Stereochemical and Computational NMR Survey of 1,2,3-Triazoles: in Search of the Original Tauto-Conformers.

The conformational analysis of nine functionalized 1,2,3-triazoles was carried out by the correlation of calculated and experimental high-level nuclear magnetic resonance (NMR) chemical shifts. In solution, the studied triazoles are in exchange dynamic equilibrium caused by their prototropic tautomerism of the NH-proton. The experimentally unresolved NMR signals were assigned for most of the compounds. A more thorough survey was conducted for 4-t-butyl-1,2,3-triazole-5-carbaldehyde oxime. The analysis performed within the framework of the DP4+ formalism completely confirmed the hypothesis of the predominance of the 2H-tautomer. Thus, the methodology for estimating stereochemical structures in the absence of some experimental data allowed the most stable conformations for dynamic systems with different tautomeric ratios to be reliably identified.

Acrylamide derivatives: A dynamic nuclear magnetic resonance study.

Substituted acrylamides have found an extensive application in organic and medical chemistry; therefore, it is very important to get insight into their features such as electronic structure, spectral properties, and stereochemical transformations. A correct interpretation of the chemical behavior and biological activity of these heteroatomic systems is impossible without knowledge of the structure of stereodynamic forms and factors determining their relative stability. The structure and peculiarities of stereodynamic behavior of substituted acrylamides and their model compounds were studied by dynamic and multinuclear 1 H, 13 C, and 15 N nuclear magnetic resonance (NMR) spectroscopy in CDCl3 and DMSO-d6 solution. It has been established that acrylamides in solution are realized as Z- and E-isomers, with the E-rotamer being somewhat predominant. The obtained experimental values of the free activation energy of rotamers vary within 15-17 kcal/mol, depending on the stereochemical structure of the molecule. 15 N NMR spectroscopy is the most reliable and fastest method for determining the structural and stereochemical features of nitrogen-containing compounds.

Arabinogalactan propargyl ethers in the A3-coupling reaction with aldehydes and secondary cyclic amines.

A family of novel pharmacologically prospective carbohydrate polymer aminomethylpropargyl derivatives has been synthesized by CuI-catalyzed A3-coupling reaction of arabinogalactan propargyl ethers with aldehydes and secondary amines in DMSO at 23-65 °Ð¡. The arabinogalactan derivatives bearing phenyl, pyridinе, and furan moieties in combination with piperidyne, pyrrolidyne, morpholine fragments are prepared in up to 95 % yield and 0.3-2.0 degree of substitution. Conversion of the propargyl groups of the starting arabinogalactan ethers reaches in most cases 80-100 %. The non-catalyzed aminoalkylation (96-100 °Ð¡, degree of substitution up to 1.1) of propargylated AG proceeds both at the ethynyl and hydroxy centers to afford mainly the products of C-substitution. Some of the synthesized compounds and their hydrochlorination products show the pronounced bacteriostatic activity with respect to Gram-negative microorganism.

Pd-catalyzed cross-coupling of arabinogalactan propargyl ethers with 5-bromosalicylic acid.

New salicylate derivatives of arabinogalactan have been synthesized in up to 90% yield by cross-coupling of propargyl ethers of arabinogalactan with 5-bromosalicylic acid salts. The reaction proceeds in the presence of the catalytic system Pd(PPh3)4/PPh3 with CuBr (CuI), and piperidine or 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU) as a base in DMSO at 70-95 °Ð¡. The propargyl groups of the starting arabinogalactan ethers are converted into propinylsalicylate ones with 35 and 50% conversion depending on the nature of the base used. Degree of arabinogalactan substitution with propynyl salicylate fragments reaches 1.0 and 0.7 in the reactions with piperidine and DBU, respectively. In the case of piperidine, along with the main process, intensive unprecedented (for Sonogashira reaction) hydroamination of propargyl groups is also observed, followed by hydrolysis of unstable enamines. The products have been characterized by IR, 1Н and 13С NMR, UV spectroscopy as well as elemental analysis and gel permeation chromatography. Salicylate derivatives of arabinogalactan show high activity in prolonging the activated partial thromboplastin time (APTT), whereas neither the arabinogalactan nor propargyl ethers of arabinogalactan nor salicylic acid salts have obvious effects.

Oxidative cross-coupling of secondary phosphine chalcogenides with amino alcohols and aminophenols: aspects of the reaction chemoselectivity.

Secondary phosphine chalcogenides react with primary amino alcohols under mild conditions (room temperature, molar ratio of the initial reagents 1 : 1) in a CCl4/Et3N oxidizing system to chemoselectively deliver amides of chalcogenophosphinic acids with free OH groups. Under similar conditions, mono-cross-coupling between secondary phosphine chalcogenides and 1,2- or 1,3-aminophenols proceeds only with the participation of phenolic hydroxyl to give aminophenylchalcogenophosphinic O-esters. The yields of the synthesized functional amides or esters are 60-85%.

Arabinogalactan propargyl ethers: Au-catalysed hydroamination by imidazols.

A method for the synthesis of pharmacologically prospective arabinogalactan (AG) imidazole- and benzimidazole derivatives in a yield of up to 97 % via Au(III)-catalyzed hydroamination of AG propargyl ethers has been developed. It is found that in the presence of 5 mol% HAuC14 and 10-fold excess imidazole relative to the propargyl groups, the hydroamination successfully competes with cross-linking processes to afford products soluble in DMSO and aqueous HC1 solutions (degree of substitution of imidazolylpropenyl fragments 0.5-1.8, yield 62-97 %). It is established that under the conditions of hydroamination Au(III) is reduced to give mainly Au(0) and minor amounts of Au(I), which are contained in AG imidazole derivatives as particles of 190-640 nm in size. Hydrochlorides of Au-containing AG imidazole derivatives show high bacteriostatic activity with respect to test gram-positive microorganisms and thus confirming their prospects as new AG-derived bioactive agents.

Cross-coupling of propargylated arabinogalactan with 2-bromothiophene.

Novel pharmacologically prospective derivatives of arabinogalactan (a polysaccharide from larch wood) containing acetylenic and thiophene moieties have been obtained in up to 90% yield by cross-coupling of propargylic ethers of arabinogalactan with 2-bromothiophene. The reaction proceeds in the presence of the catalytic system Pd(Ph3P)4/CuBr/LiBr and piperidine in DMSO at 80-85°Ð¡. An advantageous feature of the synthesis is that it requires 5-25 times lesser catalytic loading than in common Sonogashira protocols thus making the reaction particularly beneficial to synthesize pharmaceutically-oriented polysaccharides.

Modification of arabinogalactan propargyl ethers by triazolyl functions.

Polysaccharide arabinogalactan (AG) has been modified by triazolyl functions via the copper-catalyzed 1,3-dipolar addition of azides to propargyl ethers. A range of new AG triazolo derivatives bearing benzyl, 4-vinylbenzyl, 1-naphthylmethyl, (1-vinylimidazol-2-yl)methyl, (1-ethylimidazol-2-yl)methyl, (1-vinylbenzimidazol-2-yl)methyl, allyl, carboxymethyl (as Na-salt) substituents is prepared by "one-pot" approach from organic azides generated in situ and AG propargyl ethers. The latter (degree of substitution 2.0-2.2) are converted into 1,2,3-triazolo AGs in DMSO/water mixture in the presence of CuSO4·5H2O/sodium ascorbate/Et3N in 82-94% yields and with 60-100% conversion.

Propargylation of arabinogalactan with propargyl halides--a facile route to new functionalized biopolymers.

New arabinogalactan propargyl ethers with degree of substitution (DS) up to 2.8 have been obtained by propargylation of arabino-3,6-galactan (AG) with propargyl bromide (PB) in the two-phase system 30-60% КОН aqueous solution/toluene in the presence of triethylbenzylammonium chloride (TEBAС) or without catalyst (ambient temperature, 1-24 h) in 20-87% yields. The highest yields have been reached using TEBAС as phase-transfer catalyst, though the non-catalytic version proves to be also efficient (DS 2.8, 70% yield). The propargylation of AG is less effective in the systems MОН (M=Na, K)/DMSO. DS of propargyl AG reaches 1.8 (70% yield) when propargyl chloride is used as propargylating agent, while with PB under the same conditions DS value does not exceed 0.4-0.6, the yields being 44-65%. Under the action of t-BuOK/DMSO system, the AG propargyl ethers have been isomerized to the corresponding unstable allenyl ethers. AG propargyl ethers with DS 2.0 have been almost exhaustively metallated with Ag2O/NH3 system to give AG-Ag acetylides derivatives with Ag/C≡C ratio equaling 0.95.

Radical anions of flavonoids.

Several representatives of natural flavonoids and their synthetic nitro-derivatives have been investigated by polarography and electron paramagnetic resonance (EPR) spectroscopy under electrochemical reduction in acetonitrile, dimethylformamide (DMF), dimethylsulfoxide (DMSO) or 1,2-dimethoxyethane. All the compounds studied are reduced in the first stage by one-electron transfer, apart from flavanone, which accepts two electrons simultaneously. However, the primary radical anions were detected by EPR spectroscopy only for 4'-nitroflavone. It was shown that radical anions of other flavonoids quickly dimerized. The analysis of the temperature dependence of the hyperfine interaction constants and broadening of lines in EPR spectra of 4'-nitroflavone radical anions has shown that the distribution of spin density is due to both the change of polarity of the medium and rotation of the nitrophenyl moiety. The assignment of hyperfine structure constants for the 4'-nitroflavone radical anion was confirmed by density functional theory (DFT) calculations.

Tautomerism of 3-nitro-1,2,4-triazole-5-one radical anions.

The non-degenerate tautomers of 3-nitro-1,2,4-triazole-5-one (NTO) radical anions were investigated for the first time by an ESR method during electrochemical reduction of NTO in an aprotic medium.

NMR investigation of chlorophosphorylation products of N-vinylazoles.

The structure of novel phosphorus-containing N-vinylazoles prepared by action of phosphorus pentachloride has been studied by multinuclear (1)H, (13)C, (15)N, (31)P and two-dimensional (2D) NMR spectroscopy. N-vinyl-substituted 1,2-diazoles and 1,2,3-triazoles have undergone phosphorylation, exclusively, on double bond. N-vinylazoles-based hexa-coordinated phosphorus compounds have been synthesized for the first time. (31)P NMR spectroscopy provides the most convenient and unambiguous method for the investigation of E-Z-isomeric structures of phosphorylated enamines.

1H, 13C and 15N NMR spectroscopy and tautomerism of nitrobenzotriazoles.

Benzotriazole nitro derivatives were prepared by nitration of the corresponding benzotriazoles and by methylation or cyclization of appropriate nitro-1,2-phenylenediamines. Structures and tautomerism of the nitrobenzotriazoles were studied by multinuclear (1)H, (13)C, (15)N, and 2D NMR spectroscopy and quantum chemistry.

Configurational assignment of carbon, silicon and germanium containing propynal oximes by means of 13C-1H, 13C-13C and 15N-1H spin-spin coupling constants.

Configurational assignment of five carbon, silicon and germanium containing propynal oximes has been carried out by means of experimental measurements and high-level ab initio calculations of their 13C-1H, 13C-13C and 15N-1H spin-spin coupling constants. The title compounds were shown to exist in the nonequilibrium mixture of E and Z isomers with the energy difference of less than 0.3 kcal/mol calculated at the MP2/6-311G** level.

1H, 13C, 15N and 19F NMR study of acetylation products of heterocyclic thiosemicarbazones.

Novel 2-acetylamino-4-acetyl-5-aryl(heteryl)-1,3,4-thiadiazolines, 2-acetylamino-5-aryl(heteryl)-1,3,4-thiadiazoles, and some of their salts were prepared and studied by multinuclear 1H, 13C, 15N, 19F and 2D NMR spectroscopy. The acetylation of thiosemicarbazones is accompanied by ring closure to form the corresponding 1,3,4-thiadiazolines and 1,3,4-thiadiazoles. 15N NMR spectroscopy is a unique method for the identification of thiadiazole pyridinium salts.

Nonempirical calculations of NMR indirect spin-spin coupling constants. Part 14: azomethines of the alpha, beta-unsaturated aldehydes.

Configurational assignment of seven azomethines obtained from the alpha-functionally substituted and nonsubstituted alpha,beta-unsaturated aldehydes has been performed on the basis of experimental measurements and the high-level ab initio calculations of their 1J(C,C) and 1J(C,H), involving the alpha-imino carbon that demonstrated the marked stereochemical dependence of both coupling constants upon the orientation of the nitrogen lone pair in the diverse isomers of the title azomethines.

Non-empirical calculations of NMR indirect spin-spin coupling constants. Part 13: configurational assignment of aminosulfonylamidines.

Carbon-carbon coupling constants have been experimentally measured using the INADEQUATE pulse sequence in the series of N-[1,2-bis(dialkylamino)-2-arylethylidene]arylsulfonamides obtained from N-(1-aryl-2,2,2-trichloroethyl)amides of arylsulfonic acids. Comparison of the experimental J(C,C) in this series with those calculated at the SOPPA (Second-Order Polarization Propagator Approach) level in the model aminosulfonylamidine provided an unambiguous assignment at the C=N bond of the eight-title aminosulfonylamidines to E-configuration, while the unknown J(C,C) couplings in their inaccessible diverse Z isomers have been predicted with high reliability. The established marked difference between J(C,C) of the corresponding carbon-carbon bonds in cis and trans orientations to the nitrogen lone pair in aminosulfonylamidines provides a powerful tool in the configurational assignment at the C=N bond in a wide series of the related systems containing the C=N-SO2R moiety.