Lewis Acid Activated Aza-Diels-Alder Reaction ofN-(3-Pyridyl)aldimines: An Experimental and Computational Study

Crystal structure and Hirshfeld surface analysis of 1-[6-bromo-2-(4-fluoro-phen-yl)-1,2,3,4-tetra-hydroquinolin-4-yl]pyrrolidin-2-one

In the title compound, C19H18BrFN2O, the pyrrolidine ring adopts an envelope conformation. In the crystal, mol-ecules are linked by inter-molecular N-H⋯O, C-H⋯O, C-H⋯F and C-H⋯Br hydrogen bonds, forming a three-dimensional network. In addition, C-H⋯π inter-actions connect mol-ecules into ribbons along the b-axis direction, consolidating the mol-ecular packing. The inter-molecular inter-actions in the crystal structure were qu-anti-fied and analysed using Hirshfeld surface analysis.

Aza-Povarov Reaction. A Method for the Synthesis of Fused Tetracyclic Chromeno[4,3-d]pyrido[1,2-a]pyrimidines

A cornerstone in drug discovery is the development of strategies to provide privileged small molecules with specific structural and stereochemical complexity, allowing access to new potential therapeutic entities. In this work, a new strategy based on the [4 + 2] Povarov reaction involving 1,3-diazadiene was developed. This approach is applied for a straightforward procedure in the preparation of chromeno[4,3-d]pyrido[1,2-a]pyrimidine derivatives, with accessible substrates, 2-aminopyridine and unsaturated aldehydes, and excellent atom economy to obtain four fused ring heterocycles, in a regio- and diastereoselective way.

The Intramolecular Povarov Tool in the Construction of Fused Nitrogen-Containing Heterocycles

Nitrogen heterocycles are part of the structure of natural products and agents with important biological activity, such as antiviral, antibiotic, and antitumor drugs. For this reason, heterocyclic compounds are one of today's most desirable synthetic targets and the Povarov reaction is a powerful synthetic tool for the construction of highly functionalized heterocyclic systems. This process involves an aromatic amine, a carbonyl compound, and an olefin or acetylene to give rise to the formation of a nitrogen-containing heterocycle. This review illustrates advances in the synthetic aspects of the intramolecular Povarov reaction for the construction of intricate nitrogen-containing polyheterocyclic compounds. This original review presents research done in this field, with references to important works by internationally relevant research groups on this current topic, covering the literature from 1992 to 2022. The intramolecular Povarov reactions are described here according to the key processes involved, using different combinations of aromatic or heteroaromatic amines, and aliphatic, aromatic, or heteroaromatic aldehydes. Some catalytic reactions promoted by transition metals are detailed, as well as the oxidative Povarov reaction and some asymmetric intramolecular Povarov processes.

Chalcogen-Bond Catalysis: Telluronium-Catalyzed [4+2]-Cyclocondensation of (in situ Generated) Aryl Imines with Alkenes

In the chalcogen series, tellurium species exhibit the strongest chalcogen bonding (ChB) interaction with electron-rich atom. This property explains the renewed interested toward tellurium-based derivatives and their use in different applications, such as organocatalysis. In this context, the catalytic activity of telluronium salts in the Povarov reaction is presented herein. Different dienophiles, as well as imines of variable electronic nature, efficiently react in the presence of catalytic amount of either diarylmethyltelluronium or triaryltelluronium salts. Both catalysts could also readily perform the three-component Povarov reaction starting from aldehyde, aniline and dihydrofuran. The reactivity of telluroniums towards imines and aldehydes was confirmed in the solid state by the ability of Te atom to interact through ChB with the oxygen carbonyl of acetone, and in solution with significant shift variations of the imine proton and of the tellurium atom in ¹ H and ¹²⁵ Te NMR spectroscopy. For the most active telluronium catalysts bearing CF₃ groups, association constants (K) with N-phenyl phenylmethanimine in the range 22-38 M^-1 were measured in dichloromethane.

Unravelling the Mechanism and Governing Factors in Lewis Acid and Non-Covalent Diels-Alder Catalysis: Different Perspectives

In the current literature, many non-covalent interaction (NCI) donors have been proposed that can potentially catalyze Diels-Alder (DA) reactions. In this study, a detailed analysis of the governing factors in Lewis acid and non-covalent catalysis of three types of DA reactions was carried out, for which we selected a set of hydrogen-, halogen-, chalcogen-, and pnictogen-bond donors. We found that the more stable the NCI donor-dienophile complex, the larger the reduction in DA activation energy. We also showed that for active catalysts, a significant part of the stabilization was caused by orbital interactions, though electrostatic interactions dominated. Traditionally, DA catalysis was attributed to improved orbital interactions between the diene and dienophile. Recently, Vermeeren and co-workers applied the activation strain model (ASM) of reactivity, combined with the Ziegler-Rauk-type energy decomposition analysis (EDA), to catalyzed DA reactions in which energy contributions for the uncatalyzed and catalyzed reaction were compared at a consistent geometry. They concluded that reduced Pauli repulsion energy, and not enhanced orbital interaction energy, was responsible for the catalysis. However, when the degree of asynchronicity of the reaction is altered to a large extent, as is the case for our studied hetero-DA reactions, the ASM should be employed with caution. We therefore proposed an alternative and complementary approach, in which EDA values for the catalyzed transition-state geometry, with the catalyst present or deleted, can be compared one to one, directly measuring the effect of the catalyst on the physical factors governing the DA catalysis. We discovered that enhanced orbital interactions are often the main driver for catalysis and that Pauli repulsion plays a varying role.

Multicomponent Reactions for the Synthesis of Active Pharmaceutical Ingredients

Multicomponent reactions 9i.e., those that engage three or more starting materials to form a product that contains significant fragments of all of them), have been widely employed in the construction of compound libraries, especially in the context of diversity-oriented synthesis. While relatively less exploited, their use in target-oriented synthesis offers significant advantages in terms of synthetic efficiency. This review provides a critical summary of the use of multicomponent reactions for the preparation of active pharmaceutical principles.

Enantioselective catalytic Povarov reactions

Catalytic asymmetric Povarov protocols have undergone an explosive growth, especially in the last ten years, since the first example was published in 1996. The use of chiral Lewis and Brønsted acids and dual strategies based on their combination with catalysts acting by hydrogen bond formation, as well as covalent aminocatalysis, are reviewed. More recent variations such as the nitroso Povarov reaction and interrupted Povarov reactions as a route to chiral scaffolds other than tetrahydroquinolines are also discussed.

Synthesis of hybrid phosphorated indenoquinolines and biological evaluation as topoisomerase I inhibitors and antiproliferative agents

This work describes the first synthesis of diethyl 6,6a,7,11b-tetrahydro-5H-indeno[2,1-c]quinolinylphosphonates 5, diethyl 7H-indeno[2,1-c]quinolinylphosphonates 6 and diethyl 7-oxo-7H-indeno[2,1-c]quinolinylphosphonates 7, which were prepared in good to high overall yields. The synthetic route involves a multicomponent reaction of 2-phosphonateaniline, aldehydes and indene as olefin and allows the selective generation of three stereogenic centres in a short, efficient and reliable manner. The selective dehydrogenation of 1,2,3,4-tetrahydroindenoquinolines leads to the formation of corresponding indenoquinolines, and subsequent oxidation of methylene group of the indenoquinolines allows the access to indenoquinolinones.

A Dual-Sensor-Based Screening System for In Vitro Selection of TDP1 Inhibitors

DNA sensors can be used as robust tools for high-throughput drug screening of small molecules with the potential to inhibit specific enzymes. As enzymes work in complex biological pathways, it is important to screen for both desired and undesired inhibitory effects. We here report a screening system utilizing specific sensors for tyrosyl-DNA phosphodiesterase 1 (TDP1) and topoisomerase 1 (TOP1) activity to screen in vitro for drugs inhibiting TDP1 without affecting TOP1. As the main function of TDP1 is repair of TOP1 cleavage-induced DNA damage, inhibition of TOP1 cleavage could thus reduce the biological effect of the TDP1 drugs. We identified three new drug candidates of the 1,5-naphthyridine and 1,2,3,4-tetrahydroquinolinylphosphine sulfide families. All three TDP1 inhibitors had no effect on TOP1 activity and acted synergistically with the TOP1 poison SN-38 to increase the amount of TOP1 cleavage-induced DNA damage. Further, they promoted cell death even with low dose SN-38, thereby establishing two new classes of TDP1 inhibitors with clinical potential. Thus, we here report a dual-sensor screening approach for in vitro selection of TDP1 drugs and three new TDP1 drug candidates that act synergistically with TOP1 poisons.

Fused chromeno and quinolino[1,8]naphthyridines: Synthesis and biological evaluation as topoisomerase I inhibitors and antiproliferative agents

The synthesis of 1,8-naphthyridine derivatives fused with other heterocycles, such as chromenes and quinolines, as well as their behaviour as topoisomerase I inhibitors is studied. The preparation is carried out through a direct and simple process as an intramolecular [4 + 2] cycloaddition reaction between functionalized aldimines, obtained by the condensation of 2-aminopyridine and unsaturated aldehydes, and olefins. In particular, while no clear inhibitory activity is observed for chromeno[4,3-b][1,8]naphthyridine fused heterocycles, a very different result is observed for quinolino[4,3-b][1,8]naphthyridine derivatives. Experimental assays indicated that quinolino[4,3-b][1,8]naphthyridines inhibited the topoisomerase I enzymatic reaction behaving like a poison, as occurs with the natural TopI inhibitor, camptothecin. Furthermore, the cytotoxic effect on cell lines derived from human lung adenocarcinoma (A549), human ovarian carcinoma (SKOV3), and on non-cancerous lung fibroblasts cell line (MRC5) was also screened.

Visible-Light-Induced Synthesis of 1,2,3,4-Tetrahydroquinolines through Formal [4+2] Cycloaddition of Acyclic α,β-Unsaturated Amides and Imides with N,N-Dialkylanilines

1,2,3,4-Tetrahydroquinolines should be applicable to the development of new pharmaceutical agents. A facile synthesis of 1,2,3,4-tetrahydroquinolines that is achieved by a photoinduced formal [4+2] cycloaddition reaction of acyclic α,β-unsaturated amides and imides with N,N-dialkylanilines under visible-light irradiation, in which a new Ir^III complex photosensitizer, a thiourea, and an oxidant act cooperatively in promoting the reaction, is reported. The photoreaction enables the synthesis of a wide variety of 1,2,3,4-tetrahydroquinolines, while controlling the trans/cis diastereoselectivity (>99:1) and constructing contiguous stereogenic centers. A chemoselective cleavage of an acyclic imide auxiliary is demonstrated.

Fused 1,5-Naphthyridines: Synthetic Tools and Applications

Heterocyclic nitrogen compounds, including fused 1,5-naphthyridines, have versatile applications in the fields of synthetic organic chemistry and play an important role in the field of medicinal chemistry, as many of them have a wide range of biological activities. In this review, a wide range of synthetic protocols for the construction of this scaffold are presented. For example, Friedländer, Skraup, Semmlere-Wolff, and hetero-Diels-Alder, among others, are well known classical synthetic protocols used for the construction of the main 1,5-naphthyridine scaffold. These syntheses are classified according to the nature of the cycle fused to the 1,5-naphthyridine ring: carbocycles, nitrogen heterocycles, oxygen heterocycles, and sulphur heterocycles. In addition, taking into account the aforementioned versatility of these heterocycles, their reactivity is presented as well as their use as a ligand for metal complexes formation. Finally, those fused 1,5-naphthyridines that present biological activity and optical applications, among others, are indicated.

Synthetic Strategies, Reactivity and Applications of 1,5-Naphthyridines

This review covers the synthesis and reactivity of 1,5-naphthyridine derivatives published in the last 18 years. These heterocycles present a significant importance in the field of medicinal chemistry because many of them exhibit a great variety of biological activities. First, the published strategies related to the synthesis of 1,5-naphthyridines are presented followed by the reactivity of these compounds with electrophilic or nucleophilic reagents, in oxidations, reductions, cross-coupling reactions, modification of side chains or formation of metal complexes. Finally, some properties and applications of these heterocycles studied during this period are examined.

Organocatalytic Asymmetric Three-Component Povarov Reactions of Anilines and Aldehydes

We report the first highly enantio- and diastereoselective three-component Povarov reaction between anilines and aldehydes catalyzed by a chiral amine catalyst. A wide variety of substituted tetrahydroquinolines were obtained with moderate to good yields and excellent enantioselectivity and diastereoselectivity (up to 99% ee and >95:5 dr) under the reaction conditions. Furthermore, the reaction intermediates could be efficiently converted to other valuable building blocks.

Progress in the Chemistry of Tetrahydroquinolines

Tetrahydroquinoline is one of the most important simple nitrogen heterocycles, being widespread in nature and present in a broad variety of pharmacologically active compounds. This Review summarizes the progress achieved in the chemistry of tetrahydroquinolines, with emphasis on their synthesis, during the period from mid-2010 to early 2018.

Recent advances of the Povarov reaction in medicinal chemistry

The Povarov multicomponent reaction consists on the condensation of an aniline, an aldehyde, and an activated olefin to generate a tetrahydroquinoline adduct with 3 diversity points. Hereby, we report the main features of this transformation and its uses in medicinal chemistry. Relevant examples of the impact of Povarov adducts in different therapeutic areas are provided.

Study of the Hetero-[4+2]-Cycloaddition Reaction of Aldimines and Alkynes. Synthesis of 1,5-Naphthyridine and Isoindolone Derivatives

Both experimental and computational studies for the cycloaddition reaction between N-(3-pyridyl)aldimines and acetylenes where 1,5-naphthyridines are obtained are reported. The reaction of benzaldimine with a methoxycarbonyl group in position 2 with phenyl acetylene, styrene, and indene afforded polycyclic isoindolone derivatives. The mechanism of reaction of N-(3-pyridyl)aldimines with olefins can be explained by an asynchronous [4+2] cycloaddition; in the case of acetylenes, the obtained results suggest a stepwise mechanism through a 3-azatriene.

Advantages of an optical nanosensor system for the mechanistic analysis of a novel topoisomerase I targeting drug: a case study

The continuous need for the development of new small molecule anti-cancer drugs calls for easily accessible sensor systems for measuring the effect of vast numbers of new drugs on their potential cellular targets. Here we demonstrate the use of an optical DNA biosensor to unravel the inhibitory mechanism of a member of a new family of small molecule human topoisomerase I inhibitors, the so-called indeno-1,5-naphthyridines. By analysing human topoisomerase I catalysis on the biosensor in the absence or presence of added drug complemented with a few traditional assays, we demonstrate that the investigated member of the indeno-1,5-naphthyridine family inhibited human topoisomerase I activity by blocking enzyme-DNA dissociation. To our knowledge, this represents the first characterized example of a small molecule drug that inhibits a post-ligation step of catalysis. The elucidation of a completely new and rather surprising drug mechanism-of-action using an optical real time sensor highlights the value of this assay system in the search for new topoisomerase I targeting small molecule drugs.

Understanding the stereoselectivity in Brønsted acid catalysed Povarov reactions generating cis/trans CF3-substituted tetrahydroquinolines: a DFT study

Stereochemistry in the Brønsted acid catalysed Povarov reactions.

Theoretical study of the regio- and stereoselectivity of the intramolecular Povarov reactions yielding 5H-chromeno[2,3-c] acridine derivatives

The intramolecular Povarov (IMP) reactions involved in the synthesis of 5H-chromeno[2,3-c] acridine derivatives [Tetrahedron Lett., 2010, 51, 3071–3074] have been studied using density functional theory (DFT) methods.

Hetero-cycloreversions mediated by photoinduced electron transfer

Discovered more than eight decades ago, the Diels-Alder (DA) cycloaddition (CA) remains one of the most versatile tools in synthetic organic chemistry. Hetero-DA processes are powerful methods for the synthesis of densely functionalized six-membered heterocycles, ubiquitous substructures found in natural products and bioactive compounds. These reactions frequently employ azadienes and oxadienes, but only a few groups have reported DA processes with thiadienes. The electron transfer (ET) version of the DA reaction, though less investigated, has emerged as a subject of increasing interest. In the last two decades, researchers have paid closer attention to radical ionic hetero-cycloreversions, mainly in connection with their possible involvement in the repair of pyrimidine(6-4)pyrimidone photolesions in DNA by photolyases. In biological systems, these reactions likely occur through a reductive photosensitization mechanism. In addition, photooxidation can lead to cycloreversion (CR) reactions, and researchers can exploit this strategy for DNA repair therapies. In this Account, we discuss electron-transfer (ET) mediated hetero-CR reactions. We focus on the oxidative and reductive ET splitting of oxetanes, azetidines, and thietanes. Photoinduced electron transfer facilitates the splitting of a variety of four-membered heterocycles. In this context, researchers have commonly examined oxetanes, both experimentally and theoretically. Although a few studies have reported the cycloreversion of azetidines and thietanes carried out under electron transfer conditions, the number of examples remains limited. In general, the cleavage of the ionized four-membered rings appears to occur via a nonconcerted two-step mechanism. The trapping of the intermediate 1,4-radical ions and transient absorption spectroscopy data support this hypothesis, and it explains the observed loss of stereochemistry in the products. In the initial step, either C-C or C-X bond breaking may occur, and the preferred route depends on the substitution pattern of the ring, the type of heteroatom, and various experimental conditions. To better accommodate spin and charge, C-X cleavage happens more frequently, especially in the radical anionic version of the reaction. The addition or withdrawal of a single electron provides a new complementary synthetic strategy to activate hetero-cycloreversions. Despite its potential, this strategy remains largely unexplored. However, it offers a useful method to achieve C═X/olefin metathesis or, upon ring expansion, to construct six-membered heterocyclic rings.