Search results for "tautomer"
showing 10 items of 141 documents
rac-(3E,3aR,6aR)-3-(Hydroxymethylene)-3,3a,6,6a-tetrahydro-2H-cyclopenta[b]furan-2-one
2007
The crystal structure of the title compound, C8H8O3, was determined in the course of our studies of the synthesis of cyclopenta[1,2-b]furan-4-one derivatives. The title compound has two chiral C atoms and was obtained as a racemic mixture. It was found to possess a vinylogous acid group with an E configuration at the double bond. The compound exists in the hydroxymethylene and not in the tautomeric carbaldehyde form. The asymmetric unit consists of two molecules.
Addition reactions of heterocycles. VI. Reactions of 1,2-dimethylpyrrole and 1-methyl-2-carbomethoxypyrrole with nitrilimines
1978
Addition reactions of 1,2-dimethylpyrrole and 1-methyl-2-carbomethoxypyrrole with C-acetyl-N-phenylnitrilimine, have been investigated. 1,2-Dimethylpyrrole gives three different types of adducts: i.e. bis-cycloadducts (Vc) and (VIc), spirocycloadduct (IX), and non cyclic bis-adduct (XII). On the other hand, 1-methyl-2-carbomethoxypyrrole gives the bis-cycloadduct (VIb) only. Compound XII arises probably through a double 1,3-addition reaction, whereas the formation of cycloadducts Vc, VIc, and IX depends on the substituents present at C2 of the pyrrole ring and consequentially on the intermediary occurence of mono-cycloadduct (IIIc), its methylenic tautomer VII, VIc, and XL The behaviour of …
Complex tauto- and rotamerism of 2-(R-phenyl)-1,2,3,4-tetrahydroquinazolines
2005
Detailed NMR spectral analysis of CDCl3 solutions of 2-(R-phenyl)-1,2,3,4-tetrahydroquinazolines reveals three or four tautomeric forms. Apart from 2-[(benzylideneamino)methyl]aniline, the other chain tautomeric forms are present only in minor quantities. In general, electron-donating substituents increase the contribution of all chain forms. Lowering the temperature of the CDCl3 solution of 2-(R-phenyl)-1,2,3,4-tetrahydroquinazolines decreases the content of the 2-[(benzylideneamino)methyl]aniline form. At the same time, the amount of the ring form increases. Opening of the tetrahydropyrimidine ring in 2-(R-phenyl)-1,2,3,4-tetrahydroquinazolines was found to be an endothermic process espec…
(1Z,3Z)-1,4-Di(pyridin-2-yl)buta-1,3-diene-2,3-diol: The Planar Highly Conjugated Symmetrical Enediol with Multiple Intramolecular Hydrogen Bonds
2002
1H, (13)C, and (15)N NMR spectral data show that in chloroform solution (1Z,3Z)-1,4-di(pyridin-2-yl)buta-1,3-diene-2,3-diol, OO, is in ca. 9:1 equilibrium with (3Z)-3-hydroxy-1,4-di(pyridin-2-yl)but-3-en-2-one, OK, while no 1,4-di(pyridin-2-yl)-2,3-butanedione, KK, was detected. The species present in the tautomeric mixture were identified by comparing their experimental chemical shifts with those known for similar compounds as well as with the theoretically calculated (GIAO-HF/DFT) values. Ab initio calculations show that OK and especially the highly conjugated OO forms are preferred in the tautomeric mixtures both in vacuo and in chloroform solution. Comparison of experimental (Arrhenius)…
Substituent and temperature controlled tautomerism of 2-phenacylpyridine: the hydrogen bond as a configurational lock of (Z )-2-(2-hydroxy-2-phenylvi…
2000
2-Phenacylpyridines substituted in the benzene ring are in equilibrium with (Z)-2-(2-hydroxy-2-phenylvinyl)pyridines when dissolved in chloroform. The substituent affects significantly the tautomeric equilibrium [the amount of the enolimine form stabilized by the intramolecular hydrogen bond is 1 and 92% for R = p-N(CH2)4 and p-NO2, respectively]. The negative logarithm of the tautomeric equilibrium constant, KT, is linearly dependent on the Hammett σ substituent constants. The dependence of KTvs. temperature is exponential in character: the more electron-withdrawing is the substituent, the more distinct is the influence of temperature. Unexpectedly, the tautomer present in the crystalline …
ChemInform Abstract: The Chemistry of Unsaturated Nitrogen-Heterocyclic Compounds Containing Carbonyl Groups
2010
Publisher Summary Most hydroxy-substituted azaheteroaromatic compounds exist predominately in the NH/carbonyl tautomeric form, and these compounds have been widely discussed. This chapter provides an overview of azaheteroaromatic compounds and I-azetin-4-ones—these are those azaheterocycles that contain in the ring at least one carbonyl group and one or two nitrogen atoms in the form of C=N or N=N bonds. Many of these compounds are unstable but their importance as reactive intermediates is established, and many of them could be used as synthons in the preparation of pharmacologically active compounds. The chapter describes the free existence of I-azetin-4-one and the nucleophilic substituti…
The Chemistry of Unsaturated Nitrogen–Heterocyclic Compounds Containing Carbonyl Groups
1993
Publisher Summary Most hydroxy-substituted azaheteroaromatic compounds exist predominately in the NH/carbonyl tautomeric form, and these compounds have been widely discussed. This chapter provides an overview of azaheteroaromatic compounds and I-azetin-4-ones—these are those azaheterocycles that contain in the ring at least one carbonyl group and one or two nitrogen atoms in the form of C=N or N=N bonds. Many of these compounds are unstable but their importance as reactive intermediates is established, and many of them could be used as synthons in the preparation of pharmacologically active compounds. The chapter describes the free existence of I-azetin-4-one and the nucleophilic substituti…
Substituent and temperature controlled tautomerism: Multinuclear magnetic resonance, X-ray, and theoretical studies on 2-phenacylquinolines
2000
Proton-transfer equilibria in chloroform solution of twelve 2-phenacylquinolines were studied by 1H, 13C and 15N NMR spectroscopies. The (Z)-enaminone form stabilized by an intramolecular hydrogen bond was found to prevail in all cases. Electron-donating substituents in the phenacyl part of the molecule lead to an increase of the ketimine form (to 33% for p-NMe2). Variable temperature 1H NMR measurements show that higher temperatures have the same effect. The negative logarithm values of the equilibrium constant, pKT, were found to be linearly dependent on Hammett σ substituent constants. The pKTvs. temperature correlation also has a linear character. In general, strong electron-withdrawing…
Trichomonazide Wirkstoffe, 2. Mitt. Verzweigtkettig 4,6-disubstituierte 2-Cyanaminopyrimidine
1985
Aus der Umsetzung von Dicyandiamid (1) mit den β-Diketonen 2a–b gehen die 4,6-verzweigtkettig disubstituierten 2-Cyanaminopyrimidine 3a–b hervor. Fur Strukturtyp 3 last sich aus den spektroskopischen Daten, speziell aus den 1H- und 13C-NMR-Werten, das Vorliegen eines Gleichgewichts zwischen den tautomeren 2-Cyanamino- und 2-Cyaniminopyrimidinformen ableiten. 3 zeichnet sich durch trichomonazide, antivirale und antimykotische Wirkung aus. Trichomonacidal Agents, II: 4,6-Disubstituted 2-(Cyanoamino)pyrimidines Carrying Branched Substituents The reaction of dicyandiamide (1) with the β-diketones 2a–b yields the 4,6-branched disubstituted 2-(cyanoamino)pyrimidines 3a–b. From spectroscopic evide…
Synthesis of Naturally Occurring Pyrazine and Imidazole Alkaloids from Botryllus LeachiRID=?a?ID=?a? Dedicated to Prof. G . Märkl on the occasion of …
2004
The synthesis of the naturally occurring and biologically active alkaloids 1 and 2, first isolated from the red ascidian Botryllus leachi by Duran et al. [1], is described and the structure proposed for Botryllazine B (1) is confirmed. The analytical data for 2-(p-hydroxybenzoyl)-4-(p-hydroxyphenyl)imidazole (2) are discussed and compared with the literature. With special emphasis of 1H NMR data the tautomerism of aroylimidazolemethanones is described.