Study Of Tautomerism Research Articles

A New Reactive Ketenaminal: Synthesis, Coupling Reaction, Tautomeric Study, Docking and Antimicrobial Evaluation of the Products.

One of the most successful reagents used in the synthesis of the reactive enaminone is DMF-DMA, but it is very expensive with harmful effects on the human health and reacts with special compounds to generate the enaminone such as active methylene centers. In this article, we synthesized a new ketenaminal by simple method with inexpensive reagents (through desulfurization in diphenylether). Thus, a novel reactive ketenaminal (enaminone) was synthesized from the desulfurization of 2-((2-(4-chlorophenyl)-2-oxoethyl)thio)-5,7-bis(4-methoxyphenyl)pyrido[2,3-d]pyrimidin- 4(3H)-one with diphenylether. The starting keteneaminal was coupled with diazotized anilines via the known coupling conditions to give a new series of 2-(4-chlorophenyl)-1-(2-(arylhydrazono)-2- oxoethyl)-5,7-bis(4-methoxy-phenyl)pyrido[2,3-d]pyrimidin-4(1H)-ones. The structures of the new compounds were elucidated based on their IR, 1H-NMR, 13CNMR, and Mass spectra. Moreover, the potency of these compounds as antimicrobial agents has been evaluated. The results showed that some of the products have high activity nearly equal to that of the used standard antibiotic. Additionally, the docking study was done to get the binding mode of the synthesized compounds with the binding site of the DHFR enzyme. The results of molecular docking of the synthesized arylhydrazono compounds are able to fit in DHFR binding site with binding energies ranging from -4.989 to -8.178 Kcal/mol. Our goal was achieved in this context by the synthesis of new ketenaminal from inexpensive reagents, which was utilized in the preparation of bioactive arylhydrazone derivatives.

Experimental and theoretical study on structure-tautomerism among edaravone, isoxazolone, and their heterocycles derivatives as antioxidants

Edaravone is a heterocyclic pyrazolone compound. It has pronounced effect against free radicals, however renal and hepatic disorders have been reported. Isoxazolones are considered bioisosteric analogues of pyrazolones and may have comparable properties. Thus, we investigated the structural and electronic influences for edaravone, isoxazolone, and their tautomers on antioxidant process. Structure and tautomerism study among edaravone, isoxazolone and their heterocycles derivatives were related to antioxidant mechanisms by using the hybrid DFT method B3LYP with the basis sets 6-31++G(2d,2p). The CH tautomer was the most stable and energetically favored among them. Intramolecular NHN hydrogen bonds and polar medium were responsible for the low energy differences among all possible tautomers. NH tautomers in both systems proved to be better antioxidant by SET (single electron transfer), while OH tautomers were better antioxidant on HAT (homolytic hydrogen atom transfer) mechanism. Theoretical calculation showed that edaravone is more potent than phenylisoxazolone, however, both has similar antioxidant scavenging on experimental DPPH. The carbonyliminic system played a very important role in the antioxidant activity for both studied classes.

Synthesis and Study of Prototropic Tautomerism of 2-(2-Furyl)-1-hydroxyimidazoles

Novel 2-(2-furyl)imidazole derivatives were synthesised. 2-(2-Furyl)-1-methoxyimidazoles and 2-(2-furyl)-1-methylimidazole 3-oxides were used as model compounds in the study of the prototropic tautomerism of 2-(2-furyl)-1-hydroxyimidazoles by means of 1H, 13C NMR and UV/vis spectroscopies. It was demonstrated that the interaction of the π-excessive furyl moiety with an electron-withdrawing carbonyl group in position 5 of imidazole stabilised the N-hydroxy tautomeric form in both deuterated chloroform and d6-DMSO. In ethanol the N-oxide tautomer is also present along with the prevailing N-hydroxyimidazole.

Synthesis of 3-(Piperidin-4-yl)-6,7-dihydro-5H-pyrrolo-[2,1-c][1,2,4]triazole and Theoretical Study of the Hydrazone-Hydrazine Tautomerism of the Intermediate Hydrazonation Product

3-(Piperidin-4-yl)-6,7-dihydro-5H-pyrrolo[2,1-c][1,2,4]triazole was synthesized through a four-step process including etherification, hydrazonation, cyclization, and reduction with an overall yield of 39%. The final product was characterized by 1H NMR and ESI-MS/MS. The molecular structures of benzyl (Z)-4-(2-(pyrrolidin-2-ylidene)hydrazine-1-carbonyl)piperidine-1-carboxylate and related compounds were analyzed using DFT calculations at the B3LYP/6-311+G(d,p) level of theory. The results indicated a higher stability of the hydrazone tautomers.

15N, 13C and 1H NMR study of tautomerism and E/Z isomerism in 3-[(Z)-(2-phenylhydrazinylidene)methyl]quinoxalin-2(1H)-one and 3-[(E)-(2-phenylhydrazinylidene)methyl]quinoxalin-2(1H)-one

The one- and two-dimensional 15N, 13C and 1H NMR spectra of benzenediazonium chloride coupling product with 3-methylquinoxalin-2(1H)-one were measured and analysed. It was convincingly proved that two components of the reaction product obtained by coupling reaction are 3-[(Z)-(2-phenylhydrazinylidene)methyl]quinoxalin-2(1H)-one ((Z)-1, major component) and 3-[(E)-(2-phenylhydrazinylidene)methyl]quinoxalin-2(1H)-one ((E)-1, minor component). Detailed analysis of 15N, 13C and 1H NMR spectra allowed to correct previously published incorrect data describing a tautomeric equilibrium between azo and hydrazo forms. Multinuclear and multidimensional NMR studies have been proven to be extremely effective in resolving the tautomeric forms of ‘azo’ dye systems.

Unusual behavior in di-substituted piperidine and piperazine anthraquinones upon protonation – Spectral, electrochemical, and quantum chemical studies

The objective of the study is to determine the influence of a substituent on the spectroscopic and electrochemical properties of a series of novel 9,10‑anthraquinone piperidine and piperazine derivatives. We conducted UV–Vis spectroscopy, pH-spectroscopic titration, and cyclic voltammetry measurements with an interesting phenomenon observed in the case of di-substituted quinones. Compounds with substituents in position 1 and 8 exhibit a significant increase in the absorption intensity of the monoprotonated form. Spectroscopic studies in the conjunction with time-dependent density functional theory (TD-DFT) calculations were employed to explore the unusual behavior. The state-of-the-art quantum chemistry calculations including bulk solvent in the polarizable continuum model (PCM) enabled the study of prototropic tautomerization and its influence on the position of absorption bands. The results of cyclic voltammetry measurements allowed us to determine the deciding factor influencing the reduction – the number of substituents and the possibility of the hydrogen bond formation of the charged states.

Density Functional Theory (B3LYP/6-311+G(d, p)) Study of Stability, Tautomerism and Acidity of 2-Thioxanthine in Gas and Aqueous Phases

This work is a contribution of theoretical chemistry to the knowledge of 2‑thioxanthine's properties. Its aim first consists in checking the chemistry's results related to the exploitation of semi-empirical methods; it provides theoretical data on the acidity of 2‑thioxanthine tautomers. To do this, the DFT method with the B3LYP functional, associated with the 6‑311+G(d, p) basis set was used. The aqueous phase was modelled with the Polarizable Continuum Model (PCM). The results show that in gas and aqueous phases 2‑thioxanthine can exist as a mixture of four tautomers 2TX(1,3,7), 2TXX(1,3,9), 2TX(1,7,10) and 2TX(1,9,10). The relative stability decreases in the order 2TX(1,3,7)> 2TX(1,3,9)> 2TX(1,9,10)> 2TX(1,7,10). This work establishes that the tautomer 2TX(1,9,10) comes from the 2TX(1,3,7) via the 2TX(1,3,9) one. It demonstrates that the acidity of the most stable tautomer’s nitrogen 2TX(1,3,7), decreases in the order 7> 3> 1 in gas phase and in the order 3> 7>1 in aqueous phase. It provides data to elucidate the mechanisms to understand biological activities of 2‑thioxanthine.

A microwave spectroscopic and ab initio study of keto-enol tautomerism and isomerism in the cyclohexanone-water complex.

Cyclohexanone and water are two important components of secondary organic aerosol (SOA), and understanding the intermolecular interactions between these two species can provide insight to the initial formation mechanism of SOA particles. In this work, we have investigated the keto-enol tautomeric and conformational changes of the cyclohexanone monomer and its monohydrate by Fourier-transform microwave spectroscopy and ab initio calculations. Chirped-pulse and cavity-based FTMW spectrometers in the region of 7-14 GHz were used to measure rotational spectra of the most stable species prepared in a cold molecular beam. The experimental and theoretical results suggest that the chair conformer of the keto tautomer is the most stable structure. We have measured and assigned rotational spectra of ten isotopologues, including all six single 13C substitutions observed in natural abundance and four different isotopic species of water (H2O, D2O, DOH and HOD). The experimental structure of cyclohexanone-water was determined directly using this isotopic information. The analysis reveals the existence of both canonical and secondary hydrogen bonding, which we confirm using QTAIM analyses. To further elucidate the hydrogen bonding characteristics in ketone-water clusters, cyclohexanone-water is compared to a variety of other hydrated ketones, namely formaldehyde, acetone, cyclobutanone, and cyclopentanone, through utilization of the symmetry adapted perturbation theory (SAPT) energy decomposition method. The results of this study shed light on the effects of water on keto-enol tautomerization, and the role of hydrogen bonding in ketone monohydrates and the formation of related SOA particles.

Experimental and Theoretical Spectroscopic Study of Thione-Thiol Tautomerism of New Hybrides 1,3,4-Oxadiazole-2-thion with Acridine-9(10H)-one

Experimental and Theoretical Spectroscopic Study of Thione-Thiol Tautomerism of New Hybrides 1,3,4-Oxadiazole-2-thion with Acridine-9(10H)-one

Ab Initio, Transition State Theory, and Kinetic Modeling Study of the HO2-Assisted Keto-Enol Tautomerism Propen-2-ol + HO2 ⇔ Acetone + HO2 under Combustion, Atmospheric, and Interstellar Conditions.

Keto-enol tautomerisms are important reactions in gaseous and liquid systems with implications in different chemical environments, but their kinetics have not been widely investigated. These reactions can proceed via a unimolecular process or may be catalyzed by another molecule. This work presents a theoretical study of the HO2-catalyzed tautomerism that converts propen-2-ol into acetone at conditions relevant to combustion, atmospheric and interstellar chemistry. We performed CCSD(T)/aug-cc-pVTZ//M06-2X/cc-pVTZ ab initio and multistructural torsional variational transition state theory calculations to compute the forward and reverse rate constants. These rate constants have not been investigated previously, and modelers approximate the kinetics by comparison to analogue reactions. Two features of the potential energy surface of the studied tautomerism are highlighted. First, the HO2 radical exhibits a pronounced catalytic effect by inducing a double hydrogen atom transfer reaction with a much lower barrier than that of the unimolecular process. Second, a prereactive complex is formed with a strong OH···π hydrogen bond. The role of the studied reaction under combustion conditions has been assessed via chemical kinetic modeling of 2-butanol (a potential alternative fuel) oxidation. The HO2-assisted process was found to not be competitive with the unimolecular and HCOOH-assisted tautomerisms. The rate constants for the formation of the prereactive complex were calculated with the variable reaction coordinate transition state theory, and pressure effects were estimated with the system-specific quantum Rice-Ramsperger-Kassel theory; this allowed us to investigate the role of the complex by using the canonical unified statistical model. The formation and equilibration of the prereactive complex, which is also important at low pressures, enhances the reactivity by inducing a large tunneling effect that leads to a significant increase of the rate constants at cold and ultracold temperatures. These findings may help to understand and model the fate of complex organic molecules in the interstellar medium, and suggest an alternative route for the high energy barrier keto-enol tautomerism which otherwise is not kinetically favored at low temperatures.

Experimental and theoretical study of the excited-state tautomerism of 6-azauracil in water surroundings

The irradiation of water solution of 6-azauracil showed that one of the photoreactions is the phototautomerization of the oxo tautomer of the compound into the hydroxy tautomer. The photoreaction was identified by a thorough analysis of experimentally obtained UV-spectra and the theoretical spectra simulation of several hydroxy tautomers of the compound. Furthermore we performed DFT level (BLYP/aug-cc-pVDZ) investigations of the mechanism of phototautomerization of the oxo tautomer of 6-azauracil in water surroundings applying PCM. The complete excited-state reaction path of the mechanism was proposed. It was found that the phototautomerization occurs along the 1πσ∗ excited-state reaction path. This reaction path was found to connect the two conical intersections S0/S1 of the H-detachment processes in the oxo and hydroxy tautomers of 6-azauracil.

Theoretical study on excited-state multiple proton-transfer process of 7-azaindole with ammonia and mixed water-ammonia: Stepwise or concerted?

A detailed study of the excited-state tautomerization of 7AI-(NH3)n (n = 1, 2), 7AI-H2O-NH3 and 7AI-NH3-H2O complex in the gas phase were investigated at the CASSCF(10, 9)/6-31G(d, p) level theoretically. The effect of dynamic electron correlation on the barrier height and reaction energy was considered by MRPT2. The excited-state double proton transfer in 7AI-NH3 takes place in a concerted but asynchronous protolysis pathway. However, the triple proton transfer in 7AI cluster along the H-bonded ammonia dimer prefers a two-step process with a distinct intermediate. Each proton transfer step was a concerted but asynchronous path. The formation of H-bonded chain consisting of mixed water-ammonia affected the excited-state proton transfer mechanism. With the stabilization effect of the second solvent molecule, the first solvent molecule with large or small proton-accepting ability made the ESTPT occur in a stepwise or concerted pattern.

Synthesis and study of the azide-tetrazole tautomerism in 2-azido-4-(trifluoromethyl)-6-R-pyrimidines (R = H, 4-ClC6H4)

Azide-tetrazole equilibrium in azidopyrimidines bearing trifluoromethyl group on the example of 2-azidopyrimidines has been studied. The latter were synthesized via nitrosation of 2-hydrazino-4-trifluoromethylpyrimidine and reaction of NaN3 with 4-trifluoromethyl-2- chloro-6-(4-chlorophenyl)pyrimidine. The tautomeric equilibrium 5-trifluoro methyl tetrazolo[1,5-a]pyrimidine ⇆ 2-azido-4-trifluoromethylpyrimidine was observed in the absence of solvent and in DMSO-d6 solution, whereas in CDCl3 only an azide form exists. For 2-azido- 4-trifluoromethyl-6-(4-chlorophenyl)pyrimidine, only an azide isomer was detected in CDCl3 solution, and in DMSO-d6 solution it is in equilibrium with 5-(trifluoromethyl)-7-(4-chlorophenyl) tetrazolo[1,5-a]pyrimidine (the tautomer ratio is 99 : 1). Thermodynamic and kinetic parameters of 5-trifluoromethyltetrazolo[1,5-a]pyrimidine ⇆ 2-azido-4-trifluoromethylpyri midine tautomeric rearrangement in DMSO-d6 for 5-trifluoromethyltetrazolo[1,5-a]pyrimidine were determined using the exchange spectroscopy (EXSY) technique.

Theoretical Study of Tautomerization in 1,5-Dimethyl-6-Thioxo-1,3,5-Triazinane-2,4-Dione

In this work, the tautomeric transformations of a 1,5-dimethyl-6-thioxo-1,3,5-triazinane-2,4-dione molecule are explored at the M062X/6-311G(d,p) level of theory in gas and solution phases. These calculations show that the 1,5-dimethyl-6-thioxo-1,3,5-triazinane-2,4-dione isomer is more stable than its tautomer (4-hydroxy-1,5-dimethyl-6-thioxo-5,6-dihydro-1,3,5-triazin-2(1H)-one) in gas and solution phases. The frontier molecular orbitals and band gap energy calculations are performed at the M062X/6-311G(d,p) level in gas and various solvents. Solvent effects are analyzed using the self-consistent reaction field method based on the polarizable continuum model in chloroform, chlorobenzene, tetrahydrofurane, dichloromethane, and quinoline. The solvent effect on the N–H and C=O vibrations is explored. Also, natural bond orbital (NBO) analysis was used to understand the structure and bonding of the molecule.

Theoretical study of tautomerization in 1,5-dimethyl-6-thioxo-1,3,5-triazinane-2,4-dione

Theoretical study of tautomerization in 1,5-dimethyl-6-thioxo-1,3,5-triazinane-2,4-dione

Density functional evaluation and a feasibility study of intramolecular thione-thiol tautomerization

Performance of various density functionals for use in geometry and energetics calculation of thiocarbonyl systems is evaluated. The three biologically significant systems such as 2-methyl-3-thiosemicarbazide, 1-acetyl-2-thiohydantoin and bornane-2-thione are selected for the geometry analysis. Energy of tautomerization of thioacetone and enthalpy of formation of 2-thiohydantoin are considered for analyzing the performance in energetics. Minnesota functionals such as M05-2X, M06-2X and range separated hybrid functional wB97 give best results in both geometry and energetics calculations. Studies on basis set effect showed reliable performance by 6-31 + G* and 6-31++G** over higher basis sets. Feasibility of tautomerization in the aforementioned thiones has been investigated using the best performing density functionals. The proposed thione–thiol tautomerization step during PPC-DC catalyzed decarboxylation reaction has also been investigated and the computed high energy barrier shows less feasibility of the reaction.

Density functional theory study of the zeolite-mediated tautomerization of phenol and catechol

Because the structure of lignin consists mostly of inter-linked phenolic monomers, its conversion into more valuable chemicals may benefit from isomerization processes that alter the electronic structure of the aromatic rings. The tautomerization of phenolic-type compounds changes the hybridization from sp2 to sp3 of the carbon atom at the ortho position, which disables the aromaticity and facilitates the subsequent hydrogenation process. Here, we have performed a Density Functional Theory study of the tautomerization of phenol and catechol at the external surface of zeolite MFI. The tautomerization starts with the adsorption of the molecule on three-coordinated Lewis acid sites, followed by the dissociation of the phenolic hydroxyl group, with the transfer of the proton to the zeolite framework. The rotation of the deprotonated molecule enables a more favourable orientation for the back-transfer of the proton to the carbon atom at the ortho position. The energy barriers of the process are smaller than 55kJ/mol, suggesting that this transformation is easily accessible under standard reaction conditions.

Synthesis of New Dyes from Imidazo[1,2-a]Pyridine: Tautomerism, Spectroscopic Characterisation, DFT/TD-DFT Calculations, Atoms in Molecules Analyses and Antibacterial Activities

The synthesis, optical properties, theoretical calculations and antibacterial activity of a series of new heterocyclic dyes from imidazo[1,2-a] pyridine are described. The key intermediate 2-[3-(hydroxyimino)imidazo[1,2-a]pyridin-2(3H)-ylidene]malononitrile was obtained via the nucleophilic substitution of hydrogen in 3-nitroimidazo[1,2-a]pyridine with malononitrile in basic methanol solution. Tautomerism, oxidation and alkylation studies on the dye led to the synthesis of new heterocyclic indigo-coloured, purple, and orange dyes in good yields. The structures of all newly synthesised compounds were confirmed by spectral and analytical data. The optical properties of the dyes were spectrally characterised and shown to exhibit interesting photophysical properties including high extinction coefficients. Density functional theory calculations of the dyes were performed to provide the optimised geometries and relevant frontier orbitals. Calculated electronic absorption spectra were also obtained by the time-dependent density functional theory method. In addition, electrostatic potential maps and electron density maps of the dyes were evaluated by AIM (atoms in molecules) analysis. Moreover, the new dyes exhibited potent antibacterial activity and their antibacterial activities (MIC) against Gram-positive and Gram-negative bacterial species were determined.

Indirubins: A Potential Therapeutic Target in Multiple Myeloma

Current drugs in the treatment of Multiple Myeloma (MM) result in cell death via a number of mechanisms including a direct effect on plasma cells as well as alteration in the BM microenvironment. Although effective to some extent, none of the drug mechanisms of action are fully targeting a biological process essential and necessary for PC survival. Among the FDA approved kinase inhibitors, few are based on natural scaffolds. 6-bromoindirubin-39-oxime (6BIO) is a potent kinase inhibitor based on the natural 6-bromoindirubin scaffold. Indirubin and 6-bromoindirubin are two natural products that have found a particular interest in dye chemistry as the main constituent of indigo and Tyrian purple dyes. Recent findings discovered that 6BIO was a promising anti-cancer agent acting on the JAK/STAT signaling pathway mediating cell proliferation. After enhancement of the chemical structure of 6BIO, further reports exposed that MLS-2438 and MLS-2384 were Akt signaling pathway inhibitor (MLS-2438) and potent c-Src kinase direct inhibitors. A library containing 2000 natural molecules was constructed using several data platforms. Each molecule was processed through different filters such as tautomeric studies, protonation and steroisomerism status in order to be used for calculations of virtual evaluation. Two approaches were followed: the structure-based virtual screening and the ligand-based virtual screening. To achieve structural based virtual screening, binding and evaluation of the chemical relation of each molecule in the crystallographic structure of the proteasome β5 subunit was performed. In the ligand-based virtual screening, calculations were made to identify the structural similarities of each molecule with the known proteasome inhibitor, bortezomib. The results of both approaches were combined, the molecules ranked, and 100 out of 2000 were identified as strong potential bioactive hits for the β5 subunit. Out of these 100 molecules, the chemical structures of high interest were the following: indole alkaloids derivatives (indirubins), flavonoids, secoiridoids, simple phenolic acids and acetophenone. A rational selection of indirubins derivatives was conducted in order to study their cytotoxic effects on MM. Fifty indirubins derivatives were selected based on different criteria: structure, known/unknown targets, chemodiversity in substitution patterns. To explore the inhibitory effects of indirubins in MM, we performed the WST1 proliferation assay in three MM cell lines (H929, JJN3, L363). Initially, all the selected indirubins (~50 indirubins) were tested at 7.5μM in L363 cell line and proliferation results from the WST1 assay extracted after 24 hours of treatment. More than half of the indirubins tested displayed more than 50% reduction of the proliferation at 7.5μM. Interestingly, 10 out of the 50 indirubins tested reduced more than 80% proliferation after 24 hours. The most active indirubins were tested in H929 and JJN3 cell lines, where similar effects were seen after 24 hours of treatment. All tested indirubins acted in a dose-dependent manner. Based on our first set of data, we suggest that indirubins have significant anti-proliferative effects on MM cell lines. Among the most active indirubins, two molecules namely 805 and 673 emerged as attractive for further development. Compound 805 is an analog of MLS-2384 while compound 673 is an analog of MLS-2438. The latter derivative represents a promising candidate displaying an IC50below the micromolar range on H929 and JJN3 cells. To determine the kind of cell death caused by one of the most active indirubins, 673, cell cycle analysis was performed before and after treatment in H929 cell line. In particular changes in RNA expression of 84 genes key to cell cycle regulation were analyzed in H929 cell line. Our results show that among other genes, the ones which have a dramatic increase in their expression (>5 fold) are mainly involved in cell cycle arrest such as GADD45A, CDC34, TP53, CHEK1 and CHEK2. A more detailed analysis of the profiler array will be presented at the meeting. In conclusion, this is the first study to show the inhibitory effects of indirubins in MM. Further investigation of these compounds may offer a therapeutic advantage that would affect MM pathogenesis and treatment. Disclosures Kastritis:Amgen: Consultancy, Honoraria; Janssen: Consultancy, Honoraria; Takeda: Consultancy, Honoraria; Genesis: Consultancy, Honoraria. Terpos:Amgen: Consultancy, Honoraria; Takeda: Consultancy, Honoraria; BMS: Consultancy, Honoraria; Janssen: Consultancy, Honoraria. Dimopoulos:Amgen: Consultancy, Honoraria, Membership on an entity9s Board of Directors or advisory committees; Genesis: Consultancy, Honoraria; Novartis: Consultancy, Honoraria, Membership on an entity9s Board of Directors or advisory committees; Janssen: Consultancy, Honoraria, Membership on an entity9s Board of Directors or advisory committees; Celgene: Consultancy, Honoraria, Membership on an entity9s Board of Directors or advisory committees.

Theoretical and experimental study of imine-enamine tautomerism of condensation products of propanal with 4-aminobenzoic acid in ethanol

The tautomerism of the reaction products of propanal with 4-aminobenzoic acid in ethanol was studied by J-modulated spin-echo (JMOD) 13C NMR spectroscopy and gradient-enhanced heteronuclear (ge-2D) 1H–13C HSQC spectroscopy. The existence of imine and enamine tautomeric forms of the reduced compounds in solution was established. The tautomeric equilibrium of the condensation product of propanal with 4-aminobenzoic acid in ethanol was found to be shifted toward the imine form. Quantum chemical calculations by the density functional theory (DFT) method demonstrated that the 4-(N-propylidene)aminobenzoic acid molecule forms a stronger hydrogen bond with an ethanol solvent molecule compared to the enamine molecule, resulting in a higher stability of the ethanol adduct of azomethine compared to the adduct of enamine.