely required for general practice in the field of herbal medicine. To facilitate rapid identification of numerous varieties of Radix astragali that are circulating on the herb markets, a rapid molecular genetic method, named 3' untranslated region (3' UTR) sequence-based amplified polymorphism (UAP), has been developed. A cDNA library was first built from transcripts of an authentic A. membranaceus species. Several cDNA clones specific to A. membranaceus were identified through subtractive hybridization of the A. membranaceus cDNA library with Arabidopsis total cellular RNA. On the basis of these cDNA sequences of the 3' untranslated region (3' UTR) of selected cDNA clones, a Polymerase Chain Reaction (PCR) was performed on genomic DNAs of the dry roots of several putative A. membranaceus. PCR fragment length polymorphism was found between A. membranaceus and its relatives. By using this method, it was possible to differentiate the authentic A. membranaceus root from those putative ones obtained from herbal medicine markets. To the authors' knowledge, this is the first paper applying UAP in the authentication of traditional Chinese medicine plants.