Cyclisation reactions of 2-substituted benzoylphosphonates with trialkyl phosphites via nucleophilic attack on a carbonyl-containing ortho substituent

Flavonol dioxygenase chemistry mediated by a synthetic nickel superoxide

Nature exploits transition metal centers to enhance and tune the oxidizing power of natural oxidants such as O₂ and H₂O₂. The design and interrogation of synthetic metallocomplexes with similar reactivity to metalloproteins provides one strategy for gaining insight into the mechanistic underpinnings of oxygen-activating enzymes such as oxidases, oxygenases, and dioxygenases like Ni-quercetinase (Ni-QueD). Ni-QueD catalyzes the oxidative ring opening of the polyphenol quercetin, a natural product with antioxidant properties. Herein, we report the synthesis and characterization of Ni(13-DOB), a Ni(II) species complexed by an N4-macrocycle that has been characterized by single crystal X-ray crystallography. Ni(13-DOB) forms a Ni-superoxide intermediate (Ni(13-DOB)O₂^•-) upon treatment with H₂O₂ and Et₃N, as verified by resonance Raman spectroscopy. We demonstrate through UV/vis and LCMS that Ni(13-DOB)O₂^•- is capable of the 1-electron oxidation of flavonols, including both 3-hydroxyflavone (3-HF, the simplest flavonol) and quercetin itself. Incorporation of two O-atoms into the flavonol radical via superoxide from Ni(13-DOB)O₂^•- precedes oxidative cleavage of the flavonol scaffold in each case, consistent with quercetinase ring cleavage by Ni-QueD in Streptomyces sp. FLA. Conversion of 3-HF into 2-hydroxybenzoylbenzoic acid was accomplished with catalytic turnover of Ni(13-DOB) at ambient temperature, as confirmed by HPLC timecourses and GCMS analysis of isotopic labeling studies. The Ni(13-DOB)-mediated oxidative cleavage of quercetin to the corresponding biomimetic phenolic ester was also verified through ¹⁸O-isotopic labeling studies. Through the HPLC characterization of both on- and off-pathway products of flavonol dioxygenation by Ni(13-DOB)O₂^•-, the stringent reaction pathway control provided by enzyme active sites is highlighted.

Dearomative 1,6-addition of P(O)–H to in situ formed p-QM-like ion pairs from 2-benzofuryl-ols to C3-phosphinoyl hydrobenzofurans

We report a dearomative C3-phosphorylation and a tandem C3-phosphorylation/aromatization of 2-benzofuryl-ols with P(O)–H species to afford C3-phosphinoyl hydrobenzofurans and benzofurans, respectively.

Benzoylsalicylic acid derivatives as defense activators in tobacco and Arabidopsis

Systemic acquired resistance (SAR) is a long lasting inducible whole plant immunity often induced by either pathogens or chemical elicitors. Salicylic acid (SA) is a known SAR signal against a broad spectrum of pathogens in plants. In a recent study, we have reported that benzoylsalicylic acid (BzSA) is a SAR inducer in tobacco and Arabidopsis plants. Here, we have synthesized BzSA derivatives using SA and benzoyl chlorides of various moieties as substrates. The chemical structures of BzSA derivatives were elucidated using Infrared spectroscopy (IR), Nuclear magnetic spectroscopy (NMR) and High-resolution mass spectrometer (HRMS) analysis. The bioefficacy of BzSA derivatives in inducing defense response against tobacco mosaic virus (TMV) was investigated in tobacco and SA abolished transgenic NahG Arabidopsis plants. Interestingly, pre-treatment of local leaves of tobacco with BzSA derivatives enhanced the expression of SAR genes such as NPR1 [Non-expressor of pathogenesis-related (PR) genes 1], PR and other defense marker genes (HSR203, SIPK, WIPK) in systemic leaves. Pre-treatment of BzSA derivatives reduced the spread of TMV infection to uninfected areas by restricting lesion number and diameter both in local and systemic leaves of tobacco in a dose-dependent manner. Furthermore, pre-treatment of BzSA derivatives in local leaves of SA deficient Arabidopsis NahG plants induced SAR through AtPR1 and AtPR5 gene expression and reduced leaf necrosis and curling symptoms in systemic leaves as compared to BzSA. These results suggest that BzSA derivatives are potent SAR inducers against TMV in tobacco and Arabidopsis.