Thiazolopyridinium salts. Part II. Approaches to thiazolo[3,4-a]-pyridinium salts and isothiazolo[2,3-a]pyridinium salts
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Abstract:
Attempts to synthesise thiazolo[3,4-a]pyridinium salts (3) and isothiazolo[2,3-a]pyridinium salts (2) by dehydration with boiling acetic anhydride of the 5,6,7,8-tetrahydro-8-oxothiazolo[3,4-a]pyridinium salts (11) and (12) and of the 4,5,6,7-tetrahydro-4-oxoisothiazolo[2,3-a]pyridinium salt (6) respectively are reported. The reasons for the failure of the aromatisation are discussed. 7-Bromo-8-hydroxy-3-methylthiazolo[3,4-a]pyridinium bromide (17) has been prepared by an alternative route.Keywords:
Pyridinium Compounds
Under conventional and silica-supported Muffle furnace methods, water-soluble substituted trimeric triaryl pyridinium cations with various inorganic counter anions are synthesized. The solvent-free synthesis method is superior to the conventional method in terms of non-toxicity, quicker reaction times, ease of workup, and higher yields. Trimeric substituted pyridinium salts acted as excellent catalytic responses for the preparation of Gem-bisamide derivatives compared with available literature. To evaluate the molecular docking, benzyl/4-nitrobenzyl substituted triaryl pyridinium salt compounds with VEGFR-2 kinase were used with H-bonds, π-π stacking, salt bridges, and hydrophobic contacts. The results showed that the VEGFR-2 kinase protein had the most potent inhibitory activity. Intriguingly, the compound [NBTAPy]PF6- had a strongly binds to VEGFR-2 kinase and controlled its activity in cancer treatment and prevention.
Pyridinium Compounds
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Cholesterol appended pyridinium ureas 1 and 2 have been designed and synthesized. The unsymmetrical urea-based chemosensor 1 fluorimetrically distinguishes F− from the other anions examined in both CH3CN and DMSO with appreciable binding constant values. The pyridinium – based symmetrical compound 2 acts as a low molecular weight gelator (LMWG) in CHCl3 and is capable of detecting F− visually by breaking the gel. On the contrary, the chemosensor 1 in DMSO in the presence of tetrabutylammonium fluoride undergoes a change from sol to gel state that produces an unambiguous visual readout of F−.
Pyridinium Compounds
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Oxidation of 1-substituted 2-aminoimidazo[1,5-a]pyridinium salts with an excess of concentrated nitric acid gave the corresponding 2-pyridylmethylenecarbamic acid hydro-salts. Oxidation of 3-substituted -2-aminoimidazo[1,5-a]pyridinium bromide with nitric acid in acetic acid gave the corresponding 1-nitro-3-substituted imidazo[1,5-a]-pyridines. The parent 2-aminoimidazo[1,5-a] pyridinium bromide yielded either type of product depending upon the reaction conditions.Oxidation of 2-amino-1-phenylimidazo[1,5-a]pyridinium bromide with lead(IV) acetate in acetic acid solution gave 2-acetamido-3-oxo-1-phenyl-2,3-dihydroimidazo[1,5-a] pyridine.
Nitric acid
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Pyridinium Compounds
Thiazolidine
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Efficient and convenient oxidation of aliphatic and aromatic thiols to disulfides and of sulfides to sulfoxides with pyridinium hydrobromide perbromide (PHBP), nicotinic acid hydrobromide perbromide (NAHBP), and 2,6-dicarboxy pyridinium hydrobromide perbromide (DCPHBP) in a solvent or under solvent free conditions and at ambient temperature is introduced.
Hydrobromide
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Attractive interactions between a thiocarbonyl group and a pyridinium nucleus, and between a carbonyl group and a pyridinium nucleus have been proven by 1H and 13C NMR studies, UV−vis spectral analyses, and X-ray crystallographic analyses of nicotinic amides 1 and 3, and pyridinium salts 2 and 4. Comparison of the Δδ values, which are the differences in the chemical shifts with reference compounds 5 or 6, showed that the absolute Δδ values of 2 and 4 are much larger than those of 1 and 3. In the UV−vis spectra, the n→π* absorption of the CS group of 2a exhibited a significant blue shift in CHCl3. X-ray crystallographic analysis of 1−4 clearly showed that the CS group of 2a and the CO group of 4 are very close to the pyridinium moiety compared to the case of 1 and 3. In addition, the X-ray crystal packing structure of 2a showed the CS group is sandwiched between two pyridinium rings. These experimental results strongly suggested the existence of attractive (CS)···Py+ and (CO)···Py+ interactions in solution and in crystal. The optimized geometries of 1 and 2 calculated at the HF/6-311G** level are in good agreement with their X-ray geometries. MP2/6-311G** calculations for the model systems of pyridinium salts 2 and 4 predicted that the electrostatic and induction energies are the major source of the attractive interactions. Since the larger contribution of electrostatic and induction interactions are characteristic features of cation−π interactions, the (CS)···Py+ and (CO)···Py+ interactions would be classified as a cation−π interaction.
Pyridinium Compounds
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The synthesis and NMR study of fourteen N-arylpyridinium salts have been done. Chemical shifts and coupling constants ( 1 H- 1 H, 1 H- 13 C, 1 H- 19 F, and 13 C- 19 F) have been measured in hexadeuteriodimethyl sulphoxide. Conformation about the phenylpyridinium bond is discussed. The electronic properties of the three pyridinium substituents ( pyridinium, 2,4,6-trimethylpyridinium, and 2,4,6-triphenylpyridinium) have been determined (Hammet and Taft's σ values) and compared with those of the trimethylammonium substituent.
Pyridinium Compounds
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Fluorine
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The specific conductance of pyridinium , N‐methylpyridinium chloride, N‐methylpyridinium bromide, 4‐methylpyridinium bromide, and pyridinium bromide, was measured as a function of temperature. The NMR spectra of the first three fused salts were also measured over the same temperature ranges. Correlations between conductance data and spectral data were found to exist.
Pyridinium Compounds
Proton NMR
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A novel type of [2+4] capsules based on triple-ion interactions was obtained. Four monovalent anions (bromide, nitrate, acetate, and tosylate) bring together two tetrakis(pyridiniummethyl)tetramethyl cavitands by pyridinium-anion-pyridinium interactions. ESI-MS experiments have confirmed the capsule structure due to different fragmentation pathways of triple ions, cations, and ion-pairs. Each capsule encapsulates one or two anions, depending on its size. The capsules exist in equilibrium with hemicapsules containing three walls. The latter form complexes with phenols and anilines to give new unsymmetrical capsules containing both pyridinium-anion-pyridinium and pyridinium-guest-pyridinium walls.
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