Synthesis, molecular modeling, and biological studies of novel piperidine-based analogues of cocaine: evidence of unfavorable interactions proximal to the 3alpha-position of the piperidine ring

Modular access to chiral bridged piperidine-γ-butyrolactones via catalytic asymmetric allylation/aza-Prins cyclization/lactonization sequences

Chiral functionalized piperidine and lactone heterocycles are widely spread in natural products and drug candidates with promising pharmacological properties. However, there remains no general asymmetric methodologies that enable rapid assemble both critical biologically important units into one three-dimensional chiral molecule. Herein, we describe a straightforward relay strategy for the construction of enantioenriched bridged piperidine-γ-butyrolactone skeletons incorporating three skipped stereocenters via asymmetric allylic alkylation and aza-Prins cyclization/lactonization sequences. The excellent enantioselectivity control in asymmetric allylation with the simplest allylic precursor is enabled by the synergistic Cu/Ir-catalyzed protocol; the success of aza-Prins cyclization/lactonization can be attributed to the pivotal role of the ester substituent, which acts as a preferential intramolecular nucleophile to terminate the aza-Prins intermediacy of piperid-4-yl cation species. The resulting chiral piperidine-γ-butyrolactone bridged-heterocyclic products show impressive preliminary biological activities against a panel of cancer cell lines.

Redox-Neutral β-C(sp3)-H Functionalization of Cyclic Amines via Intermolecular Hydride Transfer

Herein, we report the first redox-neutral and transition-metal-free β-C(sp³)-H functionalization of cyclic amines via a consecutive intermolecular hydride transfer process. A series of N-aryl pyrrolidines and N-aryl 1,2,3,4-tetrahydropyridines decorated with CF₃ and carboxylic ester functionalities are directly accessed in good yields from pyrrolidines and piperidines. This work pushes forward the application of the intermolecular hydride transfer strategy in one-step assembly of molecular complexity.

Modular, stereocontrolled Cβ-H/Cα-C activation of alkyl carboxylic acids

The union of two powerful transformations, directed C-H activation and decarboxylative cross-coupling, for the enantioselective synthesis of vicinally functionalized alkyl, carbocyclic, and heterocyclic compounds is described. Starting from simple carboxylic acid building blocks, this modular sequence exploits the residual directing group to access more than 50 scaffolds that would be otherwise extremely difficult to prepare. The tactical use of these two transformations accomplishes a formal vicinal difunctionalization of carbon centers in a way that is modular and thus, amenable to rapid diversity incorporation. A simplification of routes to known preclinical drug candidates is presented along with the rapid diversification of an antimalarial compound series.

Discovery, cocrystallization and biological evaluation of novel piperidine derivatives as high affinity Ls-AChBP ligands possessing α7 nAChR activities

A series of novel pyridine-substituted piperidine derivatives were discovered as low nanomolar Ls-AChBP ligands with α7 nAChR partial agonism or antagonism activities. A high-resolution antagonist-bound Ls-AChBP complex was successfully resolved with a classic Loop C opening phenomenon and unique sulfur-π interactions which deviated from our previous docking mode to a large extent. With the knowledge of the co-complex, 27 novel piperidine derivatives were designed and synthesized. The structure-activity relationships (SARs) of the aromatic and pyridine regions were well established and binding modes were illustrated with the help of molecular docking which indicated that interactions with Trp 143 and the "water bridge" are essential for the high binding affinities. Halogen bonding as well as the space around 5'- or 6'- position of the pyridine ring was also proposed to influence the binding conformation of the compounds. Notably, two enantiomers of compound 2 showed opposite functions toward α7 nAChR and compound (S)-2 showed sub-nanomolar affinity (K_i = 0.86 nM) on Ls-AChBP and partial agonism (pEC₅₀ = 4.69 ± 0.11,Emax = 36.1%) on α7 nAChR with reasonable pharmacokinetics (PK) properties and fine ability of blood-brain-barrier (BBB) penetration. This study provided promising hits to develop candidates targeting nAChR-related CNS diseases.

Oxidative β-C-H sulfonylation of cyclic amines

A new and efficient process to access β-functionalisation of cyclic amines via a mild oxidative β-sulfonylation.

A transition metal-free strategy for the dehydrogenative β-sulfonylation of tertiary cyclic amines is described. N-Iodosuccinimide facilitates regioselective oxidative sulfonylation at C–H bonds positioned β to the nitrogen atom of tertiary amines, installing enaminyl sulfone functionality in cyclic systems. Mild reaction conditions, broad functional group tolerance and a wide substrate scope are demonstrated. The nucleophilic character of the enaminyl sulfone is harnessed, demonstrating potential application for scaffold diversification.

Hydride transfer initiated ring expansion of pyrrolidines toward highly functionalized tetrahydro-1-benzazepines

The first example of ring expansion from a five-membered nitrogen-containing ring to a seven-membered heterocyclic ring has been developed.

One-pot multicomponent synthesis of highly functionalized piperidines from substituted β-nitrostyrenes, Meldrum's acid, aromatic aldehydes, and ammonium acetate

An efficient and straightforward synthetic protocol has been developed for the diversity-oriented synthesis of highly functionalized piperidines containing a Meldrum's acid moiety via pseudo five-component reaction between aromatic aldehydes, ammonium acetate, substituted β-nitrostyrenes and Meldrum's acid for the generation of a wide range of structurally interesting and pharmacologically significant compounds.

Synthesis of novel 3-deoxy-3-C-triazolylmethyl-allose derivatives and evaluation of their biological activity

Recently, monosaccharide-triazole conjugates have proved to possess a large variety of useful biological activities. This paper describes synthesis of a new series of 3-deoxy-3-C-triazolylmethyl-allose derivatives. These new compounds are obtained from acetonide-protected 3-deoxy-3-azidomethyl allose and commercial alkynes via Cu(I) catalyzed 1,3-dipolar cycloaddition. The obtained molecular scaffolds differ from those described earlier by the presence of a methylene linker (-CH2-) between the C(3) of allose and the triazole moiety. It was demonstrated that acetonide-protected monosaccharide, 3-deoxy-3-C-(4-phenyl-1H-1,2,3-triazol-1-yl)methyl-1,2:5,6-di-O-isopropylidene-α-d-allofuranose, inhibited α-L-fucosidase for 26% at 0.1 mM concentration, but a deprotected analog, 3-deoxy-3-C-(4-(4-tert-butylphenyl)-1H-1,2,3-triazol-1-yl)methyl-β-d-allofuranose, showed 15% inhibition of β-glucosidase at 1 mM concentration.

Computational methods for the discovery of mood disorder therapies

INTRODUCTION

Despite the significant progress, research is still needed to reveal details of the complex and dynamic chemical processes operating in the central nervous system (CNS) and their relationship to psychological effects such as mood disorders. The incidence of behavioral depression is widely spread worldwide, with an estimated 14.8 million adults diagnosed yearly in the United States alone. The efficacy of current antidepressants on 50 - 60% of patients, their slow onset of action and the prevalence of adverse side effects highlight the need for developing a new generation of improved antidepressants. Computational methods have the potential to aid in the discovery of mood modulators.

AREAS COVERED

This review contains three main sections: historical evolution of marketed antidepressants, physicochemical and structural properties of antidepressant compounds reported in the ChEMBL database and recent efforts in the design and discovery of antidepressants using computational methods. The authors provide details of the computational methods employed, from chemoinformatic analyses to molecular modeling.

EXPERT OPINION

While there have been numerous and important findings in depression research, the high cost and time spent on research into new therapies for brain disorders is a risky undertaking. Computational methodologies can be employed to speed up the discovery of new antidepressants and to detect new sources of chemical compounds with potential antidepressant activity. Compound collections containing compounds already approved in the pharmaceutical and food industries that cover the property space and complement the structural space of CNS drugs represent a promising starting point for the discovery of new antidepressant agents.

Singular value decomposition analysis of the torsional angles of dopamine reuptake inhibitor GBR 12909 analogs: effect of force field and charges

Three-dimensional quantitative structure-activity relationship (3D-QSAR) analysis of large, flexible molecules, such as the dopamine reuptake inhibitor GBR 12909 (1), is complicated by the fact that they can take on a wide range of closely-related conformations. The first step in the analysis is to classify the conformers into groups. Over 600 conformers each of a piperazine (2) and piperidine (3) analog of 1 were generated by random search conformational analysis using the Merck Molecular Force Field (MMFF94). Singular value decomposition (SVD) was used to group the conformers of 2 and 3 by the similarity of their non-ring torsional angles. SVD uncovered subtle differences in their conformer populations due to that fact that the conformers separate along different principal components, and ultimately to the fact that different torsional angles are the chief contributors to these components. The results were compared to our previous SVD analysis (Fiorentino, et al., Journal of Computational Chemistry, 2006, 27, 609-620) of conformer populations of 2 and 3 generated by the Tripos force field and Gasteiger-Hückel charges. Except for the dominant contribution of angle B3 to principal component 8 seen with both force fields, the angles which are chiefly responsible for the grouping of the conformers of 2 and 3 are different with both force fields. This illustrates that SVD is useful in identifying unique groupings of conformers in large data sets of flexible molecules-a first step in selecting representative conformers for 3D-QSAR modeling studies.

Design and synthesis of 2- and 3-substituted-3-phenylpropyl analogs of 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperazine and 1-[2-(diphenylmethoxy)ethyl]-4-(3-phenylpropyl)piperazine: role of amino, fluoro, hydroxyl, methoxyl, methyl, methylene, and oxo substituents on affinity for the dopamine and serotonin transporters

Novel derivatives of 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperazine (GBR 12909, 1) and 1-[2-(diphenylmethoxy)ethyl]-4-(3-phenylpropyl)piperazine (GBR 12935, 2) with various substituents in positions C2 and C3 of the phenylpropyl side chain were synthesized and evaluated for their ability to bind to the dopamine transporter (DAT) and the serotonin transporter (SERT). In the C2 series, the substituent in the S-configuration, with a lone-pair of electrons, significantly enhanced the affinity for DAT, whereas the steric effect of the substituent was detrimental to DAT binding affinity. In the C3 series, neither the lone electron pair nor the steric effect of the substituent seemed to affect DAT binding affinity, while sp (2) hybridized substituents had a detrimental effect on affinity for DAT. In the series, the 2-fluoro-substituted (S)-10 had the highest DAT binding affinity and good DAT selectivity, while the 2-amino-substituted (R)-8 showed essentially the same affinity for DAT and SERT. The oxygenated 16 and 18 possessed the best selectivity for DAT.

Improved 3D-QSAR CoMFA of the dopamine transporter blockers with multiple conformations using the genetic algorithm

A 3D-QSAR/CoMFA was performed for a series of 42 piperidine-based dopamine transporter (DAT) blockers. The overall process consisted of three major steps: (1) a pharmacophore model was built using the Genetic Algorithm Similarity Program (GASP); (2) the Flexible Superposition (FlexS) technique was applied to generate multiple conformations for each of the ligands based on the pharmacophore; (3) the Genetic Algorithm was employed to optimize the selection of the ligand conformations for the CoMFA modeling. The CoMFA models were found to be more detailed in the putative binding site by exploring multiple conformations of each ligand. The comparison of the contour maps shows that, in general, these models are comparable and the differences between them result from the ability of the flexible 3alpha-substituents of the ligands to adopt multiple conformations satisfying the same pharmacophore model. These findings provide guidance for the design and improvement of compounds with DAT activity, which is important for the development of a treatment of cocaine addiction and certain neurological disorders.

Design and synthesis of promiscuous high-affinity monoamine transporter ligands: unraveling transporter selectivity

A series of 4-[2-[bis(4-fluorophenyl)methoxy]ethyl]-piperidines and 4-[2-[(bisphenyl)methoxy]ethyl]-piperidines with different types of substituents in the phenylpropyl side-chain were synthesized and examined for their ability to bind to the dopamine transporter (DAT), the serotonin transporter (SERT), and the norepinephrine transporter (NET). All of the compounds showed high binding affinities for the DAT in the low to subnanomolar range. Their ability to bind to the SERT and the NET, while maintaining their high affinity for the DAT, could be altered by substitution in positions C2 and C3 of the phenylpropyl side-chain. This approach gave rise to a new set of compounds with selectivity for the DAT, the DAT and the SERT, or the DAT and the NET. Six compounds (7, 9, 11, 12, 14, and 20) with relatively low SERT/DAT ratios were selected for additional study in biogenic amine uptake inhibition assays based on the biogenic amine transporter binding results. Some of the new ligands can serve as pharmacological tools to block DAT or DAT and another transporter simultaneously.

Structure-activity studies of 3'-4'-dichloro-meperidine analogues at dopamine and serotonin transporters

The structure-activity relationships of 3',4'-dichloro-meperidine were investigated at dopamine (DAT) and serotonin transporters (SERT). Large ester substituents and lipophilic groups at the 4-position favored molecular recognition at the SERT. The benzyl ester of 3',4'-dichloro-meperidine exhibited high potency and high selectivity for the SERT (DAT/SERT=760). Chemical modification of the ester group and N-substitution generally led to compounds with decreased DAT affinity. Only small esters and alkyl groups were tolerated at the 4-position of the meperidine ring system by the DAT. Overall, the meperidine analogues were generally more selective for the SERT than for the DAT.

Further structure-activity relationship studies of piperidine-based monoamine transporter inhibitors: effects of piperidine ring stereochemistry on potency. Identification of norepinephrine transporter selective ligands and broad-spectrum transporter inhibitors

4-(4-Chlorophenyl)piperidine analogues each bearing a thioacetamide side chain appendage similar to that found in the wake-promoting drug modafinil have been synthesized. The transporter inhibitory activity of both the cis and trans isomers of these 3,4-disubstituted piperidines in both their (+)- and (-)-enantiomeric forms was determined. These studies reveal that the (-)-cis analogues exhibit dopamine transporter/norepinephrine transporter (DAT/NET) selectivity as was previously reported for the (+)-trans analogues. On the other hand, the (-)-trans and the (+)-cis isomers show serotonin transporter (SERT) or SERT/NET selectivity. Among them, (+)-cis-5b shows a low nanomolar Ki for the NET with 39-fold and 321-fold lower potency at the DAT and SERT, respectively, thus making it a useful pharmacological research tool for exploring NET-associated behavioral signatures. On the other hand, several of the compounds described herein, such as (+)-trans-5c, show comparable activity at all three transporters. Because broad-spectrum transporter inhibitors have been hypothesized to exhibit a more rapid onset of action and/or a greater efficacy as antidepressant agents than those selective for SERT or SERT + NET, some of the present compounds will be valuable to study in animal models of depression.

CoMFA Study of Piperidine Analogues of Cocaine at the Dopamine Transporter: Exploring the Binding Mode of the 3α-Substituent of the Piperidine Ring Using Pharmacophore-Based Flexible Alignment

A 3D-QSAR CoMFA study of piperidine-based analogues of cocaine with flexible 3 alpha-substituents is described. A series of pharmacophore models were generated based on three representative compounds 1p, 2i, and 3c using the Genetic Algorithm Similarity Program (GASP) method. The flexible superposition of all studied compounds was performed for each pharmacophore model using the FlexS algorithm and the three-dimensional structure of 2i as a template. All sets of the overlaid structures with the top-ranked conformers were used for CoMFA modeling. Two best initial CoMFA models were selected and further optimized by identifying the best-fitting conformer of each compound. Compared with the initial models, the conventional correlation coefficients r(2) for the optimized models 1 and 2 were improved from 0.90 and 0.837 to 0.997 and 0.993, respectively. The leave-one-out cross-validated coefficients q(2) for the optimized models 1 and 2 were improved from 0.515 and 0.296 to 0.828 and 0.849, respectively. The results of the two CoMFA models suggest that both steric and electrostatic interactions play important roles in the binding of the 3 alpha-substituents of the piperidine-based analogues of cocaine. The contributions from steric and electrostatic fields for model 1 were 0.621 and 0.379, respectively. The contributions from steric and electrostatic fields for model 2 were 0.493 and 0.507, respectively. The two highly predictive CoMFA models indicate that the 3 alpha-substituent has two possible binding modes at the DAT. The CoMFA contour maps provide a visual representation of prospective binding modes of the 3 alpha-substituent of the piperidine-based analogues of cocaine and can be used to design novel DAT inhibitors that may be useful for the treatment of cocaine abuse and certain neurological disorders.