Nudicauline and elatine as potent norditerpenoid ligands at rat neuronal alpha-bungarotoxin binding sites: importance of the 2-(methylsuccinimido)benzoyl moiety for neuronal nicotinic acetylcholine receptor binding

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.

Synthesis and Antagonist Activity of Methyllycaconitine Analogues on Human α7 Nicotinic Acetylcholine Receptors

Methyllycaconitine (MLA), 1, is a naturally occurring norditerpenoid alkaloid that is a highly potent (IC₅₀ = 2 nM) selective antagonist of α7 nicotinic acetylcholine receptors (nAChRs). Several structural factors affect its activity such as the neopentyl ester side-chain and the piperidine ring N-side-chain. The synthesis of simplified AE-bicyclic analogues 14-21 possessing different ester and nitrogen side-chains was achieved in three steps. The antagonist effects of synthetic analogues were examined on human α7 nAChRs and compared to that of MLA 1. The most efficacious analogue (16) reduced α7 nAChR agonist responses [1 nM acetylcholine (ACh)] to 53.2 ± 1.9% compared to 3.4 ± 0.2% for MLA 1. This demonstrates that simpler analogues of MLA 1 possess antagonist effects on human α7 nAChRs but also indicates that further optimization may be possible to achieve antagonist activity comparable to that of MLA 1.

Norditerpenoid alkaloids from Aconitum and Delphinium: structural relevance in medicine, toxicology, and metabolism

Covering: 77 A.D. up to 2020Norditerpenoid alkaloids (NDA), typically N-ethylpiperidine containing C19 or C18 natural product diterpenes, are hexacycles with several contiguous often oxygenated stereocentres. As a function of their structural complexity, they display important pharmacological activities. The processed plants are used as important folk drugs and four NDAs have now been clinically approved. Many metabolism studies on Aconitum alkaloids have been reported as the understanding of their biotransformation in living systems and in cell-free systems is important for the development of these alkaloids as drugs. This Highlight sets out the missing links in NDA biosynthesis, their biological applications, SAR, toxicity, metabolism, and analytical studies.

The 1H NMR Spectroscopic Effect of Steric Compression Is Found in [3.3.1]Oxa- and Azabicycles and Their Analogues

The through-space 1H NMR effect of steric compression by the lone-pair electrons of O- and N-atoms is shown in synthetic [3.3.1]oxa- and azabicycles. The electrons of the compressed proton bond are pushed away by the repulsive force generated by the lone-pair electrons of the heteroatom. There is a corresponding significant increase in the chemical shift of the compressed proton. The intensity of this deshielding effect is related to the proximity and overlap of the lone-pair or compressing atom. The steric compression decreases when the lone-pair electrons of the heteroatom and the compressed proton are not directly overlapped, for example, in [4.3.1]- and [3.2.1]azabicycles. Steric compression is also caused by a proton, deuterium, or an ethyl group close in space to the compressed proton. The protonated [3.3.1]azabicycle adopts a true-boat/true-chair conformation in its crystal lattice, but in solution the conformation is true-chair/true-chair.

Screening a Natural Product-Inspired Library for Anti-Phytophthora Activities

Phytophthora is a genus of microorganisms that cause devastating dieback and root-rot diseases in thousands of plant hosts worldwide. The economic impact of Phytophthora diseases on crops and native ecosystems is estimated to be billions of dollars per annum. These invasive pathogens are extremely difficult to control using existing chemical means, and the effectiveness of the few treatments available is being jeopardized by increasing rates of resistance. There is an urgent need to identify new chemical treatments that are effective against Phytophthora diseases. Natural products have long been regarded as "Nature's medicine chest", providing invaluable leads for developing front-line drugs and agrochemical agents. Here, we have screened a natural product-inspired library of 328 chemicals against two key Phytophthora species: Phytophthora cinnamomi and Phytophthora agathidicida. The library was initially screened for inhibition of zoospore germination. From these screens, we identified twenty-one hits that inhibited germination of one or both species. These hits were further tested in mycelial growth inhibition studies to determine their half-maximal inhibitory concentrations (IC₅₀s). Four compounds had IC₅₀ values of approximately 10 µM or less, and our best hit had IC₅₀s of approximately 3 µM against both Phytophthora species tested. Overall, these hits may serve as promising leads for the development of new anti-Phytophthora agrochemicals.

Impacts of Steric Compression, Protonation, and Intramolecular Hydrogen Bonding on the 15N NMR Spectroscopy of Norditerpenoid Alkaloids and Their Piperidine-Ring Analogues

1H-15N HMBC spectra of norditerpenoid alkaloids and their synthetic azabicyclic analogues were obtained to investigate the impacts of the through-space effect of steric compression, protonation, and formation of intramolecular hydrogen bonding on the 15N NMR spectroscopy of these natural products and their piperidine-containing analogues. A rare 15N NMR effect of steric compression is demonstrated in half-cage A/E-rings of norditerpenoid alkaloid free bases and their synthetic azabicyclic analogues, in which the distribution of the lone pair of electrons of the tertiary amine N-atom is sterically restricted by bridged cycloalkanes, e.g., cyclopentane, cyclohexane, and cycloheptane rings. This results in significant changes in the 15N chemical shift, typically by at least ∼10 ppm. The lone pair of electrons of the N-atom in the piperidine ring are sterically compressed whether the bridged cyclohexane ring adopts a chair or boat conformation. The 15N chemical shifts of 1α-OMe norditerpenoid alkaloid free bases significantly increase (ΔδN ≥ 15.6 ppm) on alkaloid protonation and thence the formation of an intramolecular hydrogen bond between N +-H and 1α-OMe. The intramolecular hydrogen bonds between the N-atom and 1α-OH of 1α-OH norditerpenoid alkaloid free bases, karacoline, condelphine, and neoline stabilize their A-rings, adopting an unusual twisted-boat conformation, and they also significantly increase δN of the tertiary amine N-atom.

The chemistry and pharmacology of alkaloids and allied nitrogen compounds from Artemisia species: A review

Several reviews have been published on Artemisia's derived natural products, but it is the first attempt to review the chemistry and pharmacology of more than 80 alkaloids and allied nitrogen compounds obtained from various Artemisia species (covering the literature up to June 2018). The pharmacological potential and unique skeleton types of certain Artemisia's alkaloids provoke the importance of analyzing Artemisia species for bioactive alkaloids and allied nitrogen compounds. Among the various types of bioactive Artemisia's alkaloids, the main classes were the derivatives of rupestine (pyridine-sesquiterpene), lycoctonine (diterpene), pyrrolizidine, purines, polyamine, peptides, indole, piperidine, pyrrolidine, alkamides, and flavoalkaloids. The rupestine derivatives are Artemisia's characteristic alkaloids, whereas the rest are common alkaloids found in the family Asteraceae and chemotaxonomically links the genus Artemisia with the tribes Anthemideae. The most important biological activities of Artemisia's alkaloids are including hepatoprotective, local anesthetic, β-galactosidase, and antiparasitic activities; treatment of angina pectoris, opening blocked arteries, as a sleep-inducing agents and inhibition of HIV viral protease, CYP450, melanin biosynthesis, human carbonic anhydrase, [3H]-AEA metabolism, kinases, and DNA polymerase β¹ . Some of the important nitrogen metabolites of Artemisia include pellitorine, zeatin, tryptophan, rupestine, and aconitine analogs, which need to be optimized and commercialized further.

Identification and characterization of antibacterial compound(s) of cockroaches (Periplaneta americana)

Infectious diseases remain a significant threat to human health, contributing to more than 17 million deaths, annually. With the worsening trends of drug resistance, there is a need for newer and more powerful antimicrobial agents. We hypothesized that animals living in polluted environments are potential sources of antimicrobials. Under polluted milieus, organisms such as cockroaches encounter different types of microbes, including superbugs. Such creatures survive the onslaught of superbugs and are able to ward off disease by producing antimicrobial substances. Here, we characterized antibacterial properties in extracts of various body organs of cockroaches (Periplaneta americana) and showed potent antibacterial activity in crude brain extract against methicillin-resistant Staphylococcus aureus and neuropathogenic Escherichia coli K1. The size-exclusion spin columns revealed that the active compound(s) are less than 10 kDa in molecular mass. Using cytotoxicity assays, it was observed that pre-treatment of bacteria with lysates inhibited bacteria-mediated host cell cytotoxicity. Using spectra obtained with LC-MS on Agilent 1290 infinity liquid chromatograph, coupled with an Agilent 6460 triple quadruple mass spectrometer, tissues lysates were analysed. Among hundreds of compounds, only a few homologous compounds were identified that contained the isoquinoline group, chromene derivatives, thiazine groups, imidazoles, pyrrole-containing analogs, sulfonamides, furanones, and flavanones and known to possess broad-spectrum antimicrobial properties and anti-inflammatory, anti-tumour, and analgesic properties. Further identification, characterization, and functional studies using individual compounds can act as a breakthrough in developing novel therapeutics against various pathogens including superbugs.

Diterpene alkaloids with an aza-ent-kaurane skeleton from Isodon rubescens

Two compounds belonging to a new group of diterpene alkaloids, kaurines A and B (1 and 2), and an alkaloid bearing a succinimide moiety (3) were obtained from Isodon rubescens. Their structures and absolute configurations were determined by spectroscopy and quantum-chemical computational (13)C NMR and ECD data analysis. These alkaloids differ from known diterpene alkaloids and diterpenoids and are presumably biosynthesized from ent-kaurane diterpenoids.

Crystal structure of 1-[2,4-bis(4-methoxy-phenyl)-3-azabicyclo[3.3.1]nonan-3-yl]ethanone

In the title compound, C24H29NO3, the aza-bicycle contains two six-membered rings, viz. a cyclo-hexane ring and a piperidine ring. The first adopts a chair conformation and the second a half-chair conformation. The dihedral angle between their mean planes is 86.21 (13)°, indicating that they are almost perpendicular to one another. The dihedral angle between the planes of the 4-meth-oxy-phenyl rings is 17.51 (13)°, and they make dihedral angles of 81.9 (3) and 81.3 (3)° with the ethan-1-one group. In the crystal, mol-ecules are linked by C-H⋯π inter-actions forming chains along [10-1].

A review on phytochemistry, pharmacology and toxicology studies of Aconitum

Objectives

A number of species belonging to herbal genus Aconitum are well-known and popular for their medicinal benefits in Indian, Vietnamese, Korean, Japanese, Tibetan and Chinese systems of medicine. It is a valuable drug as well as an unpredictable toxic material. It is therefore imperative to understand and control the toxic potential of herbs from this genus. In this review, the ethnomedicinal, phytochemistry, pharmacology, structure activity relationship and toxicology studies of Aconitum were presented to add to knowledge for their safe application.

Key findings

A total of about 76 of all aconite species growing in China and surrounding far-east and Asian countries are used for various medical purposes. The main ingredients of aconite species are alkaloids, flavonoids, free fatty acids and polysaccharides. The tuberous roots of genus Aconitum are commonly applied for various diseases such as rheumatic fever, painful joints and some endocrinal disorders. It stimulates the tip of sensory nerve fibres. These tubers of Aconitum are used in the herbal medicines only after processing. There remain high toxicological risks of the improper medicinal applications of Aconitum. The cardio and neurotoxicities of this herb are potentially lethal. Many analytical methods have been reported for quantitatively and qualitatively characterization of Aconitum.

Summary

Aconitum is a plant of great importance both in traditional medicine in general and in TCM in particular. Much attention should be put on Aconitum because of its narrow therapeutic range. However, Aconitum's toxicity can be reduced using different techniques and then benefit from its pharmacological activities. New methods, approaches and techniques should be developed for chemical and toxicological analysis to improve its quality and safety.

Crystal structure of methyl 2-[2,4-bis-(4-fluoro-phen-yl)-3-aza-bicyclo-[3.3.1]nonan-9-yl-idene]hydrazine-carboxyl-ate

In the title compound, C22H23F2N3O2, the bicyclic ring system exists in a twin-chair conformation with an equatorial disposition of the 4-fluoro-phenyl groups on the heterocycle. These aromatic rings are inclined to one another by 19.4 (1)°. In the crystal, mol-ecules are linked by pairs of N-H⋯O and C-H⋯O hydrogen bonds into inversion dimers, incorporating R 1 (2)(7) and R 2 (2)(8) ring motifs; the same O atom accepts both hydrogen bonds. These dimers are further linked by a pair of C-H⋯F hydrogen bonds, enclosing R 2 (2)(28) ring motifs, forming supra-molecular chains along [010]. The NH group of the pyridine ring is not involved in hydrogen bonding, probably due to the steric hindrance of the fluoro-phenyl groups.

Antimicrobial evaluation of a set of heterobicyclic methylthiadiazole hydrazones: synthesis, characterization, and SAR studies

To exploit the potential antimicrobial activities of azabicyclic skeleton based compounds, a set of 2r,4c-diaryl-3-azabicyclo[3.3.1]nonan-9-one-4-methyl-1,2,3-thiadazole-5-carbonyl hydrazones were synthesized. Unambiguous structural elucidation has been carried out by investigating IR, H(1), C(13) NMR, and elemental analysis. 2D NMR spectra ((1)H-(1)H COSY, HSQC, HMBC, and NOESY) were recorded for a representative compound, 12, to confirm the proposed structure for 9-15. Antimicrobial activity assessment of synthesized hydrazones 9-15 has been evaluated by screening against selective strains. Both bacteria and fungi of various forms along with standard drug have been taken for the analysis. Difference in the potency of activity against the strains has been evaluated on the basis of SAR, and it has been revealed that substitution of electron-withdrawing halogens (chloro, fluoro, and bromo) at para positions of the phenyl (10, 12, and 13) enhanced the antifungal and antibacterial activities against tested organisms compared to other hydrazone derivatives.

Plant toxins that affect nicotinic acetylcholine receptors: a review

Plants produce a wide variety of chemical compounds termed secondary metabolites that are not involved in basic metabolism, photosynthesis, or reproduction. These compounds are used as flavors, fragrances, insecticides, dyes, hallucinogens, nutritional supplements, poisons, and pharmaceutical agents. However, in some cases these secondary metabolites found in poisonous plants perturb biological systems. Ingestion of toxins from poisonous plants by grazing livestock often results in large economic losses to the livestock industry. The chemical structures of these compounds are diverse and range from simple, low molecular weight toxins such as oxalate in halogeton to the highly complex norditerpene alkaloids in larkspurs. While the negative effects of plant toxins on people and the impact of plant toxins on livestock producers have been widely publicized, the diversity of these toxins and their potential as new pharmaceutical agents for the treatment of diseases in people and animals has also received widespread interest. Scientists are actively screening plants from all regions of the world for bioactivity and potential pharmaceuticals for the treatment or prevention of many diseases. In this review, we focus the discussion to those plant toxins extensively studied at the USDA Poisonous Plant Research Laboratory that affect the nicotinic acetylcholine receptors including species of Delphinium (Larkspurs), Lupinus (Lupines), Conium (poison hemlock), and Nicotiana (tobaccos).

2,4-Bis(furan-2-yl)-1,5-dimethyl-3-aza-bicyclo-[3.3.1]nonan-9-one

In the title compound, C18H21NO3, the bicyclic ring system adopts a twin-chair conformation. The two methyl groups attached to the bicycle are in an equatorial orientation for both rings. One of the furan rings is disordered over two orientations with an occupancy ratio of 0.686 (6):0.314 (6). In the crystal, very long N-H⋯O hydrogen bonds connect the mol-ecules into a chain perpendicular to the ac plane.

Antioxidant properties of Mannich bases

The biological importance of antioxidants influenced to synthesize some curcumin-related compounds as potential antioxidants. Accordingly, a series of 2,4-diaryl-3-azabicyco[3.3.1]nonan-9-ones were synthesized with polyphenolic and/or polymethoxyphenyl groups by modified Mannich condensations. The yield was significantly improved using BF(3)·SiO(2) as heterogeneous catalyst under mild conditions. Stereochemistry of all the synthesized compounds was established as twin-chair with an equatorial disposition of the aryl groups, through their NMR and XRD interpretations. The ABNs 8 (curcumin analog) and 10 (bis-demethoxycurcumin analog) showed an effective profile over curcumin, α-tocopherol, and vitamin C by chemical methods. Further, the efficiency of one of the active molecules, ABN 10, was demonstrated by its intracellular ROS inhibition activity on RAW 264.7 macrophage cells by FACS analysis in dose-dependent manner.

Identifying the binding site of novel methyllycaconitine (MLA) analogs at α4β2 nicotinic acetylcholine receptors

Neuronal nicotinic acetylcholine receptors (nAChR) are ligand gated ion channels that mediate fast synaptic transmission. Methyllycaconitine (MLA) is a selective and potent antagonist of the α7 nAChR, and its anthranilate ester side-chain is important for its activity. Here we report the influence of structure on nAChR inhibition for a series of novel MLA analogs, incorporating either an alcohol or anthranilate ester side-chain to an azabicyclic or azatricyclic core against rat α7, α4β2, and α3β4 nAChRs expressed in Xenopus oocytes. The analogs inhibited ACh (EC(50)) within an IC(50) range of 2.3-26.6 μM. Most displayed noncompetitive antagonism, but the anthranilate ester analogs exerted competitive behavior at the α7 nAChR. At α4β2 nAChRs, inhibition by the azabicyclic alcohol was voltage-dependent suggesting channel block. The channel-lining residues of α4 subunits were mutated to cysteine and the effect of azabicyclic alcohol was evaluated by competition with methanethiosulfonate ethylammonium (MTSEA) and a thiol-reactive probe in the open, closed, and desensitized states of α4β2 nAChRs. The azabicyclic alcohol was found to compete with MTSEA between residues 6' and 13' in a state-dependent manner, but the reactive probe only bonded with 13' in the open state. The data suggest that the 13' position is the dominant binding site. Ligand docking of the azabicyclic alcohol into a (α4)(3)(β2)(2) homology model of the closed channel showed that the ligand can be accommodated at this location. Thus our data reveal distinct pharmacological differences between different nAChR subtypes and also identify a specific binding site for a noncompetitive channel blocker.

2,4-Bis(4-chloro-phen-yl)-1-methyl-3-aza-bicyclo-[3.3.1]nonan-9-one

The title compound, C₂₁H₂₁Cl₂NO, exists in a twin-chair conformation with an equatorial orientation of the 4-chloro­phenyl groups on both sides of the secondary amino group; the dihedral angle between the 4-chloro­phenyl rings is 36.58 (2)°. The crystal packing is stabilized by an inter­molecular N—H⋯O hydrogen bond and a weak Cl⋯Cl [3.4331 (9) Å] inter­action.

2,4-Bis(2-fluoro-phen-yl)-1-methyl-3-aza-bicyclo-[3.3.1]nonan-9-one

The crystal structure of the title compound, C₂₁H₂₁F₂NO, shows that the compound exists in a twin-chair conformation with an equatorial orientation of the ortho-fluoro­phenyl groups on either side of the secondary amino group. The title compound is a 1-methyl­ated analog of 2,4-bis­(2-fluoro­phen­yl)-3-aza­bicyclo­[3.3.1]nonan-9-one; the two compound both exhibit the same stereochemistry but the orientation of the ortho-fluoro­phenyl rings differs slightly. In the title compound, the rings are orientated at a dihedral angle of 36.70 (3)° with respect to one another, whereas in the non-methyl analog, the angle is 25.68 (4)°. The crystal structure of the title compound is stabilized by an inter­molecular N—H⋯π inter­action and a weak C—H⋯F inter­action.

2,4-Bis(3-methoxy-phen-yl)-3-aza-bicyclo-[3.3.1]nonan-9-one

In the crystal structure, the title compound, C₂₂H₂₅NO₃, exists in a twin-chair conformation with equatorial orientations of the meta-methoxy­phenyl groups on both sides of the secondary amino group. The title compound is a positional isomer of 2,4-bis­(2-methoxy­phen­yl)-3-aza­bicyclo­[3.3.1]nonan-9-one and 2,4-bis­(4-methoxy­phen­yl)-3-aza­bicyclo­[3.3.1]nonan-9-one, which both also exhibit twin-chair conformations with equatorial dispositions of the anisyl rings on both sides of the secondary amino group. In the title compound, the meta-methoxy­phenyl rings are orientated at an angle of 25.02 (3)° with respect to each other, whereas in the ortho and para isomers, the anisyl rings are orientated at dihedral angles of 33.86 (3) and 37.43 (4)°, respectively. The crystal packing is dominated by van der Waals inter­actions and by an inter­molecular N—H⋯O hydrogen bond, whereas in the ortho isomer, an inter­molecular N—H⋯π inter­action (H⋯Cg = 2.75 Å) is found.

1-Methyl-2,4-bis-(2-methoxy-phen-yl)-3-aza-bicyclo-[3.3.1]nonan-9-one

The crystal structure of the title compound, C(23)H(27)NO(3), shows that the compound exists in a chair-chair conformation with an equatorial disposition of 2-methoxy-phenyl groups at an angle of 39.94 (3)° with respect to each other. An inter-molecular N-H⋯π inter-action is observed in the crystal packing.

2,4-Bis(2-fluoro-phen-yl)-3-aza-bicyclo-[3.3.1]nonan-9-one

The title compound, C₂₀H₁₉F₂NO, exists in a twin-chair conformation with an equatorial orientation of the two 2-fluoro­phenyl groups on both sides of the secondary amine group. The benzene rings are orientated at an angle of 25.68 (4)° with respect to one another and the F atoms point upwards (towards the carbonyl group). The crystal is stabilized by an inter­molecular N—H⋯π inter­action.

2,4-Bis(2-methoxy-phenyl)-3-aza-bicyclo-[3.3.1]nonan-9-one

In the title compound, C(22)H(25)NO(3), the mol-ecule has a pseudo-mirror plane. The structure is a positional isomer of 2,4-bis(4-methoxy-phenyl)-3-aza-bicyclo-[3.3.1]nonan-9-one [Cox, McCabe, Milne & Sim (1985 ▶). J. Chem. Soc. Chem. Commun. pp. 626-628]. The 3-aza-bicyclo-[3.3.1]nonan-9-one moiety adopts a double chair conformation with equatorial orientations of both 2-methoxy-phenyl substituents on either side of the secondary amino group. The benzene rings are oriented at an angle of 33.86 (4)° with respect to each other and the meth-oxy groups point towards the carbonyl group. The crystal structure is stabilized by intermolecular N-H⋯π inter-actions.

2,4-Bis(4-bromo-phen-yl)-3-aza-bicyclo-[3.3.1]nonan-9-one

The title compound, C₂₀H₁₉Br₂NO, shows a chair–chair conformation for the aza­bicycle with an equatorial disposition of the 4-bromo­phenyl groups [dihedral angle between the aromatic rings = 16.48 (3)°]. In the crystal, a short Br⋯Br contact [3.520 (4) Å] occurs and the structure is further stabilized by N—H⋯O hydrogen bonds and C—H⋯O inter­actions.

2,4-Bis(4-chloro-phen-yl)-3-aza-bicyclo-[3.3.1]nonan-9-one

In the mol­ecular structure of the title compound, C₂₀H₁₉Cl₂NO, the mol­ecule exists in a twin-chair conformation with equatorial dispositions of the 4-chloro­phenyl groups on both sides of the secondary amino group; the dihedral angle between the aromatic ring planes is 31.33 (3)°. The crystal structure is stabilized by N—H⋯O inter­actions, leading to chains of molecules.

APPLICATION OF PREPARATIVE HIGH-SPEED COUNTERCURRENT CHROMATOGRAPHY FOR SEPARATION OF ELATINE FROM DELPHINIUM SHAWURENSE

Preparative separation of elatine in Delphinium shawurense was achieved for the first time using high-speed countercurrent chromatography (HSCCC). The separation was performed with a solvent system composed of ethyl acetate-chloroform-methanol-water (3:0.1:2:3, v/v) using the lower organic phase as a mobile phase under a revolution speed of 800 rpm. This yielded 72 mg of elatine at over 97% purity with an approximately 95% recovery. The chemical structure was identified by MS and NMR.

Synthesis, nicotinic acetylcholine receptor binding, antinociceptive and seizure properties of methyllycaconitine analogs

A series of methyllycaconitine (1a, MLA) analogs was synthesized where the (S)-2-methylsuccinimidobenzoyl group in MLA was replaced with a (R)-2-methyl, 2,2-dimethyl-, 2,3-dimethyl, 2-phenyl-, and 2-cyclohexylsuccinimidobenzoyl (1b-f) group. The analogs 1b-f were evaluated for their inhibition of [(125)I]iodo-MLA binding at rat brain alpha7 nicotinic acetylcholine receptors (nAChR). In order to determine selectivity, MLA and the analogs 1b-f were evaluated for inhibition of binding to rat brain alpha,beta nAChR using [(3)H]epibatidine. At the alpha7 nAChR, MLA showed a K(i) value of 0.87 nM, analogs 1b-e possessed K(i) values of 1.67-2.16 nM, and 1f showed a K(i) value of 26.8 nM. Surprisingly, the analog 1e containing the large phenyl substituent (K(i)=1.67 nM) possessed the highest affinity. None of the compounds possessed appreciable affinity for alpha,beta nAChRs. MLA antagonized nicotine-induced seizures with an AD(50)=2 mg/kg. None of the MLA analogs were as potent as MLA in this assay. MLA and all of the MLA analogs, with the exception of 1b, antagonized nicotine's antinociceptive effects in the tail-flick assay. Compound 1c (K(i)=1.78 nM at alpha7 nAChR) with an AD(50) value of 1.8 mg/kg was 6.7 times more potent than MLA (AD(50)=12 mg/kg) in antagonizing nicotine's antinociceptive effects but was 5-fold less potent than MLA in blocking nicotine-induced seizures. Since MLA has been reported to show neuroprotection against beta-amyloid(1-42), these new analogs which have high alpha7 nAChR affinity and good selectivity relative to alpha,beta nAChRs will be useful biological tools for studying the effects of alpha7 nAChR antagonist and neuroprotection.

Methyllycaconitine analogues have mixed antagonist effects at nicotinic acetylcholine receptors

Bicyclic analogues of methyllycaconitine (MLA), such as 12, have been synthesised that incorporate the C1-OMe substituent present in the natural product. Electrophysiology experiments using Xenopus oocytes expressing nicotinic acetylcholine receptors (nAChRs) were conducted on these analogues and a related tricyclic analogue 2. The most potent compound, 2, was an antagonist at all receptors studied but displayed different antagonist effects at each receptor subtype. This study more clearly defines the biological effects of MLA analogues at nAChRs and demonstrates that these analogues are not selective ligands for the alpha7 nAChR subtype.

Nicotinic Agonists, Antagonists, and Modulators From Natural Sources

1. Acetylcholine receptors were initially defined as nicotinic or muscarinic, based on selective activation by two natural products, nicotine and muscarine. Several further nicotinic agonists have been discovered from natural sources, including cytisine, anatoxin, ferruginine, anabaseine, epibatidine, and epiquinamide. These have provided lead structures for the design of a wide range of synthetic agents. 2. Natural sources have also provided competitive nicotinic antagonists, such as the Erythrina alkaloids, the tubocurarines, and methyllycaconitine. Noncompetitive antagonists, such as the histrionicotoxins, various izidines, decahydroquinolines, spiropyrrolizidine oximes, pseudophrynamines, ibogaine, strychnine, cocaine, and sparteine have come from natural sources. Finally, galanthamine, codeine, and ivermectin represent positive modulators of nicotinic function, derived from natural sources. 3. Clearly, research on acetylcholine receptors and functions has been dependent on key natural products and the synthetic agents that they inspired.

Structural diversity and defensive properties of norditerpenoid alkaloids

We have tested the insect antifeedant and toxic activity of 43 norditerpenoid alkaloids on Spodoptera littoralis and Leptinotarsa decemlineata including eserine (physostigmine), anabasine, and atropine. Antifeedant effects of the test compounds were structure- and species-dependent. The most active antifeedants to L. decemlineata were 1,14-diacetylcardiopetaline (9) and 18-hydroxy- 14-O-methylgadesine (33), followed by 8-O-methylconsolarine (12), 14-O-acetyldelectinine (27), karakoline (7), cardiopetaline (8), 18-O-demethylpubescenine (13), 14-O-acetyldeltatsine (18), takaosamine (21), ajadine (24), and 8-O-methylcolumbianine (6) (EC50 < 1 microg/cm2). This insect showed a moderate response to atropine. S. littoralis had the strongest antifeedant response to 24, 18, 14-O-acetyldelcosine (19), and delphatine (29) (EC50 < 3 microg/cm2). None of the model substances affected the feeding behavior of this insect. The most toxic compound to L. decemlineata was aconitine (1), followed by cardiopetalidine (10) (% mortality > 60), 14-deacetylpubescenine (14), 18-O-benzoyl-18-O-demethyl-14-O-deacetylpubescenine (17), 14-O-acetyldelcosine (19), 14-deacetylajadine (25) and methyllycaconitine (30) (% mortality > 45). Orally injected S. littoralis larvae were negatively affected by 1, cardiopetaline (8), 10, 1,14-O-acetylcardiopetalidina (11), 12, 14, 1,18-O-diacetyl-19-oxo-gigactonine (41), olivimine (43), and eserine in varying degrees. Their antifeedant or insecticidal potencies did not parallel their reported nAChR binding activity, but did correlate with the agonist/antagonist insecticidal/antifeedant model proposed for nicotininc insecticides. A few compounds [14, tuguaconitine (38), 14-demethyldelboxine (40), 19, dehydrodelsoline (36), 18-O-demethylpubescenine (13), 41, 9, and delcosine (23)] had selective cytotoxic effects to ward insect-derived Sf9 cells. None were cytotoxic to mammalian CHO cells and none increased Trypanosoma cruzi mortality. The selective cytotoxic effects of some structures indicate that they can act on biological targets other than neuroreceptors.

Cholinergic nicotinic receptors: competitive ligands, allosteric modulators, and their potential applications

Discovery of the important role played by nicotinic acetylcholine receptors (nAChRs) in several CNS disorders has called attention to these membrane proteins and to ligands able to modulate their functions. The existence of different subtypes at multiple levels has complicated the understanding of this receptor's physiological role, but at the same time has increased the efforts to discover selective compounds in order to improve the pharmacological characterization of this kind of receptor and to make the possible therapeutical use of its modulators safer. This review focuses on the structure of new ligands for nAChRs, agonists, antagonists and allosteric modulators, and on their possible applications.

Effects of methyllycaconitine and related analogues on bovine adrenal alpha3beta4* nicotinic acetylcholine receptors

Adrenal secretion and binding studies were performed using ring E analogues of methyllycaconitine to assess structural determinants affecting activity on bovine adrenal alpha3beta4* nicotinic receptors. The most potent analogues are as potent as many inhibitors of adrenal secretion. Our data support the potential use of methyllycaconitine analogues to generate nicotinic receptor subtype-specific compounds.