Synthesis and Biological Evaluation of (−)- and (+)-Debromoflustramine B and Its Analogues as Selective Butyrylcholinesterase Inhibitors

Naturally Occurring Organohalogen Compounds-A Comprehensive Review

The present volume is the third in a trilogy that documents naturally occurring organohalogen compounds, bringing the total number-from fewer than 25 in 1968-to approximately 8000 compounds to date. Nearly all of these natural products contain chlorine or bromine, with a few containing iodine and, fewer still, fluorine. Produced by ubiquitous marine (algae, sponges, corals, bryozoa, nudibranchs, fungi, bacteria) and terrestrial organisms (plants, fungi, bacteria, insects, higher animals) and universal abiotic processes (volcanos, forest fires, geothermal events), organohalogens pervade the global ecosystem. Newly identified extraterrestrial sources are also documented. In addition to chemical structures, biological activity, biohalogenation, biodegradation, natural function, and future outlook are presented.

Synthesis of New 2,3-Dihydroindole Derivatives and Evaluation of Their Melatonin Receptor Binding Affinity

2,3-Dihydroindoles are promising agents for the synthesis of new compounds with neuroprotective and antioxidant properties. Usually, these compounds are obtained by direct reduction of the corresponding indoles containing acceptor groups in the indole ring for its activation. In this work, we propose a synthetic strategy to obtain new 2,3-dihydroindole derivatives from the corresponding polyfunctional 2-oxindoles. Three methods were proposed for reduction of functional groups in the 2-oxindole and 2-chloroindole molecules using various boron hydrides. The possibility of chemoselective reduction of the nitrile group in the presence of an amide was shown. The proposed synthetic strategy can be used, for example, for the synthesis of new analogs of the endogenous hormone melatonin and other compounds with neuroprotective properties.

A Multi-layered Variable Selection Strategy for QSAR Modeling of Butyrylcholinesterase Inhibitors

Background:

Alzheimer’s disease (AD), a neurological disorder, is the most common cause of senile dementia. Butyrylcholinesterase (BuChE) enzyme plays a vital role in regulating the brain acetylcholine (ACh) neurotransmitter, but in the case of Alzheimer’s disease (AD), BuChE activity gradually increases in patients with a decrease in the acetylcholine (ACh) concentration via hydrolysis. ACh plays an essential role in regulating learning and memory as the cortex originates from the basal forebrain, and thus, is involved in memory consolidation in these sites.

Methods:

In this work, we have developed a partial least squares (PLS)-regression based two dimensional quantitative structure-activity relationship (2D-QSAR) model using 1130 diverse chemical classes of compounds with defined activity against the BuChE enzyme. Keeping in mind the strict Organization for Economic Co-operation and Development (OECD) guidelines, we have tried to select significant descriptors from the large initial pool of descriptors using multi-layered variable selection strategy using stepwise regression followed by genetic algorithm (GA) followed by again stepwise regression technique and at the end best subset selection prior to development of final model thus reducing noise in the input. Partial least squares (PLS) regression technique was employed for the development of the final model while model validation was performed using various stringent validation criteria.

Results:

The results obtained from the QSAR model suggested that the quality of the model is acceptable in terms of both internal (R2= 0.664, Q2= 0.650) and external (R2 Pred= 0.657) validation parameters. The QSAR studies were analyzed, and the structural features (hydrophobic, ring aromatic and hydrogen bond acceptor/donor) responsible for enhancement of the activity were identified. The developed model further suggests that the presence of hydrophobic features like long carbon chain would increase the BuChE inhibitory activity and presence of amino group and hydrazine fragment promoting the hydrogen bond interactions would be important for increasing the inhibitory activity against BuChE enzyme.

Conclusion:

Furthermore, molecular docking studies have been carried out to understand the molecular interactions between the ligand and receptor, and the results are then correlated with the structural features obtained from the QSAR models. The information obtained from the QSAR models are well corroborated with the results of the docking study.

Discovery of the cryptic function of terpene cyclases as aromatic prenyltransferases

Catalytic versatility is an inherent property of many enzymes. In nature, terpene cyclases comprise the foundation of molecular biodiversity as they generate diverse hydrocarbon scaffolds found in thousands of terpenoid natural products. Here, we report that the catalytic activity of the terpene cyclases AaTPS and FgGS can be switched from cyclase to aromatic prenyltransferase at basic pH to generate prenylindoles. The crystal structures of AaTPS and FgGS provide insights into the catalytic mechanism of this cryptic function. Moreover, aromatic prenyltransferase activity discovered in other terpene cyclases indicates that this cryptic function is broadly conserved among the greater family of terpene cyclases. We suggest that this cryptic function is chemoprotective for the cell by regulating isoprenoid diphosphate concentrations so that they are maintained below toxic thresholds.

Anti-tyrosinase, anti-cholinesterase and cytotoxic activities of extracts and phytochemicals from the Tunisian Citharexylum spinosum L.: Molecular docking and SAR analysis

Previously phytochemical investigations carried out on the flowers and trunk bark extracts of Citharexylum spinosum L. tree, allowed the isolation of twenty molecules belonging to several families of natural substances [triterpene acids, iridoid glycosides, phenylethanoid glycosides, 8,3'-neolignan glycosides, together with other phenolic compounds]. In the present work, a biological evaluation (anti-tyrosinase, anticholinesterase and cytotoxic activities) was performed on the prepared extracts and the isolated secondary metabolites. The results showed that the EtOAc extract of the trunk bark displayed the highest anti-tyrosinase effect with a percent inhibition of 55.0 ± 1.8% at a concentration of 100 µg/mL. The highest anticholinesterase activity was presented by the same extract with an IC₅₀ value of 99.97 ± 3.01 µg/mL. The EtOAc extract of flowers and that of the trunk bark displayed the best cytotoxic property with IC₅₀ values of 96.00 ± 2.85 and 88.75 ± 2.00 µg/mL, respectively, against the human cervical cancer cell line (HeLa), and IC₅₀ values of 188.23 ± 3.88 and 197.00 ± 4.25 µg/mL, respectively, against the human lung cancer (A549) cell lines. Biological investigation of the pure compounds showed that the two 8,3'-neolignan glycosides, plucheosides D₁-D₂, generate the highest anti-tyrosinase potency with a percent inhibition of 61.4 ± 2.0 and 79.5 ± 2.3%, respectively, at a concentration of 100 µM. The iridoid glycosides exhibited a significant anticholinesterase activity with IC₅₀ values ranging from 17.19 ± 1.02 to 52.24 ± 2.50 µM. Triterpene pentacyclic acids and iridoid glycosides exerted encouraging cytotoxic effects against HeLa with IC₅₀ values ranging from 9.00 ± 1.10 to 25.00 ± 1.00 µM. The study of the structure-activity relationship (SAR) has been sufficiently and widely discussed. The natural compounds that exhibited the significant bioactivities were docked.

Cyanomethylation of Substituted Fluorenes and Oxindoles with Alkyl Nitriles

The first example of metal-free cyanomethylenation from alkyl nitriles of sp³ C-H bonds to afford quaternary carbon centers is described. This oxidative protocol is operationally simple and features good functional group compatibility. This method provides a novel approach to highly functionalized fluorene and oxindole derivatives, which are commonly used in material and pharmaceutical areas. Control experiments provide evidence of a radical reaction process.

Access to Enantioenriched Spiro-ϵ-Lactam Oxindoles by an N-Heterocyclic Carbene-Catalyzed [4+3] Annulation of Flexible Oxotryptamines with Enals

Oxotryptamines were firstly used as flexible four-atom synthons in an NHC-catalyzed formal [4+3] annulation, providing a novel enantioselective method to access structurally diverse spiro-ϵ-lactam oxindoles with excellent enantioselectivities. This metal-free reaction features a broad substrate scope, excellent functional-group tolerance and proceeds under mild reaction conditions. Importantly, enantiopure privileged hexahydropyrroloindoles could be easily constructed by a one-pot process from the resulting spiro-ϵ-lactam oxindoles.

Natural cholinesterase inhibitors from marine organisms

Covering: Published between 1974 up to 2018Inhibition of cholinesterases is a common approach for the management of several disease states. Most notably, cholinesterase inhibitors are used to alleviate the symptoms of neurological disorders like dementia and Alzheimer's disease and treat myasthenia gravis and glaucoma. Historically, most drugs of natural origin have been isolated from terrestrial sources and inhibitors of cholinesterases are no exception. However, the last 50 years have seen a rise in the quantity of marine natural products with close to 25 000 reported in the scientific literature. A number of marine natural products with potent cholinesterase inhibitory properties have also been reported; isolated from a variety of marine sources from algae to ascidians. Representing a diverse range of structural classes, these compounds provide inspirational leads that could aid the development of therapeutics. The current paper aims to, for the first time, comprehensively summarize the literature pertaining to cholinesterase inhibitors derived from marine sources, including the first papers published in 1974 up to 2018. The review does not report bioactive extracts, only isolated compounds, and a specific focus lies on compounds with reported dose-response data. In vivo and mechanistic data is included for compounds where this is reported. In total 185 marine cholinesterase inhibitors and selected analogs have been identified and reported and some of the compounds display inhibitory activities comparable or superior to cholinesterase inhibitors in clinical use.

Synthesis of 3,3-Disubstituted Heterocycles by Pd-Catalyzed Arylallylation of Unactivated Alkenes

Finding new methods of carbon-carbon bond formation is a key goal in expanding current methodology for heterocycle formation. Because of their inherently nonplanar shape, new methods of forming sp³-rich scaffolds are of particular importance. Although there are methods for combining heterocyclization and formation of new sp³-sp³ carbon-carbon bonds, these form the carbon-heteroatom bond rather than a carbon-carbon bond of the heterocycle. Here, we show a new alkene arylallylation reaction that generates a heterocycle with concomitant formation of two new carbon-carbon bonds. Furthermore, we demonstrate that this process occurs through an isohypsic (redox neutral) mechanism. Overall, this carboallylation reaction gives a new route to the synthesis of 3,3-disubstituted heterocycles.

Access to 3-Prenylated Oxindoles by α-Regioselective Prenylation: Application to the Synthesis of (±)-Debromoflustramine E

The development of a rapid, highly efficient, and one-pot synthesis of C3-α-prenylated oxindoles with simple reagents is described. The process is based on zinc-mediated α-regioselective prenylation of 3-acylidene-oxindole with commercially available prenyl bromide using inexpensive CeCl₃ as the catalyst. The new transformation tolerates a wide range of 3-acylidene-oxindoles, providing easy access to a variety of functionalized 3-prenylated oxindoles. The synthetic utility of the approach is verified by formal synthesis of the flustramine family alkaloid (±)-debromoflustramine E.

Reductive ortho C–H cyanoalkylation of aryl(heteroaryl) sulfoxides: a general approach to α-aryl(heteroaryl) nitriles

A catalyst-free low-temperature cyanoalkylation has been developed as a general protocol for the synthesis of α-aryl(heteroaryl) nitriles.

Development of the Regiodivergent Asymmetric Prenylation of 3-Substituted Oxindoles

This paper describes our efforts to design a Pd-catalyzed asymmetric prenylation of 3-substituted oxindoles that affords access to both the linear and reverse-prenylated products. Both 3-alkyl- and 3-aryloxindoles performed well under our optimized reaction conditions. The regiodivergent alkylation of monoterpene-derived electrophiles using this methodology was also investigated. The utility of this methodology in natural product synthesis was demonstrated through the efficient total syntheses of four Flustra alkaloids, which also allowed the absolute stereochemistry of the prenylated oxindole products to be assigned. Surprisingly, the same enantiomer of ligand produced linear and branched regioisomers of opposite chirality.

Synthesis and Antimicrobial Characterization of Half-Calycanthaceous Alkaloid Derivatives

A total of 29 novel tetrahydropyrroloindol-based calycanthaceous alkaloid derivatives were synthesized from indole-3-acetonitrile in good yields. The synthesized compounds were evaluated against nine strains of bacteria and a wide range of plant pathogen fungi. Bioassay results revealed that majority of the compounds displayed similar or higher in vitro antimicrobial activities than the positive control. The biological activities also indicated that substituents at R₄ and R₅ significantly affect the activities. Notably, compound c4 was found to be most active among the tested calycanthaceous analogues and might be a novel potential leading compound for further development as an antifungal agent. The results could pave the way for further design and structural modification of calycanthaceous alkaloids as antimicrobial agents.

Marine Indole Alkaloids

Marine indole alkaloids comprise a large and steadily growing group of secondary metabolites. Their diverse biological activities make many compounds of this class attractive starting points for pharmaceutical development. Several marine-derived indoles were found to possess cytotoxic, antineoplastic, antibacterial and antimicrobial activities, in addition to the action on human enzymes and receptors. The newly isolated indole alkaloids of marine origin since the last comprehensive review in 2003 are reported, and biological aspects will be discussed.

Metal-free direct cyanoisopropylation/arylation of N-arylacrylamides or N-alkyl-N-(arylsulfonyl)acrylamides with AIBN: a simple and mild approach to cyano-containing oxindoles

A general, highly efficient approach to cyano-containing oxindoles is presented that is achieved by a metal-free direct cyanoisopropylation/arylation ofN-arylacrylamides orN-alkyl-N-(arylsulfonyl)acrylamides with AIBN.

Natural products from marine organisms with neuroprotective activity in the experimental models of Alzheimer's disease, Parkinson's disease and ischemic brain stroke: their molecular targets and action mechanisms

Continuous increases in the incidence of neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), and brain stroke demand the urgent development of therapeutics. Marine organisms are well-known producers of natural products with diverse structures and pharmacological activities. Therefore, researchers have endeavored to identify marine natural products with neuroprotective effects. In this regard, this review summarizes therapeutic targets for AD, PD, and ischemic brain stroke and marine natural products with pharmacological activities on the targets according to taxonomies of marine organisms. Furthermore, several marine natural products on the clinical trials for the treatment of neurological disorders are discussed.

Synthesis of imperatorin analogs and their evaluation as acetylcholinesterase and butyrylcholinesterase inhibitors

In this study, we synthesized several imperatorin analogs using imperatorin and xanthotoxin as substrates. The anti-cholinesterase activities of all compounds were evaluated in in vitro experiments according to the modified Ellman's method. For each synthesized compound, IC50 values for both enzymes were established. Galantamine hydrobromide was used as a positive control in the enzymatic experiments. All active compounds showed selectivity toward butyrylcholinesterase (BuChE) rather than acetylcholinesterase. The most active ones were 8-(3-methylbutoxy)-psoralen and 8-hexoxypsoralen with IC50 values for BuChE of around 16.5 and 16.4 µM, respectively. The results of our study may be considered as the beginning of a search for potential anti-Alzheimer's disease drugs based on the structure of natural furocoumarins.

Synthesis and biological evaluation of 1, 3-dihydroxyxanthone mannich base derivatives as anticholinesterase agents

Background

Alzheimer’s disease (AD), a progressive and degenerative disorder, has become one of the severe problems among the aged population all over the world. To use cholinesterase inhibitor drugs has become the most predominant treatment strategy for AD.

Results

A series of novel 1, 3-dihydroxyxanthone Mannich bases derivatives (1a ~ 4e) were synthesized, structure elucidated and evaluated for anti-cholinesterase activity. The result showed that most of the target compounds exhibited moderate to good inhibitory activities with the IC₅₀ values at micromole level concentration against both acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE). The preliminary structure-activity indicated that: (i) The alkoxy or alkenoxy substituents in the position 3 of xanthone have a positive influence on the inhibition potency; (ii) types of dialkylamine methyl in position 2 of xanthone affected cholinesterase activities and AChE/BuChE selectivity. Among them, 2-((diethylamino)methyl)-1-hydroxy-3-(3-methylbut-2-enyloxy)-9H-xanthen-9-one showed potent inhibitory activity against AChE with the IC₅₀ value of 2.61 ± 0.13 μM and the best inhibitory activity against BuChE with the IC₅₀ value of 0.51 ± 0.01 μM. The results of a mixed-type manner in enzyme kinetic experiment and molecular docking study for 2-((diethylamino)methyl)-1-hydroxy-3-(3-methylbut-2-enyloxy)-9H-xanthen-9-one demonstrated that the Mannich base derivatives were likely to bind to the active site (AS) and the peripheral anionic site (PAS) of cholinesterases.

Conclusions

This study suggested that 1, 3-dihydroxyxanthone Mannich base derivatives were potential dual inhibitors of both AChE and BuChE, which may be considered as a kind of novel drug candidates for treatment of AD.

Preparation of O-Methyl Substituted 2-Oxofuro- and 2-Oxopyrrolidinoindolines by Reductive Lactonization of Oxindol-3-Ylacetic Acids

A practical procedure for the preparation of O-methyl substituted 3a,8-dialkyl-2-oxofuroindolines is described. Reductive lactonization of the corresponding oxindol-3-ylacetic acids provides a route for the formation of this class of compounds. Further transformation of 2-oxofuroindolines into 2-oxopyrrolidinoindolines, and then to pyrrolidinoindolines demonstrates their versatility as key intermediates in natural products synthesis. The results of single-crystal X-ray crystallographic analyses are given for five of the studied compounds.

Application of conformational clustering in protein-ligand docking

Protein-Ligand docking is a powerful technique routinely employed in structure-based drug design. Despite many reported success stories, docking is not always able to provide an accurate and easily interpretable prediction of the structure of the bound complex formed by a small organic molecule and a pharmacologically relevant target. Cluster analysis can represent a versatile and readily available postprocessing tool to be employed in combination with protein-ligand docking to simplify the evaluation of the results and help to overcome present limitations of docking protocols.

Exercising regiocontrol in palladium-catalyzed asymmetric prenylations and geranylation: unifying strategy toward flustramines A and B

Pd-catalyzed asymmetric prenylation of oxindoles to afford selectively either the prenyl or reverse-prenyl product has been demonstrated. Control of the regioselectivity in this transformation is governed by the choice of ligand, solvent, and halide additive. The resulting prenylated and reverse-prenylated products were transformed into ent-flustramides and ent-flustramines A and B. Additionally, control of the regio- and diastereoselectivity was obtained using π-geranylpalladium complexes.

Synthesis of Pyrrolidinoindolines from 2-(2-Oxo-3-indolyl)acetates: Scope and Limitations

A series of 1,3a,8-alkylpyrrolidinoindolines have been synthesized. The scope and limitations of the alkylation of starting methyl oxindol-3-acetates are explored employing electron-rich and electron-poor alkylating agents. Hydrolysis and reductive lactonization of the resulting carboxylic γ-oxindolic acid derivatives proceeds with good yields to afford 2-oxofuroindolines providing ready access to the pyrrolidinoindoline derivatives.

Cyclopropylazetoindolines as precursors to C(3)-quaternary-substituted indolines

This manuscript describes the generation and use of a cyclopropylazetoindoline, a novel fused heterocycle, in coupling reactions with hetero- and carbon nucleophiles to give C(3)-quaternary-substituted pyrroloindolines.

Exploration of the interrupted Fischer indolization reaction

A convergent method to access the fused indoline ring system present in a multitude of bioactive molecules has been developed. The strategy involves the condensation of hydrazines with latent aldehydes to ultimately deliver indoline-containing products by way of an interrupted Fischer indolization sequence. The method is convergent, mild, operationally simple, broad in scope, and can be used to access enantioenriched products. In addition, our approach is amenable to the synthesis of furoindoline and pyrrolidinoindoline natural products as demonstrated by the concise formal total syntheses of physovenine and debromoflustramine B. The strategy will likely enable the synthesis of more complex targets such as the communesin alkaloids.

Cascade [1,3]-sigmatropic rearrangements of ketene O,O-acetals: kinetic and DFT level mechanistic studies

The regioisomeric alpha-cyano ketene-O,O-dialkyl acetals 2a-e and 4a-e, sequential intermediates in the diazomethane induced conversion of indole alpha-cyano-gamma-lactones 1a-e to 2-indolyl cyanomalonates 5a-e, were isolated and characterized. Formation of the steady-state intermediate cycloprop[b]indoles 3a-e was evidenced by means of NMR and confirmed by the X-ray structure of 3c, demonstrating that the formation of 5a-e from 2a-e proceeds through two consecutive and one parallel unimolecular steps, with intermediates 3a-e formed in reversible processes. Evidence that the reversible reactions proceed via [1,3]-rearrangements is presented. The steady-state kinetic approach applied to intermediate 3 allowed a minimal two consecutive step 2 --> 4--> 5 kinetic model, in which the steric bulkiness of the alkyl substituent affects strongly the associated rate constants, k(1) and k(2), inverting the rate-determining step. The solvation effects enhanced the feasibility of these skeletal rearrangements as they stabilized the transition states to a great extent. The experimental determined thermodynamic parameters and DFT calculations suggest that these cascade rearrangements occur through [1,3]-sigmatropic mechanisms, in which asynchronous bond reorganization processes via four membered pseudopericyclic transition states are highly favorable.

Pyrrolidinoindoline alkaloids from Selaginella moellendorfii

Eight new pyrrolidinoindoline alkaloids (1-8) were isolated from the whole plant of Selaginella moellendorfii. Their structures were determined by mass spectrometry, 1D and 2D NMR spectroscopy, and chemical interconversions. These alkaloids have a 3-carboxybut-2-enyl group at C-3a and two methyl groups at N-8. The possible biogenetic route from selaginellic acid (1) to neoselaginellic acid (6) was postulated and chemically mimicked. Tautomerization between 6 and 6a was observed. Selected compounds were evaluated for antibacterial, cytotoxic, and acetylcholinesterase inhibitory activities.

NO-donating tacrine hybrid compounds improve scopolamine-induced cognition impairment and show less hepatotoxicity

A series of tacrine-NO donor hybrid compounds are synthesized and evaluated for cholinesterase inhibitory activity, cognition improving activity, and hepatotoxicity. The pharmacological results indicate that hybrid compounds 1, 2, and 3a potently inhibit cholinesterase in vitro and significantly improve the scopolamine-induced cognition impairment, whereas an analogue (3h) of 2 without the NO donor moiety does not. Compared to tacrine, 1 and 2 show much less hepatotoxicity. Molecular modeling studies suggest that 2 may interact with the catalytic and the peripheral anionic site of acetylcholinesterase.