C2 to C6 biobased carbonyl platforms for fine chemistry

• Jingjing Jiang,
• Muhammad Noman Haider Tariq,
• Florence Popowycz,
• Yanlong Gu and
• Yves Queneau

Beilstein J. Org. Chem. 2025, 21, 2103–2172, doi:10.3762/bjoc.21.165

• reported by Strizhov et al. [125]. The liable equilibrium transformation requires precisely pH-controlled conditions. In neutral medium, the furanone is in the form of its cis tautomer, while upon a slight increase in the pH, its cyclic original tautomer undergoes a ring opening with the formation of

Tautomerism and switching in 7-hydroxy-8-(azophenyl)quinoline and similar compounds

• Lidia Zaharieva,
• Vera Deneva,
• Fadhil S. Kamounah,
• Nikolay Vassilev,
• Ivan Angelov,
• Michael Pittelkow and
• Liudmil Antonov

Beilstein J. Org. Chem. 2025, 21, 1404–1421, doi:10.3762/bjoc.21.105

• solution as a mixture of azo and two hydrazone tautomers, as shown by the experimental and theoretical results, upon irradiation switching, based on long-range proton transfer, occurs in a limited extent. 8-(4-Hydroxy-1,2,3,5-tetrafluorophenyldiazenyl)quinolin-7-ol exists as a single enol (azo) tautomer

• stabilization of the azo tautomer and making possible long range proton transfer to the quinolyl nitrogen atom. Keywords: azo dyes; E/Z isomerization; DFT; NMR; photochemistry; proton transfer; tautomerism; UV–vis; Introduction Azo compounds have long been utilized as dyes in industries such as textiles

• tautomer, which means that the switching would not be clean. According to relative energies and relative Gibb’s free energies, collected in Table 1, compound 1 should co-exist as a three component (E, KE and KK) tautomeric mixture in both toluene and acetonitrile. If the relative energies are taken into

Entry to 2-aminoprolines via electrochemical decarboxylative amidation of N‑acetylamino malonic acid monoesters

• Olesja Koleda,
• Janis Sadauskis,
• Darja Antonenko,
• Edvards Janis Treijs,
• Raivis Davis Steberis and
• Edgars Suna

Beilstein J. Org. Chem. 2025, 21, 630–638, doi:10.3762/bjoc.21.50

• under Pd-catalyzed hydrogenolysis afforded diamino acid ester 14 (75% yield) that was likely formed by ring-opening of the unstable N-unprotected 2-aminoproline followed by the reduction of the open-chain imine tautomer. Likewise, the open-chain amino alcohol 15 was formed also upon the reduction of the

Asymmetric organocatalytic synthesis of chiral homoallylic amines

• Nikolay S. Kondratyev and
• Andrei V. Malkov

Beilstein J. Org. Chem. 2024, 20, 2349–2377, doi:10.3762/bjoc.20.201

• employing chiral Brønsted acid catalyst (S)-TRIP (118) (Scheme 25). In this approach, the racemic β-formyl amide forms the iminium intermediate that undergoes fast equilibration via the enamine tautomer to form preferentially one enantiomer which then undergoes the acid-catalysed aza-Cope rearrangement

Multicomponent syntheses of pyrazoles via (3 + 2)-cyclocondensation and (3 + 2)-cycloaddition key steps

• Ignaz Betcke,
• Alissa C. Götzinger,
• Maryna M. Kornet and
• Thomas J. J. Müller

Beilstein J. Org. Chem. 2024, 20, 2024–2077, doi:10.3762/bjoc.20.178

• pyrazole-4-carboxylates. Shen et al. used Yb(PFO)3 (PFO: perfluorooctanoate), a mild and highly efficient catalyst shown to be effective in the Mannich reaction [94], to synthesize these pyrazoles 65 (Scheme 21) [95]. The Lewis acid catalyst activates and stabilizes the enol tautomer of β-ketoesters

Regioselective alkylation of a versatile indazole: Electrophile scope and mechanistic insights from density functional theory calculations

• Pengcheng Lu,
• Luis Juarez,
• Paul A. Wiget,
• Weihe Zhang,
• Krishnan Raman and
• Pravin L. Kotian

Beilstein J. Org. Chem. 2024, 20, 1940–1954, doi:10.3762/bjoc.20.170

• position of the NH hydrogen atom: 1H-indazole (5a, benzenoid 1H-indazole tautomer) and 2H-indazole (5b, quinonoid 2H-indazole tautomer) (Figure 2) [21][22][23]. Since 1H-indazole is thermodynamically more stable than 2H-indazole, 5a is the predominant tautomer [24][25][26]. Conventionally, indazoles are

• reaction condition temperature using Gaussian 16: SMD(THF)-PBE0/def2-TZVP // SMD(THF)-PBE0/def2-SVP, def2-TZVP(Cs) at 50 °C (w/MeOPPh3+) or 90 °C (w/Cs+), utilizing Goodvibes to calculate thermochemistry. The energy of the N1- and N2-tautomers of 6 differ by 3.1 kcal/mol at 50 °C, favoring the N1-tautomer

• negligible. These data suggest that deprotonation occurs prior to alkylation and that deprotonation of either indazole tautomer leads to anions of identical or highly similar energy. Furthermore, as seen in Figure 4, a total, five coordinated complexes were found to be at least 4.5 kcal/mol more stable than

Harnessing unprotected deactivated amines and arylglyoxals in the Ugi reaction for the synthesis of fused complex nitrogen heterocycles

• Javier Gómez-Ayuso,
• Pablo Pertejo,
• Tomás Hermosilla,
• Israel Carreira-Barral,
• Roberto Quesada and
• María García-Valverde

Beilstein J. Org. Chem. 2024, 20, 1758–1766, doi:10.3762/bjoc.20.154

• tautomer, needed the addition of hydrochloric acid (1 equiv) in the case of the tert-butylamine derivatives. However, the nature of the cyclization product depended on the carboxylic acid employed in the Ugi reaction. Thus, the indole and pyrrole derivatives yielded the corresponding hemiaminal 9 as a

• reactions seem to take place through conjugated additions on the enol tautomer of the Ugi adduct. Indeed, the enolate intermediate would explain the stereochemical results, controlled by the configuration in the hemiaminal intermediate and not by the chiral information on the amine, unlike in the case of

Synthesis and biological profile of 2,3-dihydro[1,3]thiazolo[4,5-b]pyridines, a novel class of acyl-ACP thioesterase inhibitors

• Jens Frackenpohl,
• David M. Barber,
• Guido Bojack,
• Birgit Bollenbach-Wahl,
• Ralf Braun,
• Rahel Getachew,
• Sabine Hohmann,
• Kwang-Yoon Ko,
• Karoline Kurowski,
• Bernd Laber,
• Rebecca L. Mattison,
• Thomas Müller,
• Anna M. Reingruber,
• Dirk Schmutzler and
• Andrea Svejda

Beilstein J. Org. Chem. 2024, 20, 540–551, doi:10.3762/bjoc.20.46

• 11b as thiol–thione tautomer consisting of 6-bromo-5-(2-fluorophenyl)[1,3]thiazolo[4,5-b]pyridine-2-thiol and 6-bromo-5-(2-fluorophenyl)[1,3]thiazolo[4,5-b]pyridine-2(3H)thione (17.20 g, 97%). 1H NMR (400 MHz, CDCl3, δ) 9.96 (br s, 1H), 8.01 (s, 1H), 7.50–7.44 (m, 1H), 7.41–7.38 (m, 1H), 7.29–7.25 (m

• , 1H), 7.19–7.15 (m, 1H). The thiol–thione tautomer 11b (14.55 g, 42.64 mmol, 1.0 equiv) was dissolved in acetic acid (200 mL), and iron powder (35.71 g, 639.61 mmol, 15 equiv) was carefully added in portions. The resulting reaction mixture was stirred at 100 °C for 10 h. After full conversion

Synthesis of 2,2-difluoro-1,3-diketone and 2,2-difluoro-1,3-ketoester derivatives using fluorine gas

• Alexander S. Hampton,
• David R. W. Hodgson,
• Graham McDougald,
• Linhua Wang and
• Graham Sandford

Beilstein J. Org. Chem. 2024, 20, 460–469, doi:10.3762/bjoc.20.41

• ); ref codes IZICEA [47], XOPZEK and XOPZIO [48]) are known. Interestingly, in contrast to the previously described acyclic structures no OH···O(H) hydrogen bonds are present in structure 5e – the molecules are linked by OH···O(NO2) interactions. Discussion Keto–enol tautomer studies have shown that DBM

Facile approach to N,O,S-heteropentacycles via condensation of sterically crowded 3H-phenoxazin-3-one with ortho-substituted anilines

• Eugeny Ivakhnenko,
• Vasily Malay,
• Pavel Knyazev,
• Nikita Merezhko,
• Nadezhda Makarova,
• Oleg Demidov,
• Gennady Borodkin,
• Andrey Starikov and
• Vladimir Minkin

Beilstein J. Org. Chem. 2024, 20, 336–345, doi:10.3762/bjoc.20.34

• –H proton. Therefore, three tautomeric forms are possible for 5 (Scheme 4), one of which, the 7H-tautomer 7b, inevitably adopts a bipolar or biradical structure. According to the data from the DFT calculations performed at the B3LYP/6-311++G(d,p) approximation (Figure S6, Supporting Information File

•  1), the energetically preferred tautomer is the 14H-form 7a. The least stable 7H-isomer 7b conforms to a minimum on the corresponding potential energy surface. However, the stable wave function of 7b corresponds to an electronic state with a broken symmetry [20], indicating the presence of two

Metal-catalyzed coupling/carbonylative cyclizations for accessing dibenzodiazepinones: an expedient route to clozapine and other drugs

• Amina Moutayakine and
• Anthony J. Burke

Beilstein J. Org. Chem. 2024, 20, 193–204, doi:10.3762/bjoc.20.19

• -phenylenediamine (1a) with 1,2-dibromobenzene (2) in the presence of 5 mol % of PdCl2(CH3CN)2 and 5 mol % of t-BuXPhos, with Et3N (2.5 equiv) and Mo(CO)6 in DMF at 150 °C, surprisingly this afforded the 5H-dibenzo[b,e][1,4]diazepin-11-ol (5), the tautomer of DBDAP (4a) in 53% (Scheme 2). We attempted to convert

1,4-Dithianes: attractive C2-building blocks for the synthesis of complex molecular architectures

• Bram Ryckaert,
• Ellen Demeyere,
• Frederick Degroote,
• Hilde Janssens and
• Johan M. Winne

Beilstein J. Org. Chem. 2023, 19, 115–132, doi:10.3762/bjoc.19.12

• helpful when dealing with sensitive dienes. A nice illustration of this is afforded by De Lucchi’s simple synthesis of barrelene (33) from oxepin (29, Scheme 8b) [52]. Oxepin’s equally unstable valence tautomer 30 (benzene oxide) is quite reactive as a diene in Diels–Alder reactions, and can react with 7

A one-pot electrochemical synthesis of 2-aminothiazoles from active methylene ketones and thioureas mediated by NH₄I

• Shang-Feng Yang,
• Pei Li,
• Zi-Lin Fang,
• Sen Liang,
• Hong-Yu Tian,
• Bao-Guo Sun,
• Kun Xu and
• Cheng-Chu Zeng

Beilstein J. Org. Chem. 2022, 18, 1249–1255, doi:10.3762/bjoc.18.130

• produce α-iodo ketone with the molecular I2 produced by anodic oxidation. Subsequently, the nucleophilic substitution between intermediate 4 and thiourea tautomer gives α-sulfur substituted ketone 5. Intermediate 5 undergoes intramolecular nucleophilic addition to the carbonyl group and followed by

Synthesis and HDAC inhibitory activity of pyrimidine-based hydroxamic acids

• Virginija Jakubkiene,
• Gabrielius Ernis Valiulis,
• Markus Schweipert,
• Asta Zubriene,
• Daumantas Matulis,
• Franz-Josef Meyer-Almes and
• Sigitas Tumkevicius

Beilstein J. Org. Chem. 2022, 18, 837–844, doi:10.3762/bjoc.18.84

• °C (Scheme 2). It is known that there are two possible tautomeric forms of each hydroxamic acid: the keto and enol tautomer. Furthemore, each tautomer can adopt an E or Z conformation (Figure 2) [32][33][34][35][36]. The results of NMR spectra and theoretical calculations showed that hydroxamic acids

Synthesis of bis-spirocyclic derivatives of 3-azabicyclo[3.1.0]hexane via cyclopropene cycloadditions to the stable azomethine ylide derived from Ruhemann's purple

• Alexander S. Filatov,
• Olesya V. Khoroshilova,
• Anna G. Larina,
• Vitali M. Boitsov and
• Alexander V. Stepakov

Beilstein J. Org. Chem. 2022, 18, 769–780, doi:10.3762/bjoc.18.77

• betaine form 1 is the most thermodynamically stable of all tautomers (ΔG = −4.9 kcal/mol). It is also not surprising that the O-protonated form 1', which is both a ketone and an enol, is found to be the most unfavorable (ΔG = 10.8 kcal/mol). In contrast to the O-protonated tautomer 1', both the N

An isoxazole strategy for the synthesis of 4-oxo-1,4-dihydropyridine-3-carboxylates

• Timur O. Zanakhov,
• Ekaterina E. Galenko,
• Mikhail S. Novikov and
• Alexander F. Khlebnikov

Beilstein J. Org. Chem. 2022, 18, 738–745, doi:10.3762/bjoc.18.74

• -3-carboxylates. Synthesis of Isoxazoles 11–13. Synthesis of isoxazoles 1. Synthesis of pyridones 2. Transformations of pyridones 2. Gibbs free energies of the pyridole tautomer relatively to the pyridone tautomer of compounds 2 (in kcal/mol, 298 K). Supporting Information Supporting Information

Synthesis of 3,4,5-trisubstituted isoxazoles in water via a [3 + 2]-cycloaddition of nitrile oxides and 1,3-diketones, β-ketoesters, or β-ketoamides

• Md Imran Hossain,
• Md Imdadul H. Khan,
• Seong Jong Kim and
• Hoang V. Le

Beilstein J. Org. Chem. 2022, 18, 446–458, doi:10.3762/bjoc.18.47

• enol tautomer of the 1,3-diketone was predominant in CDCl3, while the keto tautomer was predominant in methanol-d4, which is a more polar solvent than CDCl3 (Figure 7). Our observations thus reinforce Meyer’s rule [34][35], which states that the keto tautomer is favored as the solvent polarity

• thermodynamical product. The solvent polarity also affects the keto–enol equilibrium of the intermediate II-D. In polar solvents, the keto tautomer is predominant as an electrophilic group for the intramolecular cyclization, while in nonpolar solvents, the enol tautomer could not accept a nucleophilic attack for

• a further cycloaddition. In addition, the conjugation between the new C=N bond and the enol double bond promotes the enol tautomer formation. This may also explain why the trifluoromethylated 1,3-diketones produced the trifluoromethyl-substituted isoxazoles with lower yields than the methyl

1,2-Naphthoquinone-4-sulfonic acid salts in organic synthesis

• Ruan Carlos B. Ribeiro,
• Patricia G. Ferreira,
• Amanda de A. Borges,
• Luana da S. M. Forezi,
• Fernando de Carvalho da Silva and
• Vitor F. Ferreira

Beilstein J. Org. Chem. 2022, 18, 53–69, doi:10.3762/bjoc.18.5

• with reddish hues. The reaction with aniline forms the substitution product of the sulfonic group with a phenylamino group in a 90% yield. In his study, he proposed that the structure of the nucleophilic addition product was tautomer 19 (Scheme 1B). In search of a reagent that could form stable adducts

• -d5, and NaOD solutions in D2O. In neutral solvents, the most stable tautomer is 4-arylamino-1,2-naphthoquinone A, while in weakly basic solvents, or ethanolic sulfuric acid, B is the most stable tautomer (Scheme 3C). Reactions employing equimolecular amounts of β-NQS and primary arylamines are

• naphthoquinones A prevail in all pH regions except for extreme acidity, where there is a shift to the form of 2-hydroxy-1,4-naphthoquinone-4-arylimines [73][74]. However, in weakly acidic or alkaline solutions, A is the most stable tautomer (Scheme 3C) [75]. Fragoso and co-workers [76] studied the tautomeric

Photophysical, photostability, and ROS generation properties of new trifluoromethylated quinoline-phenol Schiff bases

• Inaiá O. Rocha,
• Yuri G. Kappenberg,
• Wilian C. Rosa,
• Clarissa P. Frizzo,
• Nilo Zanatta,
• Marcos A. P. Martins,
• Isadora Tisoco,
• Bernardo A. Iglesias and
• Helio G. Bonacorso

Beilstein J. Org. Chem. 2021, 17, 2799–2811, doi:10.3762/bjoc.17.191

• , according to Temel and co-workers who studied a similar scaffold, namely, 4-bromo-2-((quinoline-8-yl)methyl)phenol [27], no imine–hemiaminal tautomer peak transition was observed for the Schiff bases 3. In general, there were slightly significant changes in the transitions according to the changes in the

Silica gel and microwave-promoted synthesis of dihydropyrrolizines and tetrahydroindolizines from enaminones

• Robin Klintworth,
• Garreth L. Morgans,
• Stefania M. Scalzullo,
• Charles B. de Koning,
• Willem A. L. van Otterlo and
• Joseph P. Michael

Beilstein J. Org. Chem. 2021, 17, 2543–2552, doi:10.3762/bjoc.17.170

• cyclization to the expected pyrrolizinone 13a or tautomer 13a' did not take place (Table 1, entry 1). More vigorous conditions were therefore investigated. Heating 15a in acetic acid (150 W) for 10 minutes at 140 °C produced a new product in 72% yield (Table 1, entry 2). However, the isolated product was not

Nomimicins B–D, new tetronate-class polyketides from a marine-derived actinomycete of the genus Actinomadura

• Zhiwei Zhang,
• Tao Zhou,
• Taehui Yang,
• Keisuke Fukaya,
• Enjuro Harunari,
• Shun Saito,
• Katsuhisa Yamada,
• Chiaki Imada,
• Daisuke Urabe and
• Yasuhiro Igarashi

Beilstein J. Org. Chem. 2021, 17, 2194–2202, doi:10.3762/bjoc.17.141

• canonical structures of 4 (4a–d in Figure 5). The calculated ECD spectra of 4a–d and the one of 4, which includes all contributions from each tautomer according to the energy distribution, are shown in Figure 5 and Figure 4, respectively. The experimental ECD spectrum of 4, with positive and negative Cotton

• tautomer are included in Supporting Information File 1. Biological assays Antimicrobial activity and cytotoxicity were evaluated according to the procedures previously described [29]. Structures of nomimicins A–D (4 and 1–3). COSY and key HMBC correlations for 1. Relative configuration of 1 determined by

Regioselective N-alkylation of the 1H-indazole scaffold; ring substituent and N-alkylating reagent effects on regioisomeric distribution

• Ryan M. Alam and
• John J. Keating

Beilstein J. Org. Chem. 2021, 17, 1939–1951, doi:10.3762/bjoc.17.127

• achieve regioselective indazole N-alkylation have exploited the noted difference in reactivity between the N-1 and N-2 atom of the indazole scaffold [15], as the 1H-indazole tautomer is typically considered to be more thermodynamically stable than the corresponding 2H-tautomer [16]. Using appropriate α

A recent overview on the synthesis of 1,4,5-trisubstituted 1,2,3-triazoles

• Pezhman Shiri,
• Ali Mohammad Amani and
• Thomas Mayer-Gall

Beilstein J. Org. Chem. 2021, 17, 1600–1628, doi:10.3762/bjoc.17.114

• corresponding triazole products in moderate yield (Scheme 8) [42]. The authors proposed a reaction mechanism in which the β-thioenaminone 18 tolerates deprotonation to afford anionic intermediate 21 through the tautomer 20 in the presence of a base. In the next step, the nucleophilic addition of 21 to the azide

Synthetic accesses to biguanide compounds

• Oleksandr Grytsai,
• Cyril Ronco and
• Rachid Benhida

Beilstein J. Org. Chem. 2021, 17, 1001–1040, doi:10.3762/bjoc.17.82

• sites, and eight possible tautomeric forms. The major tautomer of biguanide has long been debated and different representations have been depicted in textbooks and research articles. Historically, the biguanide structure was presented similarly to diketones (Figure 1, 1a), which led to misunderstandings

• represented as the major tautomer 1b with the conjugated system –C=N–C=N– and the numbering of the different atoms will be established as depicted in Figure 1. Biguanides are relatively strong bases, with pKa1H ≈ 11.5 (pKa of the conjugate acid of biguanide); however, significantly less basic than guanidine

Synthesis of dibenzosuberenone-based novel polycyclic π-conjugated dihydropyridazines, pyridazines and pyrroles

• Ramazan Koçak and
• Arif Daştan

Beilstein J. Org. Chem. 2021, 17, 719–729, doi:10.3762/bjoc.17.61

• was converted to pyrrole 15a,b. Finally, the phenolic parts of 15a,b were oxidized to p-quinone methides 16a,b with PIFA in excellent yield (89–97%, Scheme 8). For the formation of unexpected compound 13, we propose two different mechanisms. First, following the formation of phenolic tautomer 12A by