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

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.

Semisynthetic Transformations on (+)-Boldine Reveal a 5-HT2A/2CR Antagonist

Aporphine alkaloids have shown affinity for serotonin receptors (5-HTRs), and there has been a recent upsurge of interest in aporphines as 5-HT_2CR ligands. 1,2,9,10-Tetraoxygenated aporphine alkaloids in particular have demonstrated good affinity for 5-HTRs. In continued efforts to understand the impacts of structural modification of the 1,2,9,10-tetraoxygenated aporphine template on affinity, selectivity, and activity at 5-HT₂R subtypes, we used (+)-boldine (8) as a semisynthetic feedstock in the preparation of C-2-alkoxylated (+)-predicentrine analogues. Compound 10n, which contains a benzyloxy group at C-2, has been identified as a novel 5-HT_2CR ligand with strong affinity (4 nM) and moderate selectivity versus 5-HT_2BR and 5-HT_2AR (12-fold and 6-fold, respectively). Compound 10n functions as an antagonist at 5-HT_2A and 5-HT_2C receptors. Computational experiments indicate that several hydrophobic interactions as well as strong H-bond and salt bridge interactions between the protonated amine moiety in 10n and Asp134 are responsible for the potent 5-HT_2CR affinity of this compound. Furthermore, compound 10n displays favorable predicted drug-like characteristics, which is encouraging toward future optimization.

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.

A unified total synthesis of benzo[d][1,3]dioxole-type benzylisoquinoline alkaloids of aporphines, coptisines, and dibenzopyrrocolines

The first total synthesis of (S)-(+)-ovigerine, (S)-(+)-N-formylovigerine, and (6aS,6a'S)-(+)-ovigeridimerine of aporphine alkaloids with a benzo[d][1,3]dioxole structure feature was established. The strategy was based upon the well-known Pd-catalyzed arylation to set the aporphine framework, and Noyori asymmetric hydrogenation followed by diastereoselective resolution to achieve excellent enantioselectivity. By slightly modifying the total synthetic route and strategically combining it with a aza-Michael addition, Bischler-Napieralski reaction and N-arylation, this methodology was also applied to the total syntheses of benzo[d][1,3]dioxole-type benzylisoquinoline alkaloids of coptisines and dibenzopyrrocolines, including two impatiens, tetrahydrocoptisine, and quaternary coptisine bromide of coptisines and two dibenzopyrrocoline analogues, with the syntheses of all of these target compounds being efficient. Among the nine synthesized compounds, the total syntheses of the three aporphines and the two impatiens, all with ee values of greater than 99%, were reported for the first time. This work also represents the first unification of synthetic routes for the total synthesis of benzo[d][1,3]dioxole-type aporphines, coptisines, and dibenzopyrrocolines.

In vitro functional evaluation of isolaureline, dicentrine and glaucine enantiomers at 5-HT2 and α1 receptors

Compounds with activity at serotonin (5-hydroxytryptamine) 5-HT₂ and α₁ adrenergic receptors have potential for the treatment of central nervous system disorders, drug addiction or overdose. Isolaureline, dicentrine and glaucine enantiomers were synthesized, and their in vitro functional activities at human 5-HT₂ and adrenergic α₁ receptor subtypes were evaluated. The enantiomers of isolaureline and dicentrine acted as antagonists at 5-HT₂ and α₁ receptors with (R)-isolaureline showing the greatest potency (pK_b  = 8.14 at the 5-HT_2C receptor). Both (R)- and (S)-glaucine also antagonized α₁ receptors, but they behaved very differently to the other compounds at 5-HT₂ receptors: (S)-glaucine acted as a partial agonist at all three 5-HT₂ receptor subtypes, whereas (R)-glaucine appeared to act as a positive allosteric modulator at the 5-HT_2A receptor.

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.

Discovery of Aporphine Analogues as Potential Antiplatelet and Antioxidant Agents: Design, Synthesis, Structure-Activity Relationships, Biological Evaluations, and in silico Molecular Docking Studies

To explore the potential of aporphine alkaloids, a novel series of functionalized aporphine analogues with alkoxy (OCH₃ , OC₂ H₅ , OC₃ H₇ ) functional groups at C1/C2 of ring A and an acyl (COCH₃ and COPh) or phenylsulfonyl (SO₂ Ph and SO₂ C₆ H₄ -3-CH₃ ) functionality at the N6 position of ring B of the aporphine scaffold were synthesized and evaluated for their arachidonic acid (AA)-induced antiplatelet aggregation inhibitory activity and 2,2-diphenyl-1-picrylhydrazyl (DPPH) free-radical-scavenging antioxidant activity, with acetylsalicylic acid and ascorbic acid as standard references, respectively. The preliminary structure-activity relationship related to AA-induced platelet aggregation inhibitory activity results showed that the aporphine analogues 1-[1,2,9,10-tetramethoxy-6a,7-dihydro-4H-dibenzo[de,g]quinolin-6(5H)-yl]ethanone and 1-[2-(benzyloxy)-1,9,10-trimethoxy-6a,7-dihydro-4H-dibenzo[de,g]quinolin-6(5H)-yl]ethanone to be the best compounds of the series. Moreover, the DPPH free-radical-scavenging antioxidant activity results demonstrated that the aporphine analogues 1,2,9,10-tetramethoxy-6-(methylsulfonyl)-5,6,6a,7-tetrahydro-4H-dibenzo[de,g]quinoline, 2-ethoxy-1,9,10-trimethoxy-6-(methylsulfonyl)-5,6,6a,7-tetrahydro-4H-dibenzo[de,g]quinoline, 1-ethoxy-2,9,10-trimethoxy-6-(methylsulfonyl)-5,6,6a,7-tetrahydro-4H-dibenzo[de,g]quinoline, 2,9,10-trimethoxy-6-(methylsulfonyl)-1-propoxy-5,6,6a,7-tetrahydro-4H-dibenzo[de,g]quinoline, and 1-(benzyloxy)-2,9,10-trimethoxy-6-(methylsulfonyl)-5,6,6a,7-tetrahydro-4H-dibenzo[de,g]quinoline were the best compounds of the series. Moreover, in silico molecular docking simulation studies of the active analogues were also performed.

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.

Design, synthesis, and anticancer properties of isocorydine derivatives

Isocorydine (ICD), an aporphine alkaloid, is widely distributed in nature. Its ability to target side population (SP) cells found in human hepatocellular carcinoma (HCC) makes it and its derivative 8-amino-isocorydine (NICD) promising chemotherapeutic agents for the treatment of HCC. To improve the anticancer activity of isocorydine derivatives, twenty derivatives of NICD were designed and synthesized through chemical structure modifications of the aromatic amino group at C-8. The anti-proliferative activities of all synthesized compounds against human hepatocellular (HepG2), cervical (HeLa), and gastric (MGC-803) cancer cell lines were evaluated using an MTT assay. The results showed that all the synthetic compounds had some tumor cell growth inhibitory activity. The compound COM33 (24) was the most active with IC₅₀ values under 10 μM (IC₅₀ for HepG2 = 7.51 µM; IC₅₀ for HeLa = 6.32 μM). FICD (12) and COM33 (24) were selected for further investigation of their in vitro and in vivo activities due to their relatively good antiproliferative properties. These two compounds significantly downregulated the expression of four key proteins (C-Myc, β-Catenin, CylinD1, and Ki67) in HepG2 cells. The tumor inhibition rate of COM33 (24) in vivo was 73.8% after a dose 100 mg/kg via intraperitoneal injection and the combined inhibition rate of COM33 (24) (50 mg/kg) with sorafenib (50 mg/kg) was 66.5%. The results indicated that these isocorydine derivatives could potentially be used as targeted chemotherapy agents or could be further developed in combination with conventional chemotherapy drugs to target cancer stem cells (CSCs) and epithelial-to-mesenchymal transition (EMT), the main therapeutic targets in HCC.

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.

Aporphinoid antagonists of 5-HT2A receptors: further evaluation of ring A substituents and the size of ring C

A series of ring A-modified analogs of nantenine as well as structural variants in ring C were synthesized and evaluated for antagonist activity at 5-HT2A and α1A receptors. Halogenation improves 5-HT2A antagonist potency in molecules containing a C1 methoxyl/C2 methoxyl or C1 methoxyl/C2 hydroxyl moiety. Bromination or iodination (but not chlorination) with the latter moiety also significantly increased α1A antagonist potency. Homologation or contraction of ring C adversely affected antagonist activity at both receptors, implying that a six-membered ring C motif is beneficial for high antagonist potency at both receptors. Molecular docking studies suggest that the improved antagonist activity (by virtue of improved affinity) of C3-halogenated aporphines in this study is attributable to favorable interactions with the C3 halogen and F339 and/or F340.

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.

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.