Design, Synthesis, and Biological Evaluation of Erythrina Alkaloid Analogues as Neuronal Nicotinic Acetylcholine Receptor Antagonists

Spokewise Total Syntheses of Four Erythrina Alkaloids and Telescoped Syntheses of Six Additional Alkaloids

Based on rich sulfur-involving chemical transformations, a novel spokewise synthetic strategy, a subclass of the collective strategies, has been developed to concisely synthesize four erythrina alkaloids through a single-step transformation from a common synthetic precursor. Moreover, six additional erythrina alkaloids have also been synthesized by subsequent 1-2 steps chemical transformations. The current synthetic approaches provide a valuable platform for collective total syntheses of erythrina alkaloids and pseudo-natural erythrina alkaloids.

Tunable Approach to Diverse Phenethylamines via Reduction of 5-Aryloxazolidines with Triethylsilane

A rapid pathway for the synthesis of various β-phenethylamines from aromatic aldehydes has been developed. Initially, a wide range of 5-aryloxazolidines was prepared via the [3 + 2] cycloaddition reaction of N-methylazomethine ylide derived from sarcosine and paraformaldehyde. The subsequent reduction of 5-aryloxazolidines with triethylsilane in trifluoroacetic acid yields three types of products: N,N-dimethylphenylethanolamines, N,N-dimethylphenethylamines, and tetrahydroisoquinolines, depending on the substituents in the aromatic ring and reaction conditions. Moreover, an additional step of oxazolidine hydrolysis or ring-opening with hydrogen cyanide allowed us to synthesize N-methyl- or N-methyl-N-(cyanomethyl)phenethylamines.

Small molecule ligands for α9* and α7 nicotinic receptors: a survey and an update, respectively

The α9- and α7-containing nicotinic acetylcholine receptors (nAChRs) mediate numerous physiological and pathological processes by complex mechanisms that are currently the subject of intensive study and debate. In this regard, selective ligands serve as invaluable investigative tools and, in many cases, potential therapeutics for the treatment of various CNS disfunctions and diseases, neuropathic pain, inflammation, and cancer. However, the present scenario differs significantly between the two aforementioned nicotinic subtypes. Over the past few decades, a large number of selective α7-nAChR ligands, including full, partial and silent agonists, antagonists, and allosteric modulators, have been described and reviewed. Conversely, reports on selective α9-containing nAChR ligands are relatively scarce, also due to a more recent characterization of this receptor subtype, and hardly any focusing on small molecules. In this review, we focus on the latter, providing a comprehensive overview, while providing only an update over the last five years for α7-nAChR ligands.

Chiral phosphoric acid-catalyzed enantioselective phosphinylation of 3,4-dihydroisoquinolines with diarylphosphine oxides

Chiral phosphorous-containing compounds are playing a more and more significant role in several different research fields. Here, we show a chiral phosphoric acid-catalyzed enantioselective phosphinylation of 3,4-dihydroisoquinolines with diarylphosphine oxides for the efficient and practical construction of a family of chiral α-amino diarylphosphine oxides with a diverse range of functional groups. The phosphine products are suitable for transforming to several kinds of chiral (thio)ureas, which might be employed as chiral ligands or catalysts with potential applications in asymmetric catalysis. Control and NMR tracking experiments show that the reaction proceeds via the tert-butyl 1-(tert-butoxy)-3,4-dihydroiso-quinoline-2(1H)-carboxylate intermediate, followed by C-P bond formation. Furthermore, computational studies elucidated that the hydrogen bonding strength between the phosphonate and isoquinolinium determines the stereoselectivity of the phosphinylation reaction.

Controllable carbonyl-assisted C(sp3)–C(sp3) bond reduction and reorganization

Unprecedentedly preferential reduction of unstrained C(sp3)–C(sp3) bond over ketone, hydrogenative [2+2+2]-cycloreversion of 2,4-diacylcyclohexanols, and cyclizative degradation of poly(vinylketone) have been achieved by organolanthanide catalysis.

Iron Trichloride-Mediated Cascade Reaction of Aminosugar Derivatives for the Synthesis of Fused Tetrahydroisoquinoline-Tetrahydrofuran Systems

A method to obtain tetrahydroisoquinolines (THIQs) fused to tetrahydrofuran rings from aminosugar derivatives has been developed. The procedure relies on a key deprotection of benzyl ethers followed by a double-cyclization sequence, using FeCl₃ as the sole reagent. This tandem reaction affords the construction of novel fused polycyclic heterocycles with total stereochemical control.

Tandem H/D Exchange-SET Reductive Deuteration Strategy for the Synthesis of α,β-Deuterated Amines Using D2O

α,β-Deuterated amines are crucial for the development of deuterated drugs. We intend to introduce the novel tandem H/D exchange-single electron transfer (SET) reductive deuteration strategy with high pot- and reagent-economy by the synthesis of α,β-deuterated amine using nitrile as the precursor. The H/D exchange of the -CH₂CN group was achieved by D₂O/Et₃N, which were also the required reagents in the tandem SmI₂-mediated SET reductive deuteration of the α-deuterated nitrile. The potential application of this method was further showcased by the synthesis of bevantolol-d₄.

Erythrina Alkaloid Analogues as nAChR Antagonists-A Flexible Platform for Leads in Drug Discovery

Erythrina alkaloids and their central nervous system effects have been studied for over a century, mainly due to their potent antagonistic actions at β2-containing nicotinic acetylcholine receptors (nAChRs). In the present work, we report a synthetic approach giving access to a diverse set of Erythrina natural product analogues and present the enantioselective total synthesis of (+)-Cocculine and (+)-Cocculidine, both found to be potent antagonists of the β2-containing nAChRs.

Promising Antioxidant Activity of Erythrina Genus: An Alternative Treatment for Inflammatory Pain?

The negative impact that oxidative stress has on health is currently known. The complex mechanism of free radicals initiates a series of chain reactions that contribute to the evolution or development of different degenerative disorders. Likewise, these disorders are usually accompanied by inflammatory processes and, therefore, pain. In this sense, reactive oxygen species (ROS) have been shown to promote the nociceptive process, but effective treatment of pain and inflammation still represents a challenge. Over time, it has been learned that there is no single way to relieve pain, and as long as there are no other alternatives, the trend will continue to apply multidisciplinary management, such as promote the traditional use of the Erythrina genus to manage pain and inflammation. In this sense, the Erythrina genus produces a wide range of secondary metabolites, including flavanones, isoflavones, isoflavones, and pterocarpans; these compounds are characterized by their antioxidant activity. Phenolic compounds have demonstrated their ability to suppress pro-oxidants and inhibit inflammatory signaling pathways such as MAPK, AP1, and NFκB. Although there is preclinical evidence supporting its use, the pharmacological effect mechanisms are not entirely clear. Nowadays, there is a fast advancement in knowledge of the disciplines related to drug discovery, but most of nature's medicinal potential has not yet been harnessed. This review analyzes the decisive role that the Erythrina genus could play in managing inflammatory pain mediated by its compounds and its uses as an antioxidant.

Construction of the Erythrinane Core Skeleton via Asymmetric Catalytic Cascade Reaction of Tertiary Enamides

We report herein an efficient cascade strategy for the rapid construction of a highly enantioenriched erythrinane core skeleton. Under the sequential catalysis of a chiral Cr(III)(salen)Cl and InCl₃, cyclohexanone-derived tertiary enamides undergo an intramolecular enantioselective nucleophilic addition followed by diastereoselective Pictet-Spengler cyclization. This method is highly enantio- and diastereoselective, leading to diverse erythrina alkaloid derivatives as the sole diastereoisomer with up to 98% ee.

Artificial Erythrina Alkaloids from Three Erythrina Plants, E. variegata, E. crista-galli and E. arborescens

Fourteen unprecedented artificial Erythrina alkaloids were isolated from the Erythrina variegata, E. crista-galli and E. arborescens (Fabaceae). The structures of these alkaloids were determined by spectroscopic analyses. Their possible formations were proposed. All isolated compounds showed no cytotoxicity and hypoglycemic activity at cell screening bioassay.

Photocatalytic C-H silylation of heteroarenes by using trialkylhydrosilanes

The efficient and selective C-H silylation of heteroarenes, especially the pharmaceutically relevant electron-deficient heteroarenes, represents a great challenge in organic synthesis. Herein we wish to report a distinctive visible light-promoted photocatalytic C-H silylation approach that enables the direct coupling of trialkylhydrosilanes with both electron-deficient and -rich heteroarenes as well as with cyano-substituted arenes in moderate to high yields and with good regioselectivity. The protocol features operational simplicity, mild reaction conditions, and the use of safe and readily available Na2S2O8, bis(trimethylsilyl) peroxide (BTMSPO) or iPr3SiSH as the radical initiators. Notably, the challenging bulky and inert trialkylhydrosilanes, such as (t-butyldimethyl)silane ( t BuMe2SiH) and (triisopropyl)silane (iPr3SiH), work smoothly with the protocol. Moreover, despite the higher stability of t BuMe2Si silylation products, our studies revealed their great reactivity and versatility in diverse C-Si-based chemical transformations, providing an operationally simple, low-cost, and environmentally benign synthetic technology for molecule construction and elaboration.

Structural Simplification of Natural Products

Natural products (NPs) are important sources of clinical drugs due to their structural diversity and biological prevalidation. However, the structural complexity of NPs leads to synthetic difficulties, unfavorable pharmacokinetic profiles, and poor drug-likeness. Structural simplification by truncating unnecessary substructures is a powerful strategy for overcoming these limitations and improving the efficiency and success rate of NP-based drug development. Herein, we will provide a comprehensive review of the structural simplification of NPs with a focus on design strategies, case studies, and new technologies. In particular, a number of successful examples leading to marketed drugs or drug candidates will be discussed in detail to illustrate how structural simplification is applied in lead optimization of NPs.

Isoquinolines from Corydalis tomentella from Tibet, China, possess hepatoprotective activities

The phytochemical study on Corydalis tomentella Franch, a traditional Chinese medicinal plant in Tibet, China, led to the isolation of six previously undescribed isoquinolines, including two rarely reported N-benzyl ones, and twenty-one known ones firstly obtained from this plant. Their planar structures were elucidated by 1D, 2D NMR experiments and high resolution mass spectrometry, and the absolute configurations were determined by NOE experiments, electronic circular dichroism, and specific rotation. Seven isoquinolines exhibited stronger hepatoprotective activities than that of positive control in D-galactosamine induced L02 cells damage model, which could be served as the leading compounds for further investigations. The primary structure-activity relationship was also summarized accordingly.

Intramolecular hydride transfer onto arynes: redox-neutral and transition metal-free C(sp3)-H functionalization of amines

Transition metal-free intramolecular hydride transfer onto arynes is reported for the first time. This unique transformation is utilized in redox-neutral intermolecular α-functionalization reactions of different tertiary amines, generating C(sp3)-C(sp3/sp2/sp) bonds in a single synthetic operation. Deuterium labeling studies support initial cleavage of the α-C-H bond via intramolecular 1,5-hydride transfer onto the aryne, which leads to activation of a range of integrated pronucleophiles and ultimately affords a new approach to cross-dehydrogenative coupling reactions which utilize aryne intermediates.

A strategic approach to [6,6]-bicyclic lactones: application towards the CD fragment of DHβE

We report an effective synthetic protocol to access [6,6]-bicyclic lactone moieties through a regio- and stereoselective intramolecular Mizoroki–Heck cross-coupling reaction followed by a 6π-electrocyclization. This method enabled the first synthesis of the elusive CD fragment of the Erythrina alkaloid DHβE. Preliminary pharmacological evaluations support the notion that the key pharmacophores of DHβE are located in the A and B rings.

Biomolecular recognition of antagonists by α7 nicotinic acetylcholine receptor: Antagonistic mechanism and structure-activity relationships studies

As the key constituent of ligand-gated ion channels in the central nervous system, nicotinic acetylcholine receptors (nAChRs) and neurodegenerative diseases are strongly coupled in the human species. In recently years the developments of selective agonists by using nAChRs as the drug target have made a large progress, but the studies of selective antagonists are severely lacked. Currently these antagonists rest mainly on the extraction of partly natural products from some animals and plants; however, the production of these crude substances is quite restricted, and artificial synthesis of nAChR antagonists is still one of the completely new research fields. In the context of this manuscript, our primary objective was to comprehensively analyze the recognition patterns and the critical interaction descriptors between target α7 nAChR and a series of the novel compounds with potentially antagonistic activity by means of virtual screening, molecular docking and molecular dynamics simulation, and meanwhile these recognition reactions were also compared with the biointeraction of α7 nAChR with a commercially natural antagonist - methyllycaconitine. The results suggested clearly that there are relatively obvious differences of molecular structures between synthetic antagonists and methyllycaconitine, while the two systems have similar recognition modes on the whole. The interaction energy and the crucially noncovalent forces of the α7 nAChR-antagonists are ascertained according to the method of Molecular Mechanics/Generalized Born Surface Area. Several amino acid residues, such as B/Tyr-93, B/Lys-143, B/Trp-147, B/Tyr-188, B/Tyr-195, A/Trp-55 and A/Leu-118 played a major role in the α7 nAChR-antagonist recognition processes, in particular, residues B/Tyr-93, B/Trp-147 and B/Tyr-188 are the most important. These outcomes tally satisfactorily with the discussions of amino acid mutations. Based on the explorations of three-dimensional quantitative structure-activity relationships, the structure-antagonistic activity relationships of antagonists and the characteristics of α7 nAChR-ligand recognitions were received a reasonable summary as well. These attempts emerged herein would not only provide helpful guidance for the design of α7 nAChR antagonists, but shed new light on the subsequent researches in antagonistic mechanism.

Tying up Nicotine: New Selective Competitive Antagonist of the Neuronal Nicotinic Acetylcholine Receptors

Conformational restriction of the pyrrolidine nitrogen in nicotine by the introduction of an ethylene bridge provided a potent and selective antagonist of the α4β2-subtype of the nicotinic acetylcholine receptors. Resolution by chiral SFC, pharmacological characterization of the two enantiomers, and determination of absolute configuration via enantioselective synthesis showed that the pharmacological activity resided almost exclusively in the (R)-enantiomer.

A divergent route to 9, 10-oxygenated tetrahydroprotoberberine and 8-oxoprotoberberine alkaloids: synthesis of (±)-isocorypalmine and oxypalmatine

A new route which is germane to the synthesis of 9,10-oxygenated tetrahydroprotoberberines and 8-oxoprotoberberines is described. The route features the use of a diester (14) generated from reaction of dimethylmalonate with an aryl halide in the presence of n-butyllithium. The amide 17 prepared in subsequent steps is a versatile precursor for the synthesis of tetrahydroprotoberberine and 8-oxoprotoberberine scaffolds using standard high-yielding reactions. In this manner, (±)-isocorypalmine and oxypalmatine have been synthesized in 23% and 22 % yields respectively.

Novel strategies for catalytic asymmetric synthesis of C1-chiral 1,2,3,4-tetrahydroisoquinolines and 3,4-dihydrotetrahydroisoquinolines

This review summarizes recent progress in catalytic asymmetric construction of 1,2,3,4-tetrahydroisoquinolines and 3,4-dihydroisoquinolines, among the most important “privileged scaffolds”.

Phosphine-catalyzed dearomatizing [3+2] annulations of isoquinolinium methylides with allenes

A phosphine-catalyzed annulation provides an access to highly functionalized pyrroloisoquinolines with high regioselectivity.

Synthesis and Pharmacological Evaluation of DHβE Analogues as Neuronal Nicotinic Acetylcholine Receptor Antagonists

Dihydro-β-erythroidine (DHβE) is a member of the Erythrina family of alkaloids and a potent competitive antagonist of the α4β2-subtype of the nicotinic acetylcholine receptors (nAChRs). Guided by an X-ray structure of DHβE in complex with an ACh binding protein, we detail the design, synthesis, and pharmacological characterization of a series of DHβE analogues in which two of the four rings in the natural product has been excluded. We found that the direct analogue of DHβE maintains affinity for the α4β2-subtype, but further modifications of the simplified analogues were detrimental to their activities on the nAChRs.

Hyperbranched polyester nanorods with pyrrolo[2,1-a]isoquinoline end groups for fluorescent recognition of Fe3+

Novel hyperbranched polyester nanorods HBPE-CICA₆ and HBPE-CICA₂ were obtained and used to establish a highly sensitive fluorescent sensor for Fe³⁺ ions.