Imidazole-Catalyzed Monoacylation of Symmetrical Diamines

Dihydrobenzothiazole coupled N-piperazinyl acetamides as antimicrobial agents: Design, synthesis, biological evaluation and molecular docking studies

Substituted saturated N-heterocycles have gained momentum as effective scaffolds for the development of new drugs. In this study, we coupled partly saturated benzothiazoles with substituted piperazines and evaluated their antimicrobial activity. Following a three-step reaction sequence from commercially available cyclic 1,3-diones, a series of novel 2-[4-substituted-1-piperazinyl]-N-(7-oxo-4,5,6,7-tetrahydrobenzo[d]thiazol-2-yl)acetamides (7a-af) were synthesised. 2-Amino-5,6-dihydro-benzo[d]thiazol-7(4H)-ones, obtained through the condensation of cyclohexane-1,3-diones with thiourea, were acetylated with chloroacetic chloride and then reacted with N-substituted piperazines 6a-p to give the desired products 7a-af in excellent yields. All 32 new compounds were fully characterised by their 1 H-nuclear magnetic resonance (NMR), 13 C-NMR and high-resolution mass spectrometry spectra. The synthetic compounds 7a-af were tested in vitro for their efficacy as antimicrobials against pathogenic strains of Gram-positive and Gram-negative bacteria, Streptococcus mutans and Salmonella typhi, respectively, as well as against fungal strains, including Candida albicans 3018 and C. albicans 4748. Ciprofloxacin and fluconazole served as the reference drugs. While compounds 7c and 7l showed inhibition against fungal strains with zones of inhibition of 11 and 1 mm, respectively, four analogues (7d, 7l, 7n, and 7r) demonstrated strong antibacterial action (zone of inhibition in the range of 10-15 mm). Three compounds (7j, 7l, and 7w) also exhibited moderate antitubercular activity (MIC: 6.25 µg/mL) against Mycobacterium tuberculosis H37Rv. Molecular docking investigations and the predicted physicochemical and ADMET (absorption, distribution, metabolism, excretion, and toxicity) properties for the potent compounds made this scaffold useful as a pharmacologically active framework for the development of potential antimicrobial hits.

Simplified Procedure for General Synthesis of Monosubstituted Piperazines-From a Batch Reaction Vessel to a Flow (Microwave) Reactor

We reported a novel simplified synthetic procedure for the preparation of monosubstituted piperazine derivatives which can now be easily prepared in a one-pot-one-step way from a protonated piperazine with no need of introduction of a protecting group. Reactions, proceeding either at room or higher temperatures in common solvents, involve heterogeneous catalysis by metal ions supported on commercial polymeric resins. A general synthetic scheme was successfully applied to afford a wide range of monosubstituted piperazines. Furthermore, we picked up a set of piperazine derivatives and studied the possibilities of microwave acceleration of given synthetic reactions to make them even more efficient. Our research proceeded from a simple batch technique to the construction of a flow microwave reactor prototype and resulted in promising findings which are summarized and discussed in the article.

NMR-based investigations of acyl-functionalized piperazines concerning their conformational behavior in solution

Selected N-benzoylated piperazine compounds were synthesized to study their conformational behavior using temperature-dependent 1H NMR spectroscopy. All investigated piperazines occur as conformers at room temperature resulting from the restricted rotation of the partial amide double bond. In the case of selected mono-N-benzoylated and unsymmetrically N,N'-substituted derivatives, the appearance of the 1H NMR spectrum was further shaped by the limited interconversion of the piperazine chair conformations. Therefore, two different coalescence points T C were determined and their resulting activation energy barriers ΔG ‡ were calculated to be between 56 and 80 kJ mol-1. In most of the cases, T C and ΔG ‡ for the amide site appeared to be higher than the corresponding values for the ring inversion. The influences of substituents on rotational and inversion barriers were analyzed by correlation to Hammett constants. The obtained results are discussed and interpreted in the context of literature data. An additional aryl substituent connected at the amine site led to reduced rotational and inversion barriers compared to the free secondary amine. To support and evidence the findings from the NMR analyses, single crystals of selected piperazines were obtained and XRD analyses were performed. To underline the results, two potential TGase 2 inhibitors were investigated showing energy barriers with similar values.

1-[(2,3-Dihydro-1-benzofuran-2-yl) methyl]piperazines as novel anti-inflammatory compounds: Synthesis and evaluation on H3 R/H4 R

The histamine receptors (HRs) are members of G-protein-coupled receptor superfamily and traditional targets of huge therapeutic interests. Recently, H₃ R and H₄ R have been explored as targets for drug discovery, including in the search for dual-acting H₃ R/H₄ R ligands. The H₄ R, the most recent histamine receptor, is a promising target for novel anti-inflammatory agents in several conditions such as asthma and other chronic inflammatory diseases. Due to similarity with previously reported ligands of HRs, a set of 1-[(2,3-dihydro-1-benzofuran-2-yl)methyl]piperazines were synthesized and evaluated in competitive binding assays as H₃ R/H₄ R ligands herein. The results showed the compounds presented affinity (K_i ) for H₃ R/H₄ R in micromolar range, and they are more selective to H₃ R. All the compounds showed no important cytotoxicity to mammalian cells. The phenyl-substituted compound LINS01005 has shown the higher affinity of the set for H₄ R, but no considerable selectivity toward this receptor over H₃ R. LINS01005 showed interesting anti-inflammatory activity in murine asthma model, reducing the eosinophil counts in bronchoalveolar lavage fluid, as well as the COX-2 expression. The presented compounds are valuable prototypes for further improvements to achieve better anti-inflammatory agents.

Synthesis, dynamic NMR characterization and XRD studies of novel N,N'-substituted piperazines for bioorthogonal labeling

Novel, functionalized piperazine derivatives were successfully synthesized and fully characterized by ¹H/¹³C/¹⁹F NMR, MS, elemental analysis and lipophilicity. All piperazine compounds occur as conformers resulting from the partial amide double bond. Furthermore, a second conformational shape was observed for all nitro derivatives due to the limited change of the piperazine chair conformation. Therefore, two coalescence points were determined and their resulting activation energy barriers were calculated using ¹H NMR. To support this result, single crystals of 1-(4-nitrobenzoyl)piperazine (3a, monoclinic, space group C2/c, a = 24.587(2), b = 7.0726(6), c = 14.171(1) Å, β = 119.257(8)°, V = 2149.9(4) ų, Z = 4, D_obs = 1.454 g/cm³) and the alkyne derivative 4-(but-3-yn-1-yl)-1-(4-fluorobenzoyl)piperazine (4b, monoclinic, space group P2₁/n, a = 10.5982(2), b = 8.4705(1), c = 14.8929(3) Å, β = 97.430(1)°, V = 1325.74(4) ų, Z = 4, D_obs = 1.304 g/cm³) were obtained from a saturated ethyl acetate solution. The rotational conformation of these compounds was also verified by XRD. As proof of concept for future labeling purposes, both nitropiperazines were reacted with [¹⁸F]F^–. To test the applicability of these compounds as possible ¹⁸F-building blocks, two biomolecules were modified and chosen for conjugation either using the Huisgen-click reaction or the traceless Staudinger ligation.

Expanding the structural diversity of Bcr-Abl inhibitors: Hybrid molecules based on GNF-2 and Imatinib

In order to expand the structural diversity of Bcr-Abl inhibitors, twenty hybrids (series E and P) have been synthesized and characterized based on Imatinib and GNF-2. Their biological activities were evaluated in vitro against human leukemia cells. Most compounds exhibited potent antiproliferative activity against K562 cells, especially for compounds E4, E5 and E7. Furthermore, these new hybrids were also screened for Abl kinase inhibitory activity, and some of them inhibited Abl kinase with low micromolar IC50 values. In particular, compound P3 displayed the most potent activity with IC50 value of 0.017 μM comparable with that of Imatinib. Molecular docking studies indicated that these novel hybrids fitted well with the active site of Bcr-Abl. These results suggested the great potential of these compounds as novel Bcr-Abl inhibitors.

SNAr reaction in aqueous medium in the presence of mixed organic and inorganic bases

N-Arylation of amines with fluorobenzonitriles in aqueous medium is described. A mixture of N,N-diisopropylethyl amine and Na₂CO₃ (1 : 1) is found to achieve maximum conversion by refluxing for 3 hours in water.

One-step cyclization: synthesis of N-heteroalkyl-N'-tosylpiperazines

Piperazine derivatives are important intermediates in organic synthesis and useful building blocks in pharmaceutical and fine chemical industries. Currently available synthetic routes for these heterocyclic compounds have limited scope owing to the harsh reaction conditions, low yields, and multistep process. Herein, we reported a practical method for synthesis of alkyl-, alcohol-, amine-, and ester-extended tosylpiperazines under mild conditions with moderate to high yields. This protocol exhibits potential applicability in the synthesis of pharmaceuticals and natural products because of the operational simplicity and the conveniently available reactants. On the basis of the experimental and theoretical results, a plausible mechanism of aliphatic nucleophilic substitution (S(N)) in the cyclization has been postulated and evidence for the formation of a six-membered ring has also been confirmed by means of density functional theory (DFT) calculations.

Efficient and continuous monoacylation with superior selectivity of symmetrical diamines in microreactors

Efficient and continuous monoacylation of symmetrical diamines performed in microreactors yielded superior selectivity to that predicted by statistical considerations. It is highly valuable that the kinetically controlled product in high yields was achieved without any special catalyst at ambient temperature.

Catalytic enantioselective desymmetrization of meso-diamines: a dual small-molecule catalysis approach

The desymmetrization of meso-diamines was accomplished via enantioselective monobenzoylation facilitated by the cooperative action of two small-molecule catalysts. A chiral acyl-transfer reagent is formed in situ through the interplay of benzoic anhydride, 4-(dimethylamino)pyridine as a nucleophilic catalyst, and a chiral amide-thiourea cocatalyst.