Cyclic Imines Research Articles

The Cyclic Imine Core Common to the Marine Macrocyclic Toxins Is Sufficient to Dictate Nicotinic Acetylcholine Receptor Antagonism.

Macrocyclic imine phycotoxins are an emerging class of chemical compounds associated with harmful algal blooms and shellfish toxicity. Earlier binding and electrophysiology experiments on nAChR subtypes and their soluble AChBP surrogates evidenced common trends for substantial antagonism, binding affinities, and receptor-subtype selectivity. Earlier, complementary crystal structures of AChBP complexes showed that common determinants within the binding nest at each subunit interface confer high-affinity toxin binding, while distinctive determinants from the flexible loop C, and either capping the nest or extending toward peripheral subsites, dictate broad versus narrow receptor subtype selectivity. From these data, small spiroimine enantiomers mimicking the functional core motif of phycotoxins were chemically synthesized and characterized. Voltage-clamp analyses involving three nAChR subtypes revealed preserved antagonism for both enantiomers, despite lower subtype specificity and binding affinities associated with faster reversibility compared with their macrocyclic relatives. Binding and structural analyses involving two AChBPs pointed to modest affinities and positional variability of the spiroimines, along with a range of AChBP loop-C conformations denoting a prevalence of antagonistic properties. These data highlight the major contribution of the spiroimine core to binding within the nAChR nest and confirm the need for an extended interaction network as established by the macrocyclic toxins to define high affinities and marked subtype specificity. This study identifies a minimal set of functional pharmacophores and binding determinants as templates for designing new antagonists targeting disease-associated nAChR subtypes.

A Review of Cyclic Imines in Shellfish: Worldwide Occurrence, Toxicity and Assessment of the Risk to Consumers.

Cyclic imines are a class of lipophilic shellfish toxins comprising gymnodimines, spirolides, pinnatoxins, portimines, pteriatoxins, prorocentrolides, spiro-prorocentrimine, symbiomines and kabirimine. They are structurally diverse, but all share an imine moiety as part of a bicyclic ring system. These compounds are produced by marine microalgal species and are characterized by the rapid death that they induce when injected into mice. Cyclic imines have been detected in a range of shellfish species collected from all over the world, which raises the question as to whether they present a food safety risk. The European Food Safety Authority (EFSA) considers them to be an emerging food safety issue, and in this review, the risk posed by these toxins to shellfish consumers is assessed by collating all available occurrence and toxicity data. Except for pinnatoxins, the risk posed to human health by the cyclic imines appears low, although this is based on only a limited dataset. For pinnatoxins, two different health-based guidance values have been proposed at which the concentration should not be exceeded in shellfish (268 and 23 µg PnTX/kg shellfish flesh), with the discrepancy caused by the application of different uncertainty factors. Pinnatoxins have been recorded globally in multiple shellfish species at concentrations of up to 54 times higher than the lower guidance figure. Despite this observation, pinnatoxins have not been associated with recorded human illness, so it appears that the lower guidance value may be conservative. However, there is insufficient data to generate a more robust guidance value, so additional occurrence data and toxicity information are needed.

Cyclic Amines Coupled to Indole Derivatives With Improved Efflux Pump Inhibiting Activity in Bacteria and Cancer Cells.

Indole skeleton has become a significant tool in the field of anticancer and antibacterial therapeutic strategies. The modified aza-Friedel-Crafts reaction by direct coupling of different cyclic imines and indole derivatives has been explored. To investigate the scope and limitations of the reaction and observe the effect of structural modifications, our aim was to resynthesize selected compounds as well as prepare new derivatives starting from 6,7-dimethoxy-3,4-dihydroisoquinoline, (4aR,8aR)-4a,5,6,7,8,8a-hexahydroquinoxalin-2(1H)-one and 7-azaindole. Our further aim was the systematic biological evaluation of selected C-3-coupled indole and azaindole derivatives in favour of having a preliminary overview about the structure-activity relationships. The synthesis and resynthesis of selected compounds were accomplished by extension of aza-Friedel-Crafts reaction. The products have been tested on bacteria and cancer cells. The most significant efflux pump inhibiting (EPI) activity was observed in the case of 6,7-dihydrothieno[3,2-c]pyridine coupled indole derivative. The reaction of 6,7-dimethoxy-3,4-dihydroisoquinoline with 7-azaindole resulted in the most potent biofilm inhibitor product. Applying indole and 4,9-dihydro-3H-β-carboline, 6,7-dihydrothieno[3,2-c]pyridine led to the formation of a product with the highest anticancer activity. 6,7-Dimethoxy-3,4-dihydroisoquinoline skeleton and indole as an electron-rich aromatic compound have been found to be effective in the inhibition of ABCB1. The compounds presented in the study were investigated regarding different aspects of antibacterial and anticancer activities. Accordingly, some compounds were found to have antibacterial effect on Escherichia coli and Staphylococcus aureus strains, certain C-3-coupled derivatives showed toxicity on sensitive and ABCB1 efflux pump expressing colon adenocarcinoma and a normal, non-cancerous fibroblast cell lines.

DFT study on the Stereoselectivity of Asymmetric Synthesis of C, N‐Cyclic Azomethine Imines with Allyl Alkyl Ketones

AbstractBased on the density functional theory, the stereoselectivity of the asymmetric synthesis of tetrahydroisoquinolinopyrazole derivatives by the cycloaddition reaction of C, N‐cyclic azomethine imines with allyl alkyl ketones which was catalyzed using a chiral primary amine is studied theoretically. The results demonstrate that the four chiral transition states (TS‐1R2R, TS‐1R2S, TS‐1S2R and TS‐1S2S) are formed by the reaction of dienamine intermediate with C, N‐cyclic azomethine imine, and the reaction barrier of TS‐1R2S is in the lowest level (ΔG=6.98 kcal/mol). The consequence reveal that the 1R2S configuration product is formed by TS‐1R2S transition state. Non‐covalent interaction and electron density topology analysis show that the stereoselectivity of the chiral product is determined through van der Waals forces and hydrogen bond interaction of the transition state molecules.

Asymmetric (3+2) Cycloaddition Employing β‐Fluoroalkylated α,β‐Unsaturated 2‐Acyl Imidazole Catalyzed by a Chiral‐at‐Metal Rhodium Complex

AbstractAn asymmetric 1,3‐dipolar cycloaddition of β‐fluoroalkylated α,β‐unsaturated 2‐acyl imidazole with nitrones or N,N’‐cyclic azomethine imines to construct chiral α‐fluoroalkylated γ‐amino alcohols or chiral α‐fluoroalkylated 1,3‐diamino compounds has catalyzed by chiral‐at‐metal rhodium complexes is reported. The corresponding products with three contiguous tertiary stereocenters were obtained in 62%–98% yields with stereoselectivities up to >20:1 dr and 99% ee. Remarkably, as low as 0.06 mol% of chiral‐at‐metal rhodium complex can promote a gram‐scale reaction with nitrones or N,N’‐cyclic azomethine imines.

Synthesis and α-Functionalisation of Cyclic Imines

Abstractα-Functionalisation of cyclic imines is explored. The cyclic imine substrates are synthesised from their respective halonitrile precursors using a nucleophilic addition/cyclisation sequence. Selective monohalogenation of the cyclic imines yields α-haloimines, which serve as a platform to obtain various α-hydroxyimine derivatives. In addition, an unusual tautomerisation and oxidation sequence is observed in the attempted preparation of α-hydroxyimines.

Annuloselectivity of Dibenzoxazepines and Arynes in the Presence of CO2: [2+2] vs [2+2+2] Annulation

AbstractThe annulation of arynes with dibenzo[b,f][1,4]oxazepines (N‐aryl cyclic imines) in the presence of CO2 has been studied in comparison with 3,4‐dihydroisoquinolines (N‐alkyl cyclic imines). For the reaction with dibenzo[b,f][1,4]oxazepines, [2+2] annulation takes place specifically with arynes, with CO2 acting only as the protecting gas. 3,4‐dihydroisoquinolines, however, exclusively underwent [2+2+2] annulation with arynes and CO2, with CO2 as a reactant. Our findings offer insights into imine‐aryne annulation under CO2 and suggest that steric and nucleophilic factors control the annuloselectivity. Reactivity analysis revealed that electron‐rich and less steric N‐alkyl cyclic imines and electron‐deficient benzynes favor the [2+2+2] annulation, while electron‐deficient and bulky N‐aryl cyclic imines prefer the [2+2] annulation. In the [2+2] annulation, the Hammett analyses reveal that both the nucleophilic attack of imines to benzynes (the first step, negative ρ) and the attack of phenyl anions on the iminium species (the second step, positive ρ) affect the reaction rate.

Rh(III)‐Catalyzed sequential ring‐retentive/‐opening [4 + 2] annulations of 2H‐imidazoles towards full‐color emissive imidazo[5,1‐a]isoquinolinium salts and AIE‐active non‐symmetric 1,1′‐biisoquinolines

Rhodium-catalyzed C4aryl−H activation and ring-retentive annulation of 2H-imidazoles with internal alkynes to build imidazo[5,1-a]isoquinolinium salts with high yields and broad scope has been disclosed. These novel salts serve as new full-color emissive fluorophores (433−633 nm), just by simply modifying the substituents on C3 and C4 positions of isoquinoline ring. Furthermore, these salts can undergo ring-opening C5aryl−H activation/annulation with a different alkyne to form non-symmetric and AIE-active 1,1′-biisoquinolines, where NH4OAc plays an indispensable role that accounts for Hofmann elimination and imine formation, leading to an unprecedented imine dance: cyclic imine → N-alkenyl imine → NH imine. The 15N labelling experiments indicate that the 2nd annulation includes two pathways: N-exchange (major) and N-retention (minor).

Pyrroloindoles as Therapeutically Invaluable Agents: A Review on their Diverse Synthetic Methodologies

AbstractIn the realm of medicinal chemistry, heterocyclic compounds play a vital role due to their diverse pharmacological properties, especially, pyrroloindoles standing out as crucial motifs in drug development. Pyrroloindoles, which are formed by combining indole and pyrrole rings, exhibit unique characteristics and serve as integral components in numerous drugs and natural compounds. The synthesis of pyrroloindoles is challenging due to their complex ring structures and diverse substitution patterns. This comprehensive review compiles various synthetic strategies for obtaining pyrroloindole derivatives via metal‐catalyzed reactions; cyclopropanation/ ring opening/ iminium cyclization (CRI reaction); organocatalytic cascade addition‐cyclization; 1,3‐dipolar cycloaddition; Brønsted acid‐catalyzed [3+2] cyclodimerization; ring closure mechanisms; thermally‐induced ring‐opening and cyclization reactions; condensation reactions; and visible‐light photocatalytic asymmetric reactions providing a valuable resource for synthetic organic chemists and researchers interested in the synthesis of pyrroloindoles and their derivatives. These compounds play a pivotal role in drug discovery thus, making this review an indispensable reference.

Synergistically activating nucleophile strategy enabled organocatalytic asymmetric P-addition of cyclic imines.

Herein, we present an attractive organocatalytic asymmetric addition of P-nucleophiles to five-membered cyclic N-sulfonyl imines facilitated by phosphonium salt catalysis, enabling the highly enantioselective synthesis of tri- and tetra-substituted cyclic phosphorus-containing benzosultams. With this protocol, various cyclic α-aminophosphonates were efficiently synthesized with high yields and exceptional enantioselectivities (up to >99% ee) under mild reaction conditions. The utility and practicality of this method were demonstrated through gram-scale reactions and straightforward elaborations. Notably, the success of this approach relies on the deliberate selection of a synergistic organocatalytic system, which helps circumvent foreseeable side effects while handling secondary phosphine oxides (SPOs). Systematic mechanistic studies, incorporating experiments and DFT calculations, have revealed the critical importance of judiciously selecting bifunctional phosphonium salt catalysts for effectively activating P-nucleophiles while stereoselectively controlling the P-attack process.

Photocatalytic dehydrogenative C(sp2)–P coupling reaction between dibenzo[b,f][1,4]oxazepanes and phosphine oxides

Visible-light-induced cross-dehydrogenative coupling between cyclic imines and diaryl phosphine oxides has been developed to afford their oxyphosphide derivatives, using rhodamine B as a photocatalyst.

Enantioselective synthesis of α-tetrasubstituted (1-indolizinyl) (diaryl)-methanamines via chiral phosphoric acid catalysis.

An enantioselective Friedel-Crafts reaction of cyclic α-diaryl N-acyl imines with indolizines catalyzed by a chiral spirocyclic phosphoric acid has been developed. The asymmetric transformation proceeds smoothly to afford α-tetrasubstituted (1-indolizinyl) (diaryl)methanamines in good yields with up to 98% ee under mild conditions.

Aza-[4 + 2] annulations of α-vinyl Morita–Baylis–Hillman adducts for regiodivergent synthesis of exomethylene-bearing azatricyclic compounds

The regiodivergent synthesis of exomethylene-bearing azatricyclic products was achieved via aza-[4 + 2] annulations of cyclic imines with a single C4 synthon, the α-vinyl MBH adduct.

Radical-triggered base-free 1,3-C → C migrations: chemodivergent synthesis of cyclic imines from N-allyl enamines

Novel radical-triggered 1,3-C → C migrations of N-allyl enamines under base-free conditions and the catalyst-dependent synthesis of cyclic imines were achieved.

The Temporal Distribution of Cyclic Imines in Shellfish in the Bays of Fangar and Alfacs, Northwestern Mediterranean Region.

Spirolides (SPXs), gymnodimines (GYMs), and pinnatoxins (PnTXs) have been detected in shellfish from the northwestern Mediterranean coast of Spain. Several samples of bivalves were collected from Fangar Bay and Alfacs Bay in Catalonia over a period of over 7 years (from 2015 to 2021). Shellfish samples were analyzed for cyclic imines (CIs) on an LC1200 Agilent and 3200 QTrap triple-quadrupole mass spectrometer. In shellfish, SPX-1 was detected in two cases (of 26.5 µg/kg and 34 µg/kg), and GYM-A was only detected in trace levels in thirteen samples. Pinnatoxin G (PnTX-G) was detected in 44.6% of the samples, with its concentrations ranging from 2 µg/kg to 38.4 µg/kg. Statistical analyses revealed that seawater temperature influenced the presence or absence of these toxins. PnTX-G showed an extremely significant presence/temperature relationship in both bays in comparison to SPX-1 and GYM-A. The prevalence of these toxins in different bivalve mollusks was evaluated. A seasonal pattern was observed, in which the maximum concentrations were found in the winter months for SPX-1 and GYM-A but in the summer months for PnTX-G. The obtained results indicate that it is unlikely that CIs in the studied area pose a potential health risk through the consumption of a seafood diet. However, further toxicological information about CIs is necessary in order to perform a conclusive risk assessment.

Consecutive Ireland–Claisen Enyne-Metathesis Strategy Enables Rapid Assembly of Cyclic Imine Core Cyclohexene Motif

AbstractAn efficient strategy for rapid assembly of the complex substituted cyclohexene core that is present in several cyclic imine marine toxins is presented. Several of these toxins, including pinnatoxin A and recently discovered portimine A, have been the focus of much attention due to their fascinating biological activities. We demonstrate that the substituted cyclohexene-diene motif, which is a challenging feature to access synthetically, can be prepared through a stepwise Ireland–Claisen rearrangement/enyne metathesis procedure beginning from chiral esters. This approach enables a divergent strategy that can be implemented in syntheses of cyclic imines or derivatives thereof.

The Proton of Alcohols as Hydrogen Source in Diboron-Mediated Nickel-Catalyzed Asymmetric Transfer Hydrogenation of Cyclic N-Sulfonyl Imines.

The proton of alcohols as the sole hydrogen source in diboron-mediated nickel-catalyzed asymmetric transfer hydrogenation of cyclic N-sulfonyl imines has been developed, providing the chiral cyclic sulfamidates in excellent enantioselectivities. The mechanistic investigations suggested that the proton of alcohols could be activated by tetrahydroxydiboron to form active nickel hydride species.

Photoinduced Metal- and Photosensitizer-Free Decarbonylative C-H Alkylation of Cyclic Sulfamidate Imines.

Photoinduced decarbonylative C-C bond formation with readily accessible aldehydes as alkyl sources is described. This protocol provides a sustainable alternative for the effective construction of diverse valuable 4-alkylated sulfonyl ketimines under metal- and photosensitizer-free conditions. Significantly, in this reaction, air serves as the green oxidant, and cyclic sulfamidate imines play a dual role of substrate and photocatalyst, thus affording a concise reaction system for C-H alkylation of cyclic sulfamidate imines.

Copper(I)-Photocatalyzed Diastereoselective Aziridination of N-Sulfonyl Imines with Vinyl Azides: Application to Benzo[f][1,2,3]oxathiazepines Dioxides and Fused Isoxazolines.

An in situ generated photoactive copper(I)-complex-catalyzed aziridination reaction of cyclic N-sulfonyl imines with α-aryl-substituted vinyl azides irradiated by blue-LEDs light is reported for the first time. This novel SET process represents a mild, sustainable, and pragmatic method for accessing synthetically resourceful sulfamidate-fused aziridines in acceptable chemical yields with excellent diastereoselectivities. Delightedly, pharmacologically attractive benzo[f][1,2,3]oxathiazepine dioxides and fused isoxazoline frameworks were achieved through our newly developed metal-free based ring-expansion techniques, highlighting the synthetic value of accessed aziridines. Finally, the possible mechanism for [2+1] aza-cyclization was presented based on the conduction of a series of control experiments.

Base catalyzed abnormal [3+2]-cycloaddition reaction of isatin N,N′‑cyclic azomethine imine with malononitrile via C-3 umpolung of oxindole

In this paper, we described a base promoted annulation reaction of isatin N,N'-cyclic azomethine imine and malononitrile for the synthesis of 1′,2′-dihydrospiro[indoline-3,6′-pyrrolo[1,2-b]pyrazole] derivatives using a new abnormal [3 + 2]-cycloaddition reaction. A variety of pyrrolo-pyrazole bearing oxindole scaffolds with medicinal capability were synthesized in moderate to high yields via C3 umpolung of oxindole.