Yaequinolones, New Insecticidal Antibiotics Produced by Penicillium sp. FKI-2140

Natural 3,4-Dihydro-2(1H)-quinolinones - part III: biological activities

Natural products have played a crucial role in drug discovery, but their development is hindered by challenges such as inadequate availability and complex synthesis methods. However, both natural and synthetic compounds that have the core structure of 3,4-dihydro-2(1H)-quinolinone, also known as 2-oxo-1,2,3,4-tetrahydroquinoline (2O-THQ), display a diverse array of effects in both central and peripheral tissues, with some showing therapeutic potential in treating various disorders. Despite the significance of this family of compounds, the current literature lacks comprehensive coverage of their biological functions. This article aims to address this gap by extensively reviewing the biological activities of 2O-THQ alkaloids from diverse organisms and exploring their potential to serve as a source of innovative bioactive natural products.

Natural 3,4-dihydro-2(1h)-quinolinones- Part II: animal, bacterial, and fungal sources

While natural products have undoubtedly played a pivotal role in drug discovery, their potential as lead compounds has been hindered by challenges such as limited accessibility and complex synthesis processes. At the core of numerous natural and synthetic compounds, each exhibiting remarkable biological traits, lies the foundational structure of 3,4-dihydro-2(1H)-quinolinone, also recognised as 2-oxo-tetrahydroquinoline (2 O-THQ). This article extensively examines the occurrence of 2 O-THQ alkaloids across diverse organisms including animals, fungi, and bacteria, exploring their capacity to serve as a source for innovative bioactive natural products. Despite the undeniable significance of these compounds, the existing body of review literature has yet to provide comprehensive coverage, underscoring the pivotal contribution of this present article in investigating their prevalence in nature.

Enantioselective Construction of C4-Quaternary Quinolinones via Copper(II)-Catalyzed Asymmetric [1,3] O-to-C Rearrangement

An efficient asymmetric [1,3] O-to-C rearrangement of quinolin-2(1H)-ones enabled by a chiral bisoxazoline/copper complex has been developed. This strategy tolerated a wide range of substrates to provide a series of 1,4-dihydroquinoline-2,3-diones containing a quaternary stereocenter. A further cyclization of the [1,3] O-to-C rearrangement products was also realized, which led to various optically active 3,4-dihydroquinolin-2-ones with broad substrate scope.

Alkaloid diversity expansion of a talent fungus Penicillium raistrichii through OSMAC-based cultivation

Introduction

Alkaloidal natural products are attractive for their broad spectrum of pharmaceutical bioactivities. In the present work, the highly productive saline soil derived fungus, Penicillium raistrichii, was subjected to the strategy of OSMAC (one strain many compounds) with changes of cultivation status. Then, the work-flow led to the expansion of the alkaloid chemical diversity and subsequently induced the accumulation of four undescribed alkaloids, named raistrimides A-D (1-4), including three β-carbolines (1-3), one 2-quinolinone (4), and one new natural product, 2-quinolinone (5), along with five known alkaloid chemicals (6-10).

Methods

A set of NMR techniques including 1H, 13C, HSQC and HMBC, along with other spectroscopic data of UV-Vis, IR and HRESIMS, were introduced to assign the plain structures of compounds 1-10. The absolute configuration of 1-3 were elucidated by means of X-ray crystallography or spectroscopic analyses on optical rotation values and experimental electronic circular dichroism (ECD) data. In addition, it was the first report on the confirmation of structures of 6, 7 and 9 by X-ray crystallography data. The micro-broth dilution method was applied to evaluate antimicrobial effect of all compounds towards Staphylococcus aureus, Escherichia coli, and Candida albicans.

Results and discussion

The results indicated compounds 1, 3 and 4 to be bioactive, which may be potential for further development of anti-antimicrobial agents. The finding in this work implied that OSMAC strategy was a powerful and effective tool for promotion of new chemical entities from P. raistrichii.

Analogues of natural products yaequinolones as potential inflammatory inhibitors: Design, synthesis and biological evaluation

Inflammation is connected with a variety of diseases and there is still a need to develop more effective and safer anti-inflammatory drugs. Herein, we synthesized, resolved, and characterized eight enantiopure isomers of yaequinolone J1 (1), yaequinolone J2 (2), 4'-desmethoxyyaequinolone J1 (3), and 4'-desmethoxyyaequinolone J2 (4). The key synthetic steps were extended and 34 racemic analogues modified at the 4-aryl, the N-position, and the pyran ring were designed and synthesized. All the synthesized compounds were evaluated for their anti-inflammatory activities in RAW 264.7 cells of which 13 compounds showed significant inhibition of nitric oxide (NO) production at a concentration of 0.1 μM, which was more potent than that of indomethacin. Furthermore, compounds (-)-3, (-)-4, 5h, and 6g reduced the production of IL-6 in LPS-stimulated RAW 264.7 cells at a concentration of 50 nM. A preliminary SAR indicated that 3'-Br (5h), 4'-NO₂ (6g) on 4-phenyl and 3-bromobenzyl (7f) on the N-position were the most effective substituents. This is the first report of the anti-inflammatory yaequinolone alkaloids and the present study provided evidence for exploiting this series of highly efficacious derivatives for new anti-inflammatory agents.

HFIP-Empowered One-Pot Synthesis of C4-Aryl-Substituted Tetrahydroquinolines with Propargylic Chlorides and Anilines

Transition-metal-free, practical one-pot synthesis of C4-aryl-substituted tetrahydroquinolines from simple anilines and readily accessible propargylic chlorides has been developed. Activation of the C-Cl bond by 1,1,1,3,3,3-hexafluoroisopropanol turned out to be the key interaction, which allowed C-N bond formation under an acidic medium. Propargylated aniline is formed as an intermediate via propargylation, and subsequential cyclization and reduction gave 4-arylated tetrahydroquinolines. To demonstrate the synthetic utility, total syntheses of aflaquinolone F and I have been accomplished.

Scalable total synthesis of (+)-aniduquinolone A and its acid-catalyzed rearrangement to aflaquinolones

The strong antibacterial, antiviral and anticancer activities demonstrated by quinolones make them promising lead structures and important synthetic targets for drug discovery. Here, we report, to the best of our knowledge, the first scalable total synthesis of antiviral (+)-aniduquinolone A, possessing a 3,4-dioxygenated 5-hydroxy-4-aryl-quinolin-2(1H)-one skeleton. This synthetic strategy explores E-stereoselective Horner-Wadsworth-Emmons (HWE) olefination as the key step to assemble isopropenyl substituted tetrahydrofuran onto the 3,4-dioxygenated 5-hydroxy-4-aryl-quinolin-2(1H)-one core, which is built by highly diastereoselective intramolecular aldol reaction. Moreover, two sets of stereoisomers of aniduquinolone A with substantially overlapping NMR data were synthesized completely and assigned unambiguously by comprehensive analysis of both their spectroscopic and X-ray diffraction data. Unexpectedly, aflaquinolones A, C, and D that feature different 2,4-dimethyl cyclohexanone moieties were transformed successfully from (+)-aniduquinolone A by treating with TFA. The methodology delineated herein can be applied broadly to the synthesis of natural alkaloids containing the core structure of 3,4-dioxygenated 5-hydroxy-4-aryl-quinolin-2(1H)-one.

Biosynthesis of cyclopropane in natural products

Covering: 2012 to 2021Cyclopropane attracts wide interests in the fields of synthetic and pharmaceutical chemistry, and chemical biology because of its unique structural and chemical properties. This structural motif is widespread in natural products, and is usually essential for biological activities. Nature has evolved diverse strategies to access this structural motif, and increasing knowledge of the enzymes forming cyclopropane (i.e., cyclopropanases) has been revealed over the last two decades. Here, the scientific literature from the last two decades relating to cyclopropane biosynthesis is summarized, and the enzymatic cyclopropanations, according to reaction mechanism, which can be grouped into two major pathways according to whether the reaction involves an exogenous C1 unit from S-adenosylmethionine (SAM) or not, is discussed. The reactions can further be classified based on the key intermediates required prior to cyclopropane formation, which can be carbocations, carbanions, or carbon radicals. Besides the general biosynthetic pathways of the cyclopropane-containing natural products, particular emphasis is placed on the mechanism and engineering of the enzymes required for forming this unique structure motif.

Divergent Total Syntheses of Yaequinolone-Related Natural Products by Late-Stage C-H Olefination

Divergent total syntheses of 10 yaequinolone-related natural products have been achieved for the first time by late-stage C-H olefination of 3,4-dioxygenated 4-aryl-5-hydroxyquinolin-2(1H)-ones, core structures of this family of natural products. A robust synthetic methodology to construct the core structures has been established, and the C-H olefination reaction has been carried out with synthetically useful yields and high levels of site-selectivity under mild reaction conditions in the presence of a Pd/S,O-ligand catalyst.

Total Synthesis and Absolute Stereochemical Assignment of the Insecticidal Metabolites Yaequinolones J1 and J2

A highly stereocontrolled total synthesis of (-)-yaequinolone J1 and (+)-yaequinolone J2 was accomplished using an Evans auxiliary to establish a syn-diol unit in an acyclic appendage to a preformed benzopyran core bearing a homoprenyl group. The first total synthesis of a complex member of this family of 3,4-dioxygenated 3,4-dihydro 4-aryl quinolin-2-(1 H)-ones also allowed the assignment of absolute stereochemistry, thereby suggesting the same for several members of this family of biogenetically related alkaloids hitherto reported with relative configurations of stereogenic carbons for some and absolute assignments relying on empirical data for others.

One-pot synthesis of 1,5-diketones from 3-acetyl-4-hydroxycoumarin and effective cyclization to unexpected 3,4-dihydropyridines

A facile synthesis of 1,5-diketones from 3-acetyl-4-hydroxycoumarin, aldehydes and cyclic-ketones via a one-pot aldol condensation and subsequent Michael addition reaction in the presence of a single catalyst of l-proline under mild reaction conditions has been developed. Novel 1,5-diketones were further cyclized to unexpected 3,4-dihydropyridines rather than generally formed pyridine analogues with ammonium acetate in acetic acid. One pot, high yields (72-92%) for novel 1,5-diketones and (70-90%) for the 3,4-dihydropyridine adducts, easy work-up and purification of products are the key advantages of this method.

Catalytic mechanism and molecular engineering of quinolone biosynthesis in dioxygenase AsqJ

The recently discovered Fe^II/α-ketoglutarate-dependent dioxygenase AsqJ from Aspergillus nidulans stereoselectively catalyzes a multistep synthesis of quinolone alkaloids, natural products with significant biomedical applications. To probe molecular mechanisms of this elusive catalytic process, we combine here multi-scale quantum and classical molecular simulations with X-ray crystallography, and in vitro biochemical activity studies. We discover that methylation of the substrate is essential for the activity of AsqJ, establishing molecular strain that fine-tunes π-stacking interactions within the active site. To rationally engineer AsqJ for modified substrates, we amplify dispersive interactions within the active site. We demonstrate that the engineered enzyme has a drastically enhanced catalytic activity for non-methylated surrogates, confirming our computational data and resolved high-resolution X-ray structures at 1.55 Å resolution. Our combined findings provide crucial mechanistic understanding of the function of AsqJ and showcase how combination of computational and experimental data enables to rationally engineer enzymes.

The catalytic activity of dioxygenase AsqJ is strictly relying on the methylation of quinolone substrates. Here, the authors apply molecular simulations, X-ray crystallography and in vitro biochemical studies to the engineering of dioxygenase AsqJ with improved catalytic activity for modified non-methylated surrogates.

Photoredox-catalyzed procedure for carbamoyl radical generation: 3,4-dihydroquinolin-2-one and quinolin-2-one synthesis

A reductive approach for carbamoyl radical generation from N-hydroxyphthalimido oxamides under photoredox catalysis is outlined. This strategy was applied to the synthesis of 3,4-dihydroquinolin-2-ones via the intermolecular addition/cyclization of carbamoyl radicals with electron deficient olefins in a mild, redox-neutral manner. Under a general set of reaction conditions, diversely substituted 3,4-dihydroquinolin-2-ones, including spirocyclic systems can be prepared. By using chlorine-substituted olefins, aromatic quinolin-2-ones can also be accessed.

Oxidative Cyclization in Natural Product Biosynthesis

Oxidative cyclizations are important transformations that occur widely during natural product biosynthesis. The transformations from acyclic precursors to cyclized products can afford morphed scaffolds, structural rigidity, and biological activities. Some of the most dramatic structural alterations in natural product biosynthesis occur through oxidative cyclization. In this Review, we examine the different strategies used by nature to create new intra(inter)molecular bonds via redox chemistry. This Review will cover both oxidation- and reduction-enabled cyclization mechanisms, with an emphasis on the former. Radical cyclizations catalyzed by P450, nonheme iron, α-KG-dependent oxygenases, and radical SAM enzymes are discussed to illustrate the use of molecular oxygen and S-adenosylmethionine to forge new bonds at unactivated sites via one-electron manifolds. Nonradical cyclizations catalyzed by flavin-dependent monooxygenases and NAD(P)H-dependent reductases are covered to show the use of two-electron manifolds in initiating cyclization reactions. The oxidative installations of epoxides and halogens into acyclic scaffolds to drive subsequent cyclizations are separately discussed as examples of "disappearing" reactive handles. Last, oxidative rearrangement of rings systems, including contractions and expansions, will be covered.

Enzyme-catalyzed cationic epoxide rearrangements in quinolone alkaloid biosynthesis

Epoxides are highly useful synthons and biosynthons for the construction of complex natural products during total synthesis and biosynthesis, respectively. Among enzyme-catalyzed epoxide transformations, a reaction that is notably missing, in regard to the synthetic toolbox, is cationic rearrangement that takes place under strong acid. This is a challenging transformation for enzyme catalysis, as stabilization of the carbocation intermediate upon epoxide cleavage is required. Here, we discovered two Brønsted acid enzymes that can catalyze two unprecedented epoxide transformations in biology. PenF from the penigequinolone pathway catalyzes a cationic epoxide rearrangement under physiological conditions to generate a quaternary carbon center, while AsqO from the aspoquinolone pathway catalyzes a 3-exo-tet cyclization to forge a cyclopropane-tetrahydrofuran ring system. The discovery of these new epoxide-modifying enzymes further highlights the versatility of epoxides in complexity generation during natural product biosynthesis.

A convenient approach to an advanced intermediate toward the naturally occurring, bioactive 6-substituted 5-hydroxy-4-aryl-1H-quinolin-2-ones

An advanced intermediate for the total synthesis of the 5-hydroxy-4-aryl-3,4-dihydro-1H-quinolin-2-one natural products and a 3,4-bisdesoxy analog of peniprequinolone were synthesized.

The 3,4-dioxygenated 5-hydroxy-4-aryl-quinolin-2(1H)-one alkaloids. Results of 20 years of research, uncovering a new family of natural products

The title alkaloids are discussed. Emphasis is placed on their isolation, source microorganisms and structure, as well as relevant biological activities and synthetic progress.

A Novel One-step Synthesis of Quinoline-2(1H)-thiones and Selones by Treating 3-Aryl-3-(2-aminophenyl)-1-propyn-3-ols with a Base and Elemental Sulfur or Selenium

A one-step conversion of 3-aryl-3-(2-aminophenyl)-1-propyn-3-ols into quinoline-2(1 H)-thiones and quinoline-2(1 H)-selones was achieved only by treating the substrates with n-butyllithium and either elemental sulfur or selenium, respectively. The reactions were assumed to proceed through an intramolecular nucleophilic attack of the neighboring amino group to the plausible in situ generated reactive species related to chalcogenoketenes. The subsequent mCPBA oxidation of quinoline-2(1 H)-selones afforded quinolin-2(1 H)-ones in high yields.

Tandem prenyltransferases catalyze isoprenoid elongation and complexity generation in biosynthesis of quinolone alkaloids

Modification of natural products with prenyl groups and the ensuing oxidative transformations are important for introducing structural complexity and biological activities. Penigequinolones (1) are potent insecticidal alkaloids that contain a highly modified 10-carbon prenyl group. Here we reveal an iterative prenylation mechanism for installing the 10-carbon unit using two aromatic prenyltransferases (PenI and PenG) present in the gene cluster of 1 from Penicillium thymicola. The initial Friedel-Crafts alkylation is catalyzed by PenI to yield dimethylallyl quinolone 6. The five-carbon side chain is then dehydrogenated by a flavin-dependent monooxygenase to give aryl diene 9, which serves as the electron-rich substrate for a second alkylation with dimethylallyl diphosphate to yield stryrenyl product 10. The completed, oxidized 10-carbon prenyl group then undergoes further structural morphing to yield yaequinolone C (12), the immediate precursor of 1. Our studies have therefore uncovered an unprecedented prenyl chain extension mechanism in natural product biosynthesis.

Anti-respiratory syncytial virus prenylated dihydroquinolone derivatives from the gorgonian-derived fungus Aspergillus sp. XS-20090B15

Two new prenylated dihydroquinolone derivatives, 22-O-(N-Me-l-valyl)aflaquinolone B (1) and 22-O-(N-Me-l-valyl)-21-epi-aflaquinolone B (2), and two known analogues, aflaquinolones A (3) and D (or a diastereomer of D, 4), were isolated from the mycelia of a gorgonian-derived Aspergillus sp. fungus. The structures of the new compounds were elucidated by spectroscopic methods, ECD spectra, Marfey's method, and chemical conversion. Compounds 1 and 2 display an unusual esterification of N-Me-l-Val to the side-chain prenyl group. Compound 2 exhibited outstanding anti-RSV activity with an IC50 value of 42 nM, approximately 500-fold stronger than that of the positive control ribavirin (IC50 = 20 μM), and showed a comparatively higher therapeutic ratio (TC50/IC50 = 520).

Asymmetric synthesis of tetrahydroquinolines with quaternary stereocenters through the Povarov reaction

The asymmetric Povarov reaction with α-alkyl styrenes as dienophiles was catalyzed by an N,N'-dioxide L4-Sc(OTf)(3) complex. Enantiopure tetrahydroquinoline derivatives with a quaternary stereocenter at the C4 position were synthesized for the first time. A wide variety of α-alkyl styrenes and N-aryl aldimines were tolerated in the reaction, to give excellent diastereo- (up to 99:1 d.r.) and enantioselectivities (92 to >99% ee). In addition, the reaction could be performed on the gram scale without any loss of yield, diastereoselectivity, or enantioselectivity. An intermolecular hydrogen-shift reaction was found to be a side reaction, which offered a method to synthesize the corresponding quinoline derivatives with chiral quaternary sterocenters.

Yaequinolones, New Insecticidal Antibiotics Produced by Penicillium sp. FKI-2140

The structure and relative stereochemistry of yaequinolones, fungal insecticidal antibiotics, were elucidated by spectroscopic studies, including NMR spectral analyses. Yaequinolones possess a p-methoxyphenylquinolinone skeleton modified with different isoprenyl-derived side chains.