Dopamine/Serotonin Receptor Ligands. 10: SAR Studies on Azecine-type Dopamine Receptor Ligands by Functional Screening at Human Cloned D1, D2L, and D5 Receptors with a Microplate Reader Based Calcium Assay Lead to a Novel Potent D1/D5 Selective Antagonist

Advances in synthesis of novel annulated azecines and their unique pharmacological properties

Annulated azecines, mostly partially saturated benzo[d]azecine and dibenzo[c,g]azecine fusion isomers, constitute a unique class of alkaloids and nature-inspired azaheterocyclic compounds with interesting reactivity, physicochemical and biological properties. Due to difficulties associated with the synthesis of the benzazecine (or bioisosteric) scaffold they are not the focus of organic and medicinal chemists' consideration, whereas it is worth noting the range of their pharmacological activities and their potential application in medicinal chemistry. Herein, we reviewed the synthetic methodologies of arene-fused azecine derivatives known up to date and reported about the progress in disclosing their potential in drug discovery. Indeed, their conformational restriction or liberation drives their selectivity towards diverse biological targets, making them versatile scaffolds for developing drugs, including antipsychotic and anticancer drugs, but also small molecules with potential for anti-neurodegenerative treatments, as the recent literature shows.

Synthesis of 10-Membered Azecines through Pd-Catalyzed Formal [6+4] Cycloaddition and Their Transannular Reaction to Polycyclic Compounds

Azecine fragments are frequently presented in natural products and bioactive compounds. However, minor efforts have been devoted to these 10-membered N-heterocycles, and their synthesis is still challenging. Reported herein is the first catalytic formal [6+4] cycloaddition for the synthesis of 10-membered azecines. Under palladium catalysis, the reaction of δ-vinylvalerolactones and benzofuran-derived azadienes proceeds smoothly to afford benzofuran-fused azecines with high diastereoselectivity in moderate to good yields. A unique transannular reaction of these 10-membered azecines for the construction of polycyclic compounds is also demonstrated.

Synthesis of Tetracyclic C2-Symmetric Ketiminium Salts and Evaluation as Catalysts for Epoxidation

Short synthetic routes to the nonplanar tetracyclic ketiminium salt A and two close analogs are described. These are helical nonplanar structures as shown by X-ray crystallography.

A Liquid Chromatography Tandem Mass Spectrometry Method for the Simultaneous Estimation of the Dopamine Receptor Antagonist LE300 and Its N-methyl Metabolite in Plasma: Application to a Pharmacokinetic Study

LE300 is a novel dopamine receptor antagonist used to treat cocaine addiction. In the current study, a sensitive and fast liquid chromatography-tandem mass spectrometry (LC-MS/MS) has been established and validated for the simultaneous analysis of LE300 and its N-methyl metabolite, MLE300, in rat plasma with an application in a pharmacokinetic study. The chromatographic elution of LE300, MLE300, and Ponatinib (IS, internal standard), was carried out on a 50 mm C₁₈ analytical column (ID: 2.1 mm and particle size: 1.8 μm) maintained at 22 ± 2 °C. The run time was 5 min at a flow rate of 0.3 mL/min. The mobile phase consisted of 42% aqueous solvent (10 mM ammonium formate, pH: 4.2 with formic acid) and 58% organic solvent (acetonitrile). Plasma samples were pretreated using protein precipitation with acetonitrile. The electrospray ionization (ESI) source was used to generate an ion-utilizing positive mode. A multiple reaction monitoring mass analyzer mode was utilized for the quantification of analytes. The linearity of the calibration curves in rat plasma ranged from 1 to 200 ng/mL (r² = 0.9997) and from 2 to 200 ng/mL (r² = 0.9984) for LE300 and MLE300, respectively. The lower limits of detection (LLOD) were 0.3 ng/mL and 0.7 ng/mL in rat plasma for LE300 and MLE300, respectively. Accuracy (RE%) ranged from -1.71% to -0.07% and -4.18% to -1.48% (inter-day), and from -3.3% to -1.47% and -4.89% to -2.15% (intra-day) for LE300 and MLE300, respectively. The precision (RSD%) was less than 2.43% and 1.77% for the inter-day, and 2.77% and 1.73% for intra-day of LE300 and MLE300, respectively. These results are in agreement with FDA guidelines. The developed LC-MS/MS method was applied in a pharmacokinetic study in Wistar rats. T_max and C_max were 2 h and 151.12 ± 12.5 ng/mL for LE300, and 3 h and 170.4 ± 23.3 ng/mL for MLE300.

Access to Indole-Fused Benzannulated Medium-Sized Rings through a Gold(I)-Catalyzed Cascade Cyclization of Azido-Alkynes

Because benzannulated and indole-fused medium-sized rings are found in many bioactive compounds, combining these fragments might lead to unexplored areas of biologically relevant and uncovered chemical space. Herein is shown that α-imino gold carbene chemistry can play an important role in solving the difficulty in the formation of medium-sized rings. Namely, phenylene-tethered azido-alkynes undergo arylative cyclization through the formation of a gold carbene intermediate to afford benzannulated indole-fused medium-sized tetracycles. The reactions allow a range of different aryl substitution patterns and efficient access to these otherwise difficult-to-obtain medium-sized rings. This study also demonstrates the feasibility of the semihollow-shaped C-dtbm ligand for the construction of a nine-membered ring.

Azido-Alkynes in Gold(I)-Catalyzed Indole Syntheses

The exploitation of nitrogen-functionalized reactive intermediates plays an important role in the synthesis of biologically relevant scaffolds in the field of pharmaceutical sciences. Those based on gold carbenes carry a strong potential for the design of highly efficient cascade processes toward the synthesis of compounds containing a fused indole core structure. This personal account gives a detailed explanation of our contribution to this sector, and embraces the reaction development of efficient gold-catalyzed cascade processes based on diversely functionalized azido-alkynes. Challenging cyclizations and their subsequent application in the synthesis of pharmaceutically relevant scaffolds and natural products conducted in an intra- or intermolecular fashion are key features of our research.

Robust and Fast UV–HPLC Method for Biotransformation Analysis of Azecines

Dibenzoazecines are a new class of drug candidates for the treatment of schizophrenia. Compared to the drugs currently used in therapy, the azecines have a novel mechanism of action. Thus, they have the potential to cause fewer side effects compared to the standard therapy with a constant high neuroleptic potency. This theory was substantiated by comparative in vivo tests with haloperidol and risperidone. Seventeen new azecine derivatives have already been tested furthermore of stability, physicochemical parameters, pharmacokinetics including esterase cleavage, stability in simulated gastrointestinal fluid, stability at different pH values and determination of octanol/water-partition coefficients. For these substances, class is still a lack of information concerning the metabolism. Therefore, the present study investigated and developed a reliable and reproducible gradient reversed-phase HPLC–UV method to determinate the lead structure LE404 alongside emerging metabolites in compliance with international requirements like ICH guidelines and the European Pharmacopoeia. Up to now, there is no innovative method suitable for such a determination. Chromatographic separations were achieved with a phenomenex™ Gemini column (5 µm C18 110 Å, 250 × 4.60 mm) using a mixture of acetonitrile/potassium dihydrogen phosphate buffer (4 mmol L−1, pH 2.5) as mobile phase. The gradient method flow rate was 1.0 mL min−1, and UV detection was made at 220 nm. The optimized HPLC method was found to be specific, accurate, reproducible and robust for determination of LE404.

Silver Trifluoromethanesulfonate-Catalyzed Annulation of Propargylic Alcohols with 3-Methyleneisoindolin-1-one

A silver-catalyzed formal [3 + 3] annulation of 3-methyleneisoindolin-1-one with alkynol for the synthesis of 1,5-dihydroindolizin-3(2H)-one derivatives is disclosed. The protocol allows practical synthesis of N-heterocyclic scaffolds with a broad scope of functional groups and could be efficiently scaled up to gram scale, which incarnates a potential application of this methodology. In addition, a range of chlorine anion substitution of alkenes can be constructed by adjusting the structure of the alkynol substrates with the use of TMSCl.

Transition-Metal-Free Deconstructive Lactamization of Piperidines

One of the major challenges in organic synthesis is the activation or deconstructive functionalization of unreactive C(sp³ )-C(sp³ ) bonds, which requires using transition or precious metal catalysts. We present here an alternative: the deconstructive lactamization of piperidines without using transition metal catalysts. To this end, we use 3-alkoxyamino-2-piperidones, which were prepared from piperidines through a dual C(sp³ )-H oxidation, as transitory intermediates. Experimental and theoretical studies confirm that this unprecedented lactamization occurs in a tandem manner involving an oxidative deamination of 3-alkoxyamino-2-piperidones to 3-keto-2-piperidones, followed by a regioselective Baeyer-Villiger oxidation to give N-carboxyanhydride intermediates, which finally undergo a spontaneous and concerted decarboxylative intramolecular translactamization.

Gold(i)-catalyzed pathway-switchable tandem cycloisomerizations to indolizino[8,7-b]indole and indolo[2,3-a]quinolizine derivatives

A common strategy was developed to access both indolizino[8,7-b]indole and indolo[2,3-a]quinolizine derivatives from tryptamine-N-ethynylpropiolamide substrates in a switchable fashion via tuning both the electronic effects and steric effects.

First synthesis of heterocyclic allenes – benzazecine derivatives

Benzazecines with an allene fragment were prepared for the first time and in high yields via tandem reaction of 1-phenylethynyl-1-methyl(benzyl)-1,2,3,4-tetrahydroisoquinolines with activated alkynes in trifluoroethanol.

Polypharmacology of dopamine receptor ligands

Most neurological diseases have a multifactorial nature and the number of molecular mechanisms discovered as underpinning these diseases is continuously evolving. The old concept of developing selective agents for a single target does not fit with the medical need of most neurological diseases. The development of designed multiple ligands holds great promises and appears as the next step in drug development for the treatment of these multifactorial diseases. Dopamine and its five receptor subtypes are intimately involved in numerous neurological disorders. Dopamine receptor ligands display a high degree of cross interactions with many other targets including G-protein coupled receptors, transporters, enzymes and ion channels. For brain disorders like Parkinsońs disease, schizophrenia and depression the dopaminergic system, being intertwined with many other signaling systems, plays a key role in pathogenesis and therapy. The concept of designed multiple ligands and polypharmacology, which perfectly meets the therapeutic needs for these brain disorders, is herein discussed as a general ligand-based concept while focusing on dopaminergic agents and receptor subtypes in particular.

Organocatalytic enantioselective Friedel–Crafts reaction: an efficient access to chiral isoindolo-β-carboline derivatives

An organocatalytic asymmetric Friedel–Crafts reaction between indole and indole-derived hydroxylactams has been realized to furnish isoindolo-β-carbolines in good to excellent yields (up to >99%) and generally high enantioselectivities (up to >99% ee).

D1-like receptors distinguishing thieno-azecine regioisomers

Design of novel azecine derivatives with modulated dopaminergic receptor selectivity and affinity profiles.

Fishing for accessory binding sites at GPCRs with 'loop-hooks' - an approach towards selectivity? Part I

Receptor-subtype selectivity is an important issue in medicinal chemistry and can become very difficult to achieve if the actual binding pockets of the respective receptors are highly conserved. For such cases, known unselective ligands could be equipped with a spacer that sticks outside the actual orthosteric binding pocket towards the extracellular loops. The end of the spacer bears certain functional groups to enable specific or unspecific interactions with the receptor residues outside the binding cavity. Our experiments indicated that it is possible to achieve selectivity within the dopamine D1 family with such 'loop-hooks'.

Epigenetic histone modification regulates developmental lead exposure induced hyperactivity in rats

Lead (Pb) exposure was commonly considered as a high environmental risk factor for the development of attention-deficit/hyperactivity disorder (ADHD). However, the molecular basis of this pathological process still remains elusive. In light of the role of epigenetics in modulating the neurological disease and the causative environment, the alterations of histone modifications in the hippocampus of rats exposed by various doses of lead, along with concomitant behavioral deficits, were investigated in this study. According to the free and forced open field test, there showed that in a dosage-dependent manner, lead exposure could result in the increased locomotor activity of rats, that is, hyperactivity: a subtype of ADHD. Western blotting assays revealed that the levels of histone acetylation increased significantly in the hippocampus by chronic lead exposure, while no dramatic changes were detected in terms of expression yields of ADHD-related dopaminergic proteins, indicating that histone acetylation plays essential roles in this toxicant-involved pathogenesis. In addition, the increased level of histone acetylation might be attributed to the enzymatic activity of p300, a typical histone acetyltransferase, as the transcriptional level of p300 was significantly increased upon higher-dose Pb exposure. In summary, this study first discovered the epigenetic mechanism bridging the environmental influence (Pb) and the disease itself (ADHD) in the histone modification level, paving the way for the comprehensive understanding of ADHD's etiology and in further steps, establishing the therapy strategy of this widespread neurological disorder.

Novel fused pyrrole heterocyclic ring systems as structure analogs of LE 300: Synthesis and pharmacological evaluation as serotonin 5-HT(2A), dopamine and histamine H(1) receptor ligands

LE 300 represents a structurally novel type of antagonists acting preferentially at the dopamine D(1)/D(5 )receptors and the serotonin 5-HT(2A )receptor. This compound consists of a ten-membered central azecine ring fused to an indole ring on one side and a benzene moiety on the other side. To estimate the importance of the indole and / or phenyl moieties in this highly active benz-indolo-azecine, both rings were removed and replaced with a 1H-pyrrole counterpart. Accordingly, some new analogs of LE 300 namely, pyrrolo[2,3-g]indolizine, pyrrolo[3,2-a]quinolizine rings and their corresponding dimethylpyrrolo[2,3-d]azonine, and dimethylpyrrolo[2,3-d]azecine were synthesized to be evaluated for their activity at the 5-HT(2A) and dopamine D(1), D(2L), D(4), D(5) receptors in relation to LE 300. In addition, their activity at the H(1)-histamine receptors was also determined. The results suggested that the rigid pyrrolo[2,3-g]indolizine 7 and pyrrolo[3,2-a]quinolizine 8 analogs lacked biological activity in the adopted three bioassays. However, their corresponding flexible pyrrolo[2,3-d]azonine 11 and pyrrolo[2,3-d]azecine 12 derivatives revealed weak partial agonistic activity and weak antagonistic potency at the serotonin 5-HT(2A )and histamine H(1 )receptors, respectively. Meanwhile, they showed no affinity to any of the four utilized dopamine receptors. Variation in ring size did not contribute to a significant influence on the three tested bioactivities. Removal of the hydrophobic moiety (phenyl ring) and replacement of the indole moiety with a 1H-pyrrole counterpart led to a dramatic alteration in the profile of activity of such azecine-type compounds.