Microwave-assisted synthesis of biologically relevant steroidal 17-exo-pyrazol-5'-ones from a norpregnene precursor by a side-chain elongation/heterocyclization sequence

• Gergő Mótyán,
• László Mérai,
• Márton Attila Kiss,
• Zsuzsanna Schelz,
• Izabella Sinka,
• István Zupkó and
• Éva Frank

Beilstein J. Org. Chem. 2018, 14, 2589–2596, doi:10.3762/bjoc.14.236

• , keto–enol tautomerism can be challenging and is of special importance in biological systems, chemical reactivity, and molecular recognition [18]. The tautomeric equilibrium in solution is strongly influenced by the substitution pattern of the heterocyclic ring, the polarity and protic nature of the

• , a subsequent derivatization with acetic anhydride in pyridine to afford 8, allowed its structure verification indirectly. This derivatization did not only improve the solubility of the compound, but also eliminated the possibility of prototropic tautomerism through acetylation of both the amino and

Determining the predominant tautomeric structure of iodine-based group-transfer reagents by ¹⁷O NMR spectroscopy

• Nico Santschi,
• Cody Ross Pitts,
• Benson J. Jelier and
• René Verel

Beilstein J. Org. Chem. 2018, 14, 2289–2294, doi:10.3762/bjoc.14.203

• for the design of electrophilic transfer reagents. A particularly intriguing aspect is the fundamental II–IIII tautomerism about the hypervalent bond, which has led in certain cases to a surprising re-evaluation of the classic hypervalent structure. Thus, through a combination of 17O NMR spectroscopy

• can be used to gain insight into the solution-phase tautomerism observed in iodine-based group-transfer reagents. In particular, we confirm that Shen’s revised, electrophilic SCF3-transfer reagent adopts an "acyclic" thioperoxide tautomeric form in solution whereas an electrophilic cyanide source

• structural analysis. Tautomerism in iodine-based group-transfer reagents probed by 17O NMR spectroscopy (A) and key structures investigated herein (B). Assignment of acyclic (b) and cyclic (a) structures to 5 and 6, respectively, based on computed isotropic shift values (δiso) and experimental 17O NMR

¹⁵N-Labelling and structure determination of adamantylated azolo-azines in solution

• Sergey L. Deev,
• Alexander S. Paramonov,
• Tatyana S. Shestakova,
• Igor A. Khalymbadzha,
• Oleg N. Chupakhin,
• Julia O. Subbotina,
• Oleg S. Eltsov,
• Pavel A. Slepukhin,
• Vladimir L. Rusinov,
• Alexander S. Arseniev and
• Zakhar O. Shenkarev

Beilstein J. Org. Chem. 2017, 13, 2535–2548, doi:10.3762/bjoc.13.250

• this approach for the syntheses of 15N-labelled tetrazolo[1,5-b][1,2,4]triazines and tetrazolo[1,5-a]pyrimidines [25] starting from 15N-labelled 5-aminotetrazole. However, due to proton tautomerism, the use of single-labelled [2-15N]-5-aminotetrazole led to the formation of isotopomer mixtures, which

Asymmetric synthesis of propargylamines as amino acid surrogates in peptidomimetics

• Matthias Wünsch,
• David Schröder,
• Tanja Fröhr,
• Lisa Teichmann,
• Sebastian Hedwig,
• Nils Janson,
• Clara Belu,
• Jasmin Simon,
• Shari Heidemeyer,
• Philipp Holtkamp,
• Jens Rudlof,
• Lennard Klemme,
• Alessa Hinzmann,
• Beate Neumann,
• Hans-Georg Stammler and
• Norbert Sewald

Beilstein J. Org. Chem. 2017, 13, 2428–2441, doi:10.3762/bjoc.13.240

• structure analysis. Due to the extended π-system of the α,β-unsaturated imines 12i and 12k, the progress of the propargylamide–allenylamide rearrangement that eventually leads to the formation of the α,β-unsaturated imines by tautomerism could be easily monitored by UV–vis spectroscopy. The reaction mixture

• turned brightly yellow when treated with bases like piperidine. The X-ray crystal structures of 11i and 12i confirm unequivocally the structure of the products and thus the postulated two-step rearrangement. Both, the rearrangement of the alkyne to the allene and the subsequent tautomerism to the α,β

Synthesis of tetrasubstituted pyrazoles containing pyridinyl substituents

• Josef Jansa,
• Ramona Schmidt,
• Ashenafi Damtew Mamuye,
• Laura Castoldi,
• Alexander Roller,
• Vittorio Pace and
• Wolfgang Holzer

Beilstein J. Org. Chem. 2017, 13, 895–902, doi:10.3762/bjoc.13.90

• tautomerism of compound 1a. Supporting Information Supporting Information File 344: Experimental details and characterization data. Acknowledgements We thank Mrs. Fatehme Keramati for experimental contributions. Josef Jansa thanks Palacký University for awarding a study visit at the University of Vienna

Thiazol-4-one derivatives from the reaction of monosubstituted thioureas with maleimides: structures and factors determining the selectivity and tautomeric equilibrium in solution

• Alena S. Pankova,
• Pavel R. Golubev,
• Alexander F. Khlebnikov,
• Alexander Yu. Ivanov and
• Mikhail A. Kuznetsov

Beilstein J. Org. Chem. 2016, 12, 2563–2569, doi:10.3762/bjoc.12.251

• ; regioselectivity; tautomerism; thiazolidinones; thioureas; Introduction The thiazolidine core is one of the privileged scaffolds in a variety of pharmaceuticals with exclusively broad range of biological activities [1][2][3][4][5][6][7]. 2-Aminothiazolidin-4-ones can be easily prepared treating thiourea

• the nature of the dynamic process observed in solutions of 2-aminothiazolidines. This phenomenon was first detected by UV spectroscopy [19] and believed to be caused by a proton transfer between nitrogen atoms (amino–imino tautomerism), however, contradictory information on the equilibrium ratio of

• investigate the regioselectivity of the reaction between monosubstituted thioureas and maleimides and the tautomerism of the products. The results of this investigation are reported herein. Results and Discussion First, we repeated some experiments reported by Marrian [9], namely, the reactions of N

Protonated paramagnetic redox forms of di-o-quinone bridged with p-phenylene-extended TTF: A EPR spectroscopy study

• Nikolay O. Chalkov,
• Vladimir K. Cherkasov,
• Gleb A. Abakumov,
• Andrey G. Starikov and
• Viacheslav A. Kuropatov

Beilstein J. Org. Chem. 2016, 12, 2450–2456, doi:10.3762/bjoc.12.238

• Scheme 2 and Supporting Information File 1). The pairwise equivalence of H1–H2 and H3–H4 p-phenylene protons in the EPR spectrum originates from a prototropic tautomerism within a single dioxolene coordination site [7][8]. Meanwhile, the exchange of this semiquinone proton between the different

Et₃B-mediated and palladium-catalyzed direct allylation of β-dicarbonyl compounds with Morita–Baylis–Hillman alcohols

• Ahlem Abidi,
• Yosra Oueslati and
• Farhat Rezgui

Beilstein J. Org. Chem. 2016, 12, 2402–2409, doi:10.3762/bjoc.12.234

• monoallylation products 3d–i in moderate to good yields [44]. The analysis of 1H NMR spectra of the β-dicarbonyl derivatives 3a–i in CDCl3 revealed that a keto–enol tautomerism exists only for the acetylacetone derivative 3g and its enolic form 5g in a 54:46 ratio, respectively (Table 2, entry 7), whereas the

Biosynthesis of α-pyrones

• Till F. Schäberle

Beilstein J. Org. Chem. 2016, 12, 571–588, doi:10.3762/bjoc.12.56

• systems. Pyrone ring formation. Examples for the three types of PKS systems are shown in A–C. In D the mechanism catalyzed by a free-standing ketosynthase is depicted. Herein the keto–enol tautomerism is shown. A) Polyketide synthase (PKS) type I: The end part of the phenylnannolone A biosynthesis is

• -tethered chains are interconnected by the catalytic activity of a free-standing KS. In the second step the lactonization takes place, facilitated by the keto–enol tautomerism. Thereby the α-pyrone 78 is formed. Structures of csypyrones. Schematic drawing of the T-shaped catalytic cavities of the related

Versatile deprotonated NHC: C,N-bridged dinuclear iridium and rhodium complexes

• Albert Poater

Beilstein J. Org. Chem. 2016, 12, 117–124, doi:10.3762/bjoc.12.13

• with experiments that indicated 3H-T is 5.2 kcal/mol more stable than 3H-H, which means that with free tautomerism only the thermodynamic isomer 3H-T would not coexist with 3H-H, but as the unique isomer [59]. Furthermore, the strength of the two H-H and H-T arrangements of the bridging pNHC ligand

• ) centre, shows that the effect on system 3 of the released electron is mainly paid by the metal centres, but also partially spread over the ligands [59]. To follow up the experimental results the tautomerism/metallotropism between pNHC and imidazole ligands in these iridium complexes bearing a doubly C,N

2-Hetaryl-1,3-tropolones based on five-membered nitrogen heterocycles: synthesis, structure and properties

• Yury A. Sayapin,
• Inna O. Tupaeva,
• Alexandra A. Kolodina,
• Eugeny A. Gusakov,
• Vitaly N. Komissarov,
• Igor V. Dorogan,
• Nadezhda I. Makarova,
• Anatoly V. Metelitsa,
• Valery V. Tkachev,
• Sergey M. Aldoshin and
• Vladimir I. Minkin

Beilstein J. Org. Chem. 2015, 11, 2179–2188, doi:10.3762/bjoc.11.236

• diffraction. 2-(2-Benzoxa(thia)zolyl)-6-alkoxycarbonylphenols display intense green fluorescence with anomalous Stokes shifts caused by the excited state intramolecular proton transfer (ESIPT) effects. Keywords: β-tropolones; fluorescence; intramolecular hydrogen bond; tautomerism; X-ray analysis

Derivatives of the triaminoguanidinium ion, 3. Multiple N-functionalization of the triaminoguanidinium ion with isocyanates and isothiocyanates

• Jan Szabo,
• Kerstin Karger,
• Nicolas Bucher and
• Gerhard Maas

Beilstein J. Org. Chem. 2014, 10, 2255–2262, doi:10.3762/bjoc.10.234

• range, and therefore, a complete assignment of most of these spectra was not possible. Due to the likely occurrence of several dynamic processes in solution (conformational changes with hindered rotation around single bonds, prototropic tautomerism in the guanidine moiety [1], dynamics of intra- and

An experimental and theoretical NMR study of NH-benzimidazoles in solution and in the solid state: proton transfer and tautomerism

• Carla I. Nieto,
• Pilar Cabildo,
• M. Ángeles García,
• Rosa M. Claramunt,
• Ibon Alkorta and
• José Elguero

Beilstein J. Org. Chem. 2014, 10, 1620–1629, doi:10.3762/bjoc.10.168

• C5/C6) of NH-benzimidazoles that, in some solvents and in the solid state, appear different (blocked tautomerism). In the case of 1H-benzimidazole itself we have measured the prototropic rate in HMPA-d18. Keywords: CPMAS; DNMR; GIAO; proton transfer; tautomerism; Introduction Of almost any class of

• properties [4]. Particularly relevant for the present work is their proton conducting abilities, based of the 1,3-N–H···N hydrogen bonds, not only in benzimidazole polymers but in molecular compounds [5][6]. Degenerated tautomerism (autotrope) [7][8] simultaneously simplifies and complicates the NMR spectra

• of molecules in solution to the point that the assignment of some signals that become magnetically equivalent (isochronous) [9][10] by fast proton exchange has been much neglected. With the advent of solid-state NMR spectroscopy and the suppression of prototropic tautomerism, the assignment problem

Use of activated enol ethers in the synthesis of pyrazoles: reactions with hydrazine and a study of pyrazole tautomerism

• Denisa Tarabová,
• Stanislava Šoralová,
• Martin Breza,
• Marek Fronc,
• Wolfgang Holzer and
• Viktor Milata

Beilstein J. Org. Chem. 2014, 10, 752–760, doi:10.3762/bjoc.10.70

• tautomerism and supramolecular structure. A reverse addition of the reagents led to the isolation of two novel products, namely bis-enehydrazines 6 with an unsymmetrical arrangement of the formally equivalent subunits. Keywords: bis-enehydrazines; enol ethers; NMR; pyrazole; tautomerism; Introduction Enol

• , which is capable of annular prototropic tautomerism [5][6]. In contrast, the use of „asymmetric“ enol ethers, that is, R1, R1 in 2 and 3 are not equal, may lead to two different pyrazoles, both of which exhibit tautomerism). However, depending on the different reactivities of the electron-withdrawing

• -pyrazole-4-carboxylate (5c) [14][15][16][17][18]. The Reaction of enol ethers 3b and 3c with hydrazine hydrate at rt for 10 min gave products of an SNV reaction, 4b and 4c. These then underwent intramolecular cyclization to yield 5b and 5c. Due to oxo–enol tautomerism eight tautomers are theoretically

One-pot three-component synthesis and photophysical characteristics of novel triene merocyanines

• Christian Muschelknautz,
• Robin Visse,
• Jan Nordmann and
• Thomas J. J. Müller

Beilstein J. Org. Chem. 2014, 10, 599–612, doi:10.3762/bjoc.10.51

• triggering the allenyl enol–dienone tautomerism [30]. Therefore, we propose a similar mechanism for the formation of the merocyanines 8 and 10 (Scheme 3). The sequences commence after oxidative addition of the Pd species in the carbon–iodine bond of 5 with a 5-exo-dig insertion of the appended alkynoyl

An oxidative amidation and heterocyclization approach for the synthesis of β-carbolines and dihydroeudistomin Y

• Suresh Babu Meruva,
• Akula Raghunadh,
• Raghavendra Rao Kamaraju,
• U. K. Syam Kumar and
• P. K. Dubey

Beilstein J. Org. Chem. 2014, 10, 471–480, doi:10.3762/bjoc.10.45

• reaction with tryptamine in presence of NaI. The Schiff base on in situ oxidation with cumene hydroperoxide afforded an unstable oxaziridine derivative 15. Ring opening of the oxaziridine derivative 15 in presence of base afforded ketoiminol 16, which on iminol–amide tautomerism provided the required α

Four-component reaction of cyclic amines, 2-aminobenzothiazole, aromatic aldehydes and acetylenedicarboxylate

• Hong Gao,
• Jing Sun and
• Chao-Guo Yan

Beilstein J. Org. Chem. 2013, 9, 2934–2939, doi:10.3762/bjoc.9.330

• (D) and piperidine. Then 2,3-pyrrolidinedione D transforms to the more stable enol-form through the keto–enol tautomerism. Because the enol connects to both ester and amide groups, it has much stronger acidity and is deprotonated by piperidine in the solution to give the piperidinium 2-pyrrolidinon-3

Structure of 1,5-benzodiazepinones in the solid state and in solution: Effect of the fluorination in the six-membered ring

• Marta Pérez-Torralba,
• Rosa M. Claramunt,
• M. Ángeles García,
• Concepción López,
• M. Carmen Torralba,
• M. Rosario Torres,
• Ibon Alkorta and
• José Elguero

Beilstein J. Org. Chem. 2013, 9, 2156–2167, doi:10.3762/bjoc.9.253

• solid state. Keywords: benzodiazepinones; DFT; GIAO calculations; inversion barriers; multinuclear NMR; tautomerism; X-ray structures; Introduction In our previous paper [1] we already reported the relevance of 1,5-benzodiazepine derivatives in central nervous system pathologies as well as for other

• . More significant for the present work, the N-methyl substituent folded considerably the ring, compare 2 with 1, this being related to the inversion process discussed below. Energies and tautomerism For the 1H-derivatives five possible tautomers exist while for the N-methyl ones only three different

• the expected diazepinone that they represent using the tautomer 3d again without any reported proof, neither in the main text nor in the supplementary data [13]. A comprehensive theoretical study of the tautomerism of 3 was carried out by Okovytyy et al. in 2010, including monoethanol and diethanol

Synthesis and antibacterial activity of monocyclic 3-carboxamide tetramic acids

• Yong-Chul Jeong and
• Mark G. Moloney

Beilstein J. Org. Chem. 2013, 9, 1899–1906, doi:10.3762/bjoc.9.224

• pyroglutamates by a conjugate addition of amines [22]. Tautomerism Tautomerism in tricarbonyl 3-acyltetramate systems is known to be complex and strongly dependent on the identity of the side chain acyl group [23]. 3-Acyl (X = CH2) [10][24][25], 3-carboxamide (X = NH) [8][10] and 3-alkoxycarbonyl (X = O

• was determined by visually reading the first concentration where no growth (no turbidity) appeared. Some antibiotic natural and unnatural tetramic acids. Tautomerism of tetramates. Synthesis of simple 3-carboxamide tetramic acids. Reaction conditions: (a) triethylamine (2.0 equiv), 1-bromohexane (0.5

Synthesis and quantitative structure–activity relationship study of substituted imidazophosphor ester based tetrazolo[1,5-b]pyridazines as antinociceptive/anti-inflammatory agents

• Wafaa M. Abdou,
• Neven A. Ganoub and
• Eman Sabry

Beilstein J. Org. Chem. 2013, 9, 1730–1736, doi:10.3762/bjoc.9.199

• been reported that 3,6-diazidopyridazine presents azido (1a) tetrazolo tautomerism (1b) whereby it is mainly in the tetrazole form 1b [27][28][29]. Accordingly, and as indicated from the spectral data of our products, we considered that the substrate 3,6-diazidopyridazine (1a) is exclusively

A computational study of base-catalyzed reactions of cyclic 1,2-diones: cyclobutane-1,2-dione

• Nargis Sultana and
• Walter M. F. Fabian

Beilstein J. Org. Chem. 2013, 9, 594–601, doi:10.3762/bjoc.9.64

• tautomerism [32][33]. Product P3a essentially is the ion-dipole complex between the ring tautomer of neutral γ-oxobutanoic acid with [OH(H2O)5]– , and hence, its formation is completely reasonable. The actual pathway C obtained by the calculations is indicated in Scheme 5. In fact, Int4 is barely stable if

Recent advances towards azobenzene-based light-driven real-time information-transmitting materials

• Jaume García-Amorós and
• Dolores Velasco

Beilstein J. Org. Chem. 2012, 8, 1003–1017, doi:10.3762/bjoc.8.113

• different classes: trans–cis–trans isomerisations, photo-induced ring-closing reactions and photo-tautomerism [17]. Depending on the thermal stability of the photogenerated isomer, photochromic systems can be classified in two categories [18]: ● P-type (photochemically reversible type); these do not revert

• section. Type-III azoderivatives: optical switches acting within the microsecond time scale, based on azo-dyes exhibiting both azo-hydrazone tautomerism and a push–pull electronic distribution Among all the azoderivatives presented up to now in this review, azopyridinium methyl iodide salts are the

The Eschenmoser coupling reaction under continuous-flow conditions

• Sukhdeep Singh,
• J. Michael Köhler,
• Andreas Schober and
• G. Alexander Groß

Beilstein J. Org. Chem. 2011, 7, 1164–1172, doi:10.3762/bjoc.7.135

• Information File 1). This is due to the two different possibilities for the intramolecular H-bond formation caused by the keto–enol tautomerism of 4. In this scheme, the reaction of 2-mercapto-pyrimidin-4-ol derivatives 2a, 2b and 2c proceeded smoothly with high yields. 6-Aza-2-thiothymine (2d) was

Advances in synthetic approach to and antifungal activity of triazoles

• Kumari Shalini,
• Nitin Kumar,
• Sushma Drabu and
• Pramod Kumar Sharma

Beilstein J. Org. Chem. 2011, 7, 668–677, doi:10.3762/bjoc.7.79

• nitrogen atoms. However, flash vacuum pyrolysis at 500 °C leads to loss of molecular nitrogen (N2) to produce aziridine. Certain triazoles are relatively easy to cleave by ring–chain tautomerism. Synthesis of triazoles Substituted 1,2,3-triazoles can be produced by the azide–alkyne Huisgen cycloaddition in

C-Arylation reactions catalyzed by CuO-nanoparticles under ligand free conditions

• Mazaahir Kidwai,
• Saurav Bhardwaj and
• Roona Poddar

Beilstein J. Org. Chem. 2010, 6, No. 35, doi:10.3762/bjoc.6.35

• residue was chromatographed to afford pure 3-phenyl-2,4-pentadione (Entry 1, Table 3). From the spectral data it was observed that some products show 1,3-keto-enol tautomerism. The structures of all the products were unambiguously established on the basis of their spectral properties (1H NMR, MS and 13C