(+/-)-Nantenine analogs as antagonists at human 5-HT(2A) receptors: C1 and flexible congeners

Total Synthesis of the Racemate of Laurolitsine

The total synthesis of laurolitsine was achieved for the first time. This reaction was accomplished in 14 steps with a 2.3% yield (this was calculated using 3-hydroxy-4-methoxybenzaldehyde as the starting material) starting from two simple materials, 3-hydroxy-4-methoxybenzaldehyde and 2-(3-hydroxy-4-methoxyphenyl)acetic acid, and the longest linear sequence consisted of 11 steps. The key steps included an electrophilic addition reaction in which a nitro group was reduced to an amino group using lithium tetrahydroaluminum and a Pd-catalyzed direct biaryl coupling reaction. In this paper, many of the experimental steps were optimized, and an innovative postprocessing method in which 2-(3-(benzyloxy)-4-methoxyphenyl)ethanamine is salted with oxalic acid was proposed.

Aporphines: A privileged scaffold in CNS drug discovery

Aporphine alkaloids embedded in 4H-dibenzo[de,g]quinoline four-ring structures belong to one of the largest subclasses of isoquinoline alkaloids. Aporphine is a privileged scaffold in the field of organic synthesis and medicinal chemistry for the discovery of new therapeutic agents for central nervous system (CNS) diseases, cancer, metabolic syndrome, and other diseases. In the past few decades, aporphine has attracted continuing interest to be widely used to develop selective or multitarget directed ligands (MTDLs) targeting the CNS (e.g., dopamine D_1/2/5, serotonin 5-HT_1A/2A/2C and 5-HT₇, adrenergic α/β receptors, and cholinesterase enzymes), thereby serving as valuable pharmacological probes for mechanism studies or as potential leads for CNS drug discovery. The aims of the present review are to highlight the diverse CNS activities of aporphines, discuss their SAR, and briefly summarize general synthetic routes, which will pave the way for the design and development of new aporphine derivatives as promising CNS active drugs in the future.

Unprecedented TMEDA-Catalyzed Regioselective Decarboalkoxy C-N Bond Formation: A Unified Direct Access to Indolo[2,1-a]isoquinoline and Dibenzopyrrocoline Alkaloids

An unprecedented TMEDA-catalyzed, regioselective, decarboethoxy direct C-N coupling protocol towards the synthesis of dibenzopyrocolines 17a - i and 5,6-dihydroindolo[2,1 a ]isoquinoline 15a - f / 18a-c alkaloids via the identification of N,N,N',N'-tetramethylethylenediamine ( TMEDA ) as a homogeneous catalyst is reported. The transition-metal-free, TMEDA-catalytic novel protocol is operationally simple and showed a wide range of functional group tolerance and substrate compatibility. The gram-scale application and synthesis of naturally occurring Cryptaustoline (dibenzopyrocoline) alkaloid, further highlights the importance and versatile nature of the developed protocol. This finding also offers a TMEDA-catalyzed direct synthesis of dibenzopyrocolines and substituted 5,6dihydroindolo[2,1 -a ]isoquinoline compounds in a one-pot. The probable reaction pathway involves the free-radical sequential approach via a single electron transfer (SET) mechanism.

Structure-activity profiling of alkaloid natural product pharmacophores against a Schistosoma serotonin receptor

Serotonin (5-HT) is an important regulator of numerous aspects of flatworm biology, ranging from neuromuscular function to sexual maturation and egg laying. In the parasitic blood fluke Schistosoma mansoni, 5-HT targets several G-protein coupled receptors (GPCRs), one of which has been demonstrated to couple to cAMP and regulate parasite movement. This receptor, Sm.5HTR_L, has been successfully co-expressed in mammalian cells alongside a luminescent cAMP-biosensor, enabling pharmacological profiling for candidate anti-schistosomal drugs. Here, we have utilized this assay to perform structure-activity investigations of 143 compounds containing previously identified alkaloid natural product pharmacophores (tryptamines, aporphines and protoberberines) shown to regulate Sm.5HTR_L. These experiments mapped regions of the tryptamine pharmacophore amenable and intolerant to substitution, highlighting differences relative to orthologous mammalian 5-HT receptors. Potent Sm.5HTR_L antagonists were identified, and the efficacy of these compounds were evaluated against live adult parasites cultured ex vivo. Such structure-activity profiling, characterizing the effect of various modifications to these core ring systems on Sm.5HTR_L responses, provides greater understanding of pharmacophores selective for this target to aid future drug development efforts.

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Various alkaloids were screened against a schistosome serotonin receptor, Sm.5HTR_L.

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Compounds with a tryptamine core displayed agonist activity at Sm.5HTR_L.

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Aporphine and protoberberine compounds displayed antagonist activity at Sm.5HTR_L.

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Compound activity at Sm.5HTR_L is broadly mirrored by motility effects on adult worms.

Synthesis and evaluation of nuciferine and roemerine enantiomers as 5-HT2 and α1 receptor antagonists

In this study, the (S)-enantiomers of the aporphine alkaloids, nuciferine and roemerine, were prepared via a synthetic route involving catalytic asymmetric hydrogenation and both stereoisomers were evaluated in vitro for functional activity at human 5-HT2 and adrenergic α1 receptor subtypes using a transforming growth factor-α shedding assay. Both enantiomers of each of the compounds were found to act as antagonists at 5-HT2 and α1 receptors. (R)-roemerine was the most potent compound at 5-HT2A and 5-HT2C receptors (pKb = 7.8-7.9) with good selectivity compared to (S)-roemerine at these two receptors and compared to its activity at 5-HT2B, α1A, α1B and α1D receptors.

Facile total synthesis of lysicamine and the anticancer activities of the RuII, RhIII, MnII and ZnII complexes of lysicamine

Lysicamine is a natural oxoaporphine alkaloid, which isolated from traditional Chinese medicine (TCM) herbs and has been shown to possess cytotoxicity to hepatocarcinoma cell lines. Reports on its antitumor activity are scarce because lysicamine occurs in plants at a low content. In this work, we demonstrate a facile concise total synthesis of lysicamine from simple raw materials under mild reaction conditions, and the preparation of the Ru(II), Rh(III), Mn(II) and Zn(II) complexes 1–4 of lysicamine (LY). All the compounds were fully characterized by elemental analysis, IR, ESI-MS, ¹H and ¹³C NMR, as well as single-crystal X-ray diffraction analysis. Compared with the free ligand LY, complexes 2 and 3 exhibited superior in vitro cytotoxicity against HepG2 and NCI-H460. Mechanistic studies indicated that 2 and 3 blocked the cell cycle in the S phase by decreasing of cyclins A2/B1/D1/E1, CDK 2/6, and PCNA levels and increasing levels of p21, p27, p53 and CDC25A proteins. In addition, 2 and 3 induced cell apoptosis via both the caspase-dependent mitochondrial pathway and the death receptor pathway. in vivo study showed that 2 inhibited HepG2 tumor growth at 1/3 maximum tolerated dose (MTD) and had a better safety profile than cisplatin.

Synthesis and evaluation of aporphine analogs containing C1 allyl isosteres at the h5-HT(2A) receptor

A series of C1 aporphine analogs related to compound 5 and that contain substituted allylic, alkynyl, nitrile, ester and benzyl groups was synthesized and evaluated for affinity at h5HT2A and α1A receptors in functional activity assays that measure calcium release. The presence of branched allylic substituent groups diminished affinity for the h5HT2A receptor. Likewise, the alkynyl, nitrile and ester derivatives evaluated displayed lower 5-HT2A receptor affinity as compared to 5. Hydrophobic, steric and electronic effects impact the affinity of p-substituted benzyl derivatives 8i-8k but in different ways. High hydrophobicity and size favor 5-HT2A affinity whereas, high electronegativity disfavors 5-HT2A affinity. p-Bromobenzyl analog 8k was identified as a 5-HT2A receptor selective ligand, with the highest 5-HT2A receptor affinity of any aporphine known to date. Most of the other analogs were selective for the 5-HT2A versus the α1A receptor. ChemScore binding energies from docking studies correlated qualitatively with the observed trends in affinity for 8i-8k, although the binding energies were not well differentiated quantitatively.

C4 phenyl aporphines with selective h5-HT(2B) receptor affinity

A group of aporphine alkaloids related to (±)-nantenine (1) and bearing a C4 phenyl and various C1 or N-substituents, was synthesized and evaluated for affinity to h5-HT receptors. In general, unlike nantenine, the analogs lack affinity for the h5-HT(2A) receptor and other 5-HT receptors but bind selectively to the h5-HT(2B) receptor. With regards to 5-HT(2B) affinity, there appears to be a low tolerance for bulky C1 or N-substituents when the C4 phenyl moiety is present. Compound 5a had the highest 5-HT(2B) affinity of the compounds tested, was found to be an antagonist and is selective vs other CNS receptors.

Semisynthetic Studies on and Biological Evaluation of N-Methyllaurotetanine Analogues as Ligands for 5-HT Receptors

N-Methyllaurotetanine (1) has been reported to display good affinity for the 5-HT1A receptor, but no structure-affinity studies have been performed to date. The commercially available alkaloid boldine (2) was used as the starting material for synthesis of various C-9 alkoxy analogues of N-methyllaurotetanine in order to gauge the effect of C-9 alkylation on affinity and selectivity at 5-HT1A, 5-HT2A, and 5-HT7 receptors. Mitsunobu reactions were implemented in the alkylation steps leading to the analogues. Modest improvement in 5-HT1A affinity was observed upon alkylation for most analogues. Thus, the C-9 hydroxy group of 1 is not critical for affinity to the 5-HT1A receptor. Some analogues displayed high affinity for the 5-HT7 receptor, comparable to N-methyllaurotetanine, with moderate selectivity vs 5-HT1A and 5-HT2A receptors.

Identification of tris-(phenylalkyl)amines as new selective h5-HT2B receptor antagonists

A series of tris-(phenylalkyl)amines was synthesized and evaluated for affinity to 5-HT₂ receptors. In general, the compounds displayed high affinity (4 of 11 analogs had K_i values < 10 nM) and good selectivity for the 5-HT_2B receptor vs. other 5-HT₂ receptors. Functional assays revealed that the compounds are 5-HT_2B antagonists.

Evaluation of structural effects on 5-HT(2A) receptor antagonism by aporphines: identification of a new aporphine with 5-HT(2A) antagonist activity

A set of aporphine analogs related to nantenine was evaluated for antagonist activity at 5-HT2A and α1A adrenergic receptors. With regards to 5-HT2A receptor antagonism, a C2 allyl group is detrimental to activity. The chiral center of nantenine is not important for 5-HT2A antagonist activity, however the N6 nitrogen atom is a critical feature for 5-HT2A antagonism. Compound 12b was the most potent 5-HT2A aporphine antagonist identified in this study and has similar potency to previously identified aporphine antagonists 2 and 3. The ring A and N6 modifications examined were detrimental to α1A antagonism. A slight eutomeric preference for the R enantiomer of nantenine was observed in relation to α1A antagonism.

Facile synthesis of 4,5,6a,7-tetrahydrodibenzo[de,g]chromene heterocycles and their transformation to phenanthrene alkaloids

Oxa-Pictet-Spengler cyclization and microwave-assisted C-H arylation have been implemented as key steps in the synthesis of new isochroman heterocycles containing a 4,5,6a,7-tetrahydrodibenzo[de,g]chromene motif. These isochromans may be easily transformed to phenanthrene alkaloids via acidic cleavage of the isochroman ring and standard synthetic manipulations thereafter. The route described is attractive in that it provides access to two biologically interesting scaffolds in simple and high yielding synthetic steps.

A New Route to Azafluoranthene Natural Products via Direct Arylation

Microwave-assisted direct arylation was successfully employed in the synthesis of azafluoranthene alkaloids for the first time. Direct arylation reactions on a diverse set of phenyltetrahydroisoquinolines produces the indeno[1,2,3-ij]isoquinoline nucleus en route to a high yielding azafluoranthene synthesis. The method was used as a key step in the efficient preparation of the natural products rufescine and triclisine. As demonstrated herein, this synthetic approach should be generally applicable to the preparation of natural and un-natural azafluoranthene alkaloids as well as "azafluoranthene-like" isoquinoline alkaloids.

New aporphinoid 5-HT2A and α1A antagonists via structural manipulations of nantenine

A series of C1, C2, C3 and N6 analogs of nantenine (2) was synthesized and evaluated in 5-HT(2A) and α(1A) receptor functional assays. Alkyl substitution of the C1 and N6 methyl groups of nantenine provided selective 5-HT(2A) and α(1A) antagonists, respectively. The C2 alkyloxy analogs studied were generally selective for α(1A) versus 5-HT(2A). The C3 bromo analog 15 is one of the most potent aporphinoid 5-HT(2A) antagonists known presently.

Cytotoxicity of aporphines in human colon cancer cell lines HCT-116 and Caco-2: an SAR study

A series of synthetic aporphine derivatives structurally related to domesticine and nantenine (ring A, N6 and ring C truncated analogs), was evaluated in MTS cytotoxicity assays against the human colon cancer cell lines, HCT-116 and Caco-2. In general, the C1 position of ring A is tolerant of alkoxy substituents as well as a benzoyl ester functionality. Other modifications evaluated resulted in a decrease in cytotoxic activity. The most potent compounds identified had IC(50) values in the range 23-38 μM, comparable to the known cytotoxic agent, etoposide.

Synthesis of C-Homoaporphines via Microwave-Assisted Direct Arylation

Representatives of the C-homoaporphine class of alkaloids have shown interesting biological activities. To date the synthesis of these molecules has never been attempted via a direct arylation strategy. We report herein the first Pd-mediated intramolecular direct arylation in the synthesis of C-homoaporphines via the use of microwaves. Use of tricyclohexylphosphine tetrafluoroborate as ligand gave good percentage conversions and suppressed competing debromination with the substrates evaluated. This arylation strategy should be broadly useful in the synthesis of C-homoaporphine alkaloids as demonstrated herein in the synthesis of (±)-homonantenine.

Nantenine as an acetylcholinesterase inhibitor: SAR, enzyme kinetics and molecular modeling investigations

Nantenine, as well as a number of flexible analogs, were evaluated for acetylcholinesterase (AChE) inhibitory activity in microplate spectrophotometric assays based on Ellman's method. It was found that the rigid aporphine core of nantenine is an important structural requirement for its anticholinesterase activity. Nantenine showed mixed inhibition kinetics in enzyme assays. Molecular docking experiments suggest that nantenine binds preferentially to the catalytic site of AChE but is also capable of interacting with the peripheral anionic site (PAS) of the enzyme, thus accounting for its mixed inhibition profile. The aporphine core of nantenine may thus be a useful template for the design of novel PAS or dual-site AChE inhibitors. Inhibiting the PAS is desirable for prevention of aggregation of the amyloid peptide Aβ, a major causative factor in the progression of Alzheimer's disease (AD).

Affinity of aporphines for the human 5-HT2A receptor: insights from homology modeling and molecular docking studies

Analogs of nantenine were docked into a modeled structure of the human 5-HT(2A) receptor using ICM Pro, GLIDE, and GOLD docking methods. The resultant docking scores were used to correlate with observed in vitro apparent affinity (K(e)) data. The GOLD docking algorithm when used with a homology model of 5-HT(2A), based on a bovine rhodopsin template and built by the program MODELLER, gives results which are most in agreement with the in vitro results. Further analysis of the docking poses among members of a C1 alkyl series of nantenine analogs, indicate that they bind to the receptor in a similar orientation, but differently than nantenine. Besides an important interaction between the protonated nitrogen of the C1 alkyl analogs and residue Asp155, we identified Ser242, Phe234, and Gly238 as key residues responsible for the affinity of these compounds for the 5-HT(2A) receptor. Specifically, the ability of some of these analogs to establish a H-bond with Ser242 and hydrophobic interactions with Phe234 and Gly238 appears to explain their enhanced affinity as compared to nantenine.

Synthetic studies and pharmacological evaluations on the MDMA ('Ecstasy') antagonist nantenine

The naturally occurring aporphine alkaloid nantenine, has been shown to antagonize behavioral and physiological effects of MDMA in mice. We have synthesized (+/-)-nantenine via an oxidative cyclization reaction with PIFA and evaluated its binding profile against a panel of CNS targets. To begin to understand the importance of the chiral center of nantenine with regards to its capacity to antagonize the effects of MDMA in vivo, (R)- and (S)-nantenine were prepared and evaluated in a food-reinforced operant task in rats. Pretreatment with either nantenine enantiomer (0.3mg/kg ip) completely blocked the behavioral suppression induced upon administration of 3.0mg/kg MDMA. (+/-)-Nantenine displayed high affinity and selectivity for the alpha(1A) adrenergic receptor among several other receptors suggesting that this alpha(1) subtype may be significantly involved in the anti-MDMA effects of the enantiomers.