Cyclic Imines Research Articles

Synthesis of pyrrole-fused dibenzoxazepine/dibenzothiazepine/triazolobenzodiazepine derivatives via isocyanide-based multicomponent reactions

An efficient and facile synthesis of pyrrole-fused dibenzoxazepine/dibenzothiazepine/triazolobenzodiazepine derivatives was developed through the isocyanide-based multicomponent reaction of isocyanides, gem-diactivated olefins, and cyclic imines such as dibenzoxazepine, dibenzothiazepine, and triazolobenzodiazepine under solvent- and catalyst-free conditions. Purposefully, this approach produced various bioactive scaffolds using environmentally friendly, mild, and simple conditions. Due to their bioactive moieties, these compounds with exclusive fluorescence properties may attract great attention in biomedical applications, clinical diagnostics, and conjugate materials.

Asymmetric Construction of Chiral 2‐Azetines and Axially Chiral Tetrasubstituted Allenes via Phosphine Catalysis

Chiral 2‐azetines and allenes are highly valuable structural units in natural products and useful chemicals. However, enantioselective synthesis of both 2‐azetines and allenes has been extremely challenging. Herein, we present asymmetric construction of chiral 2‐azetines (70–98% yields and up to 96% ee) through chiral phosphine‐catalyzed [2+2] annulation of yne‐enones with sulfamate‐derived cyclic imines. These 2‐azetines were easily transformed into chiral allenes upon treatment with Et3SiH, BF3•Et2O and water at rt for 2 minutes. Based on the above transformations, a concise one‐pot synthetic procedure combining [2+2] annulation of yne‐enones and sulfamate‐derived cyclic imines under phosphine catalysis and sequential reduction/isomerization/ring‐opening reaction through Et3SiH, BF3•Et2O and water was thus set up, providing axially chiral tetrasubstituted allenes in satisfactory yields and enantioselectivities (56–90% yields and up to 91% ee).

Kinetico‐Mechanistic Studies of Cu(II)‐Mediated Cyclization of Imines via C−H Bond Activations

AbstractHerein, we focus on the kinetico‐mechanistic studies of a carboxylate and Cu(II)‐assisted synthesis of imidazo[1,5‐a]pyridines encompassing C(sp3)−H amination and cyclization of imines coupled with C(sp2)−H cyanation using time‐resolved UV‐Vis reaction monitoring and ex‐situ analyses. Thus, kinetic studies have been carried out, providing a proof of the elementary steps involved, allowing for batch analysis of the compounds present in solution via MS. The experimental data obtained are consistent with the formation of a cyclometalated complex (k1) involving a C(sp3)−H bond cleavage followed by a reductive elimination/proton abstraction event (k2), both showing high enthalpy‐demanding transition states. The two processes show a well‐defined Eyring behavior with values of ΔH≠ and ΔS≠ within the expected range. Further insights on the intermediate fast C(sp2)–cyanation are also presented together with control reactions.

Asymmetric Construction of Chiral 2-Azetines and Axially Chiral Tetrasubstituted Allenes via Phosphine Catalysis.

Chiral 2-azetines and allenes are highly valuable structural units in natural products and useful chemicals. However, enantioselective synthesis of both 2-azetines and allenes has been extremely challenging. Herein, we present asymmetric construction of chiral 2-azetines (70-98% yields and up to 96% ee) through chiral phosphine-catalyzed [2+2] annulation of yne-enones with sulfamate-derived cyclic imines. These 2-azetines were easily transformed into chiral allenes upon treatment with Et3SiH, BF3•Et2O and water at rt for 2 minutes. Based on the above transformations, a concise one-pot synthetic procedure combining [2+2] annulation of yne-enones and sulfamate-derived cyclic imines under phosphine catalysis and sequential reduction/isomerization/ring-opening reaction through Et3SiH, BF3•Et2O and water was thus set up, providing axially chiral tetrasubstituted allenes in satisfactory yields and enantioselectivities (56-90% yields and up to 91% ee).

Computation‐Guided Scope Exploration of a Conjugate Addition/Truce‐Smiles Cascade Reaction for Scaffold Diversification

Based on the successful application of a nitroaryl transfer cascade reaction in the total synthesis of various monoterpene indole alkaloids proceeding via fused bicyclic intermediate, we investigated analogous cascade reactions involving spiro, bridged, and alternatively fused bicyclic intermediates. Unfortunately, none of these was found to afford the desired nitroaryl transfer product. DFT studies of the original cascade process revealed that the Truce‐Smiles rearrangement is the rate‐determining step. In addition, we found that the subsequent SO2 extrusion is accompanied by retro‐Mannich ring opening, resulting in a stabilized enolate that only recyclizes after activation of the cyclic imine moiety. Computation of the reaction profiles of the proposed alternative cascade reactions showed that the barrier for the Truce‐Smiles rearrangement is unreasonably high for the bridged and alternatively fused systems, but only moderately higher for the five‐membered spiro system. Reasoning that even more electron‐deficient arenesulfonamides should have a lower barrier for the Truce‐Smiles rearrangement, we synthesized the corresponding 2,4‐dinitrobenzenesulfonamide precursor and found that it indeed smoothly undergoes the nitroaryl transfer cascade at room temperature. In this case, however, the cascade reaction produces a cyclic imine product, as the intermediate 2,4‐dinitrophenyl‐substituted enolate is insufficiently nucleophilic to undergo the Mannich cyclization.

AgOTf-catalyzed three-component coupling for the synthesis of C-alkynyl iminosugars

A3 coupling of tosylribose, primary amine and alkyne has been accomplished for the synthesis of C-alkynyl iminosugars using a catalytic amount of AgOTf under extremely mild conditions. This method is compatible with acid sensitive acetonide and cyclohexylidene and also TBS and PMB ethers The reaction proceeds through a cyclic iminium ion, which is formed from tosylribose and a primary amine followed by the attack of alkyne resulting in the formation of C-alkynyl iminosugar, which is a key intermediate of many biologically important natural products.

Catalytic Asymmetric Cycloadditions of Cyclic Sulfamidate Imines: Straightforward Access to Chiral N‐Heterocycles

AbstractChiral N‐heterocyclic compounds are key structures in natural compounds and pharmaceuticals, and they serve as essential building blocks of functional materials. Catalytic asymmetric cycloaddition reactions represent one of the most efficient synthetic strategies for constructing optically active heterocycles. Cyclic sulfamidate imines have recently come to be extensively studied and widely utilized in both organocatalytic and transition metal‐catalyzed asymmetric cycloadditions, where they have been shown to provide various N‐fused‐heterocycles and spiro‐cycles exhibiting high efficiencies with excellent stereoselectivities. This review highlights recent advancements in catalytic asymmetric cycloadditions of cyclic sulfamidate imines for the stereoselective synthesis of biologically active sulfamidate‐containing heterocycles since 2012 while focusing on the diverse reactivities of cyclic sulfamidate imines and the mechanisms of chiral induction in catalysis.

Enhancing the Detection of Cyclic Imines in Marine Mollusks Using Advanced LC/MRM-MS

Enhancing the Detection of Cyclic Imines in Marine Mollusks Using Advanced LC/MRM-MS

Visible light-driven decarboxylative alkylation of azomethine imines with carboxylic acids

The alkylation of cyclic five-membered azomethine imines with carboxylic acids proceeds under irradiation with 400 nm light using a dual catalytic system consisting of 9-arylacridine and tetrabutylammonium decatungstate. Azomethine imines containing aromatic, heterocyclic and aliphatic groups are suitable for the process providing pyrazolidin-3-ones in 42–98% yields.

Toxicological and Pharmacological Activities, and Potential Medical Applications, of Marine Algal Toxins.

Marine algal toxins have garnered significant attention in the research community for their unique biochemical properties and potential medical applications. These bioactive compounds, produced by microalgae, pose significant risks due to their high toxicity, yet offer promising therapeutic benefits. Despite extensive research identifying over 300 marine algal toxins, including azaspiracids, brevetoxins, cyclic imines, and yessotoxins, gaps remain in the understanding of their pharmacological potential. In this paper, we critically review the classification, bioactive components, toxicology, pharmacological activities, and mechanisms of these toxins, with a particular focus on their clinical applications. Our motivation stems from the increasing interest in marine algal toxins as candidates for drug development, driven by their high specificity and affinity for various biological receptors. We aim to bridge the gap between toxicological research and therapeutic application, offering insights into the advantages and limitations of these compounds in comparison to other bioactive substances. This review not only enhances the understanding of marine algal toxins' complexity and diversity, but also highlights their extensive application potential in medicine and bioscience, providing a foundation for future research and development in this field.

Theoretical investigation of the nickel-catalyzed Mizoroki-Heck/Amination reaction towards the synthesis of biologically active Indolo[2,1-a] isoquinolines derivatives

Density functional theory (DFT) was employed to computationally investigate the nickel-catalyzed Mizoroki-Heck/amination cascade reaction of o-dihaloarenes with cyclic imines. The computational results reveal that this reaction proceeds via the generation of a sterically bulky N-heterocylcic carbene ligand-coordinated Ni(0) active catalyst, which triggers the Ni(0)-Ni(II)-Ni(0) catalytic cycle. Furthermore, during the cyclic imine insertion, the formation of hydrogen bond (NH…Cl) plays a pivotal role in facilitating the reaction. Additionally, theoretical analysis elucidates the origin of high regioselectivity and provides valuable insights for experiments and theoretical research in the future.

Synthesis of Angular Polycyclic Aromatic Molecules Via Pd‐Catalyzed Decarboxylative Cycloaddition of 2‐Alkylidenetrimethylene Carbonates with Quinazoline‐Derived Azomethine Imines

AbstractA formal [4+3] cycloaddition of 2‐alkylidenetrimethylene carbonates with quinazoline‐derived azomethine imines is described. Under Pd catalysis, the reaction works efficiently under room temperature to afford angular quinazoline‐fused seven‐membered N,N,O‐heterocycles. A variety of cyclic carbonates and azomethine imines were well tolerated under mild reaction conditions. This work demonstrates the synthetic utility of decarboxylative cycloadditions of 2‐alkylidenetrimethylene carbonates for the synthesis of angular polycyclic aromatic molecules.

1,3-Hydro-di/monofluoromethylation of N,N'-Cyclic Azomethine Imines with HCF2SO2Na/H2CFSO2Na via Photocatalytic Radical Addition.

The radical 1,3-hydro-di/monofluoromethylation of N,N'-cyclic azomethine imines with HCF2SO2Na/H2CFSO2Na via photoredox catalysis is described. This reaction exhibits broad functional group compatibility, providing the desired products in good yields. However, CF3SO2Na failed to produce the trifluoromethyl product. DFT calculations revealed that the transition state activation energy for radical trifluoromethylation was significantly higher and the isotropic charge repulsion makes it difficult for the CF3 radical to transfer electrons.

Structure-Based Design of Ultrapotent Tricyclic Ligands for FK506-Binding Proteins.

Access to small, rigid, and sp3-rich molecules is a major limitation in the drug discovery for challenging protein targets. FK506-binding proteins hold high potential as drug targets or enablers of molecular glues but are fastidious in the chemotypes accepted as ligands. We here report an enantioselective synthesis of a highly rigidified pipecolate-mimicking tricyclic scaffold that precisely positions functional groups for interacting with FKBPs. This was enabled by a 14-step gram-scale synthesis featuring anodic oxidation, stereospecific vinylation, and N-acyl iminium cyclization. Structure-based optimization resulted in the discovery of FKBP inhibitors with picomolar biochemical and subnanomolar cellular activity that represent the most potent FKBP ligands known to date.

Enzymatic Synthesis of Unprotected α,β-Diamino Acids via Direct Asymmetric Mannich Reactions.

α,β-Diamino acids are important structural motifs and building blocks for numerous bioactive natural products, peptidomimetics, and pharmaceuticals, yet efficient asymmetric synthesis to access these stereoarrays remains a challenge. Herein, we report the development of a pyridoxal 5'-phosphate (PLP)-dependent enzyme that is engineered to catalyze stereoselective Mannich-type reactions between free α-amino acids and enolizable cyclic imines. This biocatalyst enabled one-step asymmetric enzymatic synthesis of the unusual pyrrolidine-containing amino acid L-tambroline at gram-scale with high enantio- and diastereocontrol. Furthermore, this enzymatic platform is capable of utilizing a diverse range of α-amino acids as the Mannich donor and various cyclic imines as the acceptor. By coupling with different imine-generating enzymes, we established versatile biocatalytic cascades and demonstrated a general, concise, versatile, and atom-economic approach to access unprotected α,β-diamino acids, including structurally complex α,α-disubstituted α,β-diamino acids with contiguous stereocenters.

R‐BINOL‐Chiral Phosphoric Acid Catalyzed Regioselective Synthesis of 7‐Oxo‐Pyrazolo[1,2‐a]Pyrazoles via [3+2] Cycloaddition Reaction of Morita Baylis Hillman Adducts with N, N’‐Cyclic Azomethine Imines

AbstractWe report an efficient [3+2] cycloaddition protocol for the synthesis of pyrazolo[1,2‐a]pyrazolones from Morita‐Baylis‐Hillman adducts and N,N’‐cyclic azomethine imines. The R‐BINOL chiral phosphoric acid was identified as the suitable catalyst for the reported cycloaddition protocol. Almost all MBH adducts regioselectively delivered 7‐oxo‐pyrazolo[1,2‐a]pyrazoles at an efficient yield of 69–91 %. With exception, the MBH adducts derived from acrylonitrile deliver 5‐oxo‐pyrazolo[1,2‐a]pyrazoles comparatively at a higher yield than the 7‐oxo‐pyrazolo[1,2‐a]pyrazoles. The density functional theory (DFT) studies demonstrate that the reaction is thermodynamically more feasible in the presence of R‐BINOL chiral phosphoric acid.

Annulation Reactions of Isoquinolinium Ylides with Sulfamate-Derived Cyclic Imines toward Polycyclic 1,3-Benzoxazepine Heterocycles

AbstractA dearomatization-guided and (3+2) cycloaddition-triggered annulation reaction of in situ formed isoquinolinium ylides with sulfamate-derived cyclic imines is reported, offering a cogent synthesis of tetracyclic 1,3-benzoxazepine frameworks in high yields. The protocol features successive ring-forming and ring-breaking events in a cascade fashion and is also scalable.

Detection of Cyclic Imines (CIs) Toxins in Whole Body of Shellfishes: First Monitoring Report of CIs in South Korea Shellfishes

Cyclic imines (CIs) produced by microalgal species that accumulate in the food chains of marine organisms are novel biotoxins that do not belong to the classic group of marine biotoxins. In the past, CIs were found only in limited areas; however, in recent years, rapid changes in marine ecosystems have led to widespread CIs and increased exposure to toxic risks. In this study, we analyzed seven CI toxins, GYM-A, SPX (13-desmethyl spirolide C, 13, 19-dideMe spirolide C, 20-methyl spirolide G), and PnTX-E, F, and G, using LC/MRM-MS. Shellfish samples were purchased from a domestic Korean fish market (67 samples in 2021 and 216 samples in 2022). The entire body of the shellfish was ground and extracted with 50% methanol, followed by lipophilic-specific SPE. Only GYM-A, PnTX-G, and 13-desmethyl spirolide C were detected in all analyzed samples. The maximum concentrations of GYM-A is maximum 179 ppt (ng/kg) in Crassostrea nippona (March 2022), PnTX-G is maximum 7 ppt in Anadara broughtonii (April 2022), 13-desmethyl SPX C is maximum 58 ppt in Crassostrea nippona (April 2022). The southern coast exhibited the highest frequency of detection of these toxins, which was attributed to elevated sea-surface temperatures, aligned with conducive conditions for toxin-producing phytoplankton. According to the monitoring results, there were no significant CI toxins in the shellfish; however, it is important to monitor CI toxin accumulation in shellfish because of their high risk of toxicity.

Synthesis of Nitrones and Nitroalkanes via Chiral Cyclic Imines.

A series of structurally chiral cyclic imines efficiently yields chiral nitrones and nitroalkanes. This is the first report of the synthesis of nitro groups by C═N bond cleavage of imines through a nitrone intermediate.

Does climate change increase the risk of marine toxins? Insights from changing seawater conditions.

Marine toxins produced by marine organisms threaten human health and impose a heavy public health burden on coastal countries. Lately, there has been an emergence of marine toxins in regions that were previously unaffected, and it is believed that climate change may be a significant factor. This paper systematically summarizes the impact of climate change on the risk of marine toxins in terms of changes in seawater conditions. From our findings, climate change can cause ocean warming, acidification, stratification, and sea-level rise. These climatic events can alter the surface temperature, salinity, pH, and nutrient conditions of seawater, which may promote the growth of various algae and bacteria, facilitating the production of marine toxins. On the other hand, climate change may expand the living ranges of marine organisms (such as algae, bacteria, and fish), thereby exacerbating the production and spread of marine toxins. In addition, the sources, distribution, and toxicity of ciguatoxin, tetrodotoxin, cyclic imines, and microcystin were described to improve public awareness of these emerging marine toxins. Looking ahead, developing interdisciplinary cooperation, strengthening monitoring of emerging marine toxins, and exploring more novel approaches are essential to better address the risks of marine toxins posed by climate change. Altogether, the interrelationships between climate, marine ecology, and marine toxins were analyzed in this study, providing a theoretical basis for preventing and managing future health risks from marine toxins.