Transition-metal-free intramolecular Friedel–Crafts reaction by alkene activation: A method for the synthesis of some novel xanthene derivatives

• Tülay Yıldız,
• İrem Baştaş and
• Hatice Başpınar Küçük

Beilstein J. Org. Chem. 2021, 17, 2203–2208, doi:10.3762/bjoc.17.142

• organic Brønsted acid-catalyzed cyclization reactions [41][42], herein we report a highly efficient intramolecular FCA of appropriate unactivated alkenes with a polyaromatic structure in order to synthesize xanthene derivatives. We developed a new intramolecular FCA method by activating alkenes working

• as AlCl3, H2SO4, or H3PO4, which have generally corrosive properties, were used, in this study, an intramolecular ring closure reaction was carried out under easy operating conditions with an organic Brønsted acid catalyst with high yields. So, the xanthene synthesis with alkene activation was

Catalyzed and uncatalyzed procedures for the syntheses of isomeric covalent multi-indolyl hetero non-metallides: an account

• Ranadeep Talukdar

Beilstein J. Org. Chem. 2021, 17, 2102–2122, doi:10.3762/bjoc.17.137

• )silane 38 that involved n-BuLi as the base [58]. The strategy was later adopted by Ohshita in 2004 (40a, Scheme 6) [59]. Between 2016 and 2018, some acid-catalyzed syntheses of bis(indol-3-yl)silanes appeared [60][61][62][63]. Chen and co-workers demonstrated a Brønsted acid-catalyzed Friedel–Crafts

• , which possess AhR affinity [93]. Janosik presented a synthesis of such selenopyrans 132 via the bis(indol-2-yl)selanes 131 [73]. Treating these compounds with orthoformate esters in the presence of the Brønsted acid MeSO3H led to the target selenopyrans (Scheme 17b). The methylated analogs of 132

Asymmetric organocatalyzed synthesis of coumarin derivatives

• Natália M. Moreira,
• Lorena S. R. Martelli and
• Arlene G. Corrêa

Beilstein J. Org. Chem. 2021, 17, 1952–1980, doi:10.3762/bjoc.17.128

• dihydrocinchonine 26 in combination with trifluoracetic acid (TFA) as Brønsted acid [39]. The reaction provides pyranocoumarins 25 with three vicinal stereogenic centers in high regio-, diastereo- and enantioselectivities through a tandem allylic alkylation/intramolecular oxa-Michael addition (Scheme 7). A

Development of N-F fluorinating agents and their fluorinations: Historical perspective

• Teruo Umemoto,
• Yuhao Yang and
• Gerald B. Hammond

Beilstein J. Org. Chem. 2021, 17, 1752–1813, doi:10.3762/bjoc.17.123

• ’-Difluorobipyridinium salts In 1998, the Umemoto group reported a new series of N,N’-difluorobipyridinium salts 24-2, N,N’-difluoro-2,2’-, -2,4’-, -3,3’-, and -4,4’-bipyridinium salts. These were synthesized by the direct fluorination of bipyridines 24-1 with 10–20% F2/N2 in the presence of a Lewis acid, a Brønsted

• acid, or its metal salt in acetonitrile or as a 50:1 mixture of acetonitrile/formic acid at −40 to 0 °C. The yields were good to excellent [86] (Figure 7). The trimer 24-3 and polymer homologues 24-4 were also prepared. The N,N’-difluorobipyridinium salts are stable and generally furnished non

Methodologies for the synthesis of quaternary carbon centers via hydroalkylation of unactivated olefins: twenty years of advances

• Thiago S. Silva and
• Fernando Coelho

Beilstein J. Org. Chem. 2021, 17, 1565–1590, doi:10.3762/bjoc.17.112

• [18][19][20][21]. In the presence of a Brønsted acid source, the alkylmetal bond can be transformed into a C–H bond to afford a hydrofunctionalized olefin product (Scheme 1) [22]. The relevance of the formation of new C(sp3)–C(sp3) bonds has led to great efforts to enable the use of carbon

Prins cyclization-mediated stereoselective synthesis of tetrahydropyrans and dihydropyrans: an inspection of twenty years

• Asha Budakoti,
• Pradip Kumar Mondal,
• Prachi Verma and
• Jagadish Khamrai

Beilstein J. Org. Chem. 2021, 17, 932–963, doi:10.3762/bjoc.17.77

• the synthesis of diol by the condensation of styrene and paraformaldehyde in the presence of a Brønsted acid [23][24]. The major breakthrough for this reaction was reported by Hanschke in 1955, when the THP ring was selectively constructed through a Prins reaction involving 3-butene-1-ol and a variety

• cyclopropane carbaldehydes and propargyl alcohol. Mullen and Gagné's (R)-[(tolBINAP)Pt(NC6F5)2][SbF6]2-catalyzed asymmetric Prins cyclization strategy to chromans. Yu and co-workers’ DDQ-catalyzed asymmetric Prins cyclization strategy to trisubstituted THPs. Lalli and Weghe’s chiral-Brønsted-acid- and achiral

• -Lewis-acid-promoted asymmetric Prins cyclization strategy. List and co-workers’ iIDP Brønsted acid-promoted asymmetric Prins cyclization strategy. Zhou and co-workers’ strategy for chiral phosphoric acid (CPA)-catalyzed cascade Prins cyclization. List and co-workers’ approach for asymmetric Prins

The synthesis of chiral β-naphthyl-β-sulfanyl ketones via enantioselective sulfa-Michael reaction in the presence of a bifunctional cinchona/sulfonamide organocatalyst

• Deniz Tözendemir and
• Cihangir Tanyeli

Beilstein J. Org. Chem. 2021, 17, 494–503, doi:10.3762/bjoc.17.43

• transition state model to explain the origin of the stereoinduction was proposed (Scheme 2), according to the Houk’s Brønsted acid hydrogen bonding model. Guo’s computational work in 2017 on the sulfa-Michael addition of thiols to enones in the presence of cinchona alkaloid-type organocatalysts showed that

• while the deprotonated nucleophile is oriented by the Brønsted acid moiety. The substrate scope was extended to substituted chalcones, under the optimized conditions (Table 3). The chalcone derivatives used in this work were obtained by Claisen–Schmidt condensation, using known procedures [44]. Among

Helicene synthesis by Brønsted acid-catalyzed cycloaromatization in HFIP [(CF₃)₂CHOH]

• Takeshi Fujita,
• Noriaki Shoji,
• Nao Yoshikawa and
• Junji Ichikawa

Beilstein J. Org. Chem. 2021, 17, 396–403, doi:10.3762/bjoc.17.35

• metal catalysts. Recently, we have developed a Brønsted acid-catalyzed cycloaromatization in 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP) as solvent [20][21][22]. Fluoroalcohols, such as HFIP, exhibit high ionization power and low nucleophilicity, based on the electron-withdrawing inductive effect of

• silica gel column chromatography (hexane/CH2Cl2 = 3:1) gave 1b (1.31 g, 88%). Conventional methods for the synthesis of helicenes. Brønsted acid-catalyzed cycloaromatization of biaryls bearing an acetal moiety. Two strategies for the helicene synthesis via Suzuki–Miyaura coupling/cycloaromatization

CF₃-substituted carbocations: underexploited intermediates with great potential in modern synthetic chemistry

• Anthony J. Fernandes,
• Armen Panossian,
• Bastien Michelet,
• Agnès Martin-Mingot,
• Frédéric R. Leroux and
• Sébastien Thibaudeau

Beilstein J. Org. Chem. 2021, 17, 343–378, doi:10.3762/bjoc.17.32

• progressive conversion of the starting material into the C6-derivative 64 (Scheme 18). Chen et al. reported the synthesis of C2-phosphorylated indoles via 1,2-phosphorylation of 3-indolylmethanols with H-phosphine oxides or H-phosphonates under Brønsted acid activation [72]. The scope of the reaction includes

• a longer reaction time, allenes 127 undergo a subsequent intramolecular hydroarylation reaction leading to indenes 128. The authors suggested the formation of FeCl3–HFIP complexes being involved in a Lewis acid-assisted Brønsted acid catalysis. The CF3-substituted propargyl alcohol 129 was found to

• generate CF3-substituted iminium ions able to promote Friedel–Crafts alkylations [110][111]. Ma et al. exploited this mode of activation in a Brønsted acid-catalyzed three-component asymmetric reaction [112]. Mixing hemiacetal 175, arylaniline 176, and indole derivatives 149 in the presence of a catalytic

Hydrazides in the reaction with hydroxypyrrolines: less nucleophilicity – more diversity

• Dmitrii A. Shabalin,
• Evgeniya E. Ivanova,
• Igor A. Ushakov,
• Elena Yu. Schmidt and
• Boris A. Trofimov

Beilstein J. Org. Chem. 2021, 17, 319–324, doi:10.3762/bjoc.17.29

• , namely a Brønsted acid-activated hydroxypyrroline, depend on the reaction conditions and the structure of the hydrazides. Keywords: hydrazides; pyridazines; pyrrolines; recyclization; ring expansion; Introduction Di- and tetrahydropyridazines are valuable heterocyclic motifs which are utilized as key

• saturated pyridazine scaffolds is both highly topical and necessary. Recently, we have developed a convenient approach to the 1,4-dihydropyridazine core based on the Brønsted acid-catalyzed regioselective recyclization of 5-hydroxypyrrolines (assembled in a one-pot manner from ketoximes and acetylene gas [8

Three-component reactions of aromatic amines, 1,3-dicarbonyl compounds, and α-bromoacetaldehyde acetal to access N-(hetero)aryl-4,5-unsubstituted pyrroles

• Wenbo Huang,
• Kaimei Wang,
• Ping Liu,
• Minghao Li,
• Shaoyong Ke and
• Yanlong Gu

Beilstein J. Org. Chem. 2020, 16, 2920–2928, doi:10.3762/bjoc.16.241

• obtained with AlCl3 (Table 1, entries 4 and 5). p-Toluenesulfonic acid (PTSA), a strong Brønsted acid, also exhibited a promising catalytic ability, and the yield of 4a reached 73% (Table 1, entry 6). When HOAc was used, only unreacted starting materials were recovered (Table, entry 7). The effect of the

Recent developments in enantioselective photocatalysis

• Callum Prentice,
• James Morrisson,
• Andrew D. Smith and
• Eli Zysman-Colman

Beilstein J. Org. Chem. 2020, 16, 2363–2441, doi:10.3762/bjoc.16.197

• there is much progress yet to be made. Brønsted acid catalysis Using chiral amines and NHCs as catalysts to generate asymmetry relies upon the formation of covalently bonded intermediates such as enamines, iminium ions or Breslow intermediates within the catalytic cycle. The first example of merging non

• "Brønsted acid catalysis", yet interestingly their conjugate bases can also be used as efficient hydrogen bonding catalysts. Knowles et al. showed that a tricatalytic system using chiral phosphate 235 can mediate the deracemisation of cyclic urea rac-236 (Scheme 36) [97]. The proposed mechanism involves a

Fluorohydration of alkynes via I(I)/I(III) catalysis

• Jessica Neufeld,
• Constantin G. Daniliuc and
• Ryan Gilmour

Beilstein J. Org. Chem. 2020, 16, 1627–1635, doi:10.3762/bjoc.16.135

• -TolIF2 17 demonstrated the importance of the exogenous amine·HF as a Brønsted acid activator (Figure 3C). Similar yields were observed when comparing the stoichiometric reaction and the catalytic process using an amine/HF ratio of 1:7.5. To interrogate the importance of the benzoate group in the

• -fluoroketone motif from alkynes. Selectfluor®-mediated oxidation of p-TolI in the presence of an amine/HF source, which plays a dual function as fluoride source and Brønsted acid activator, enables in situ generation of p-TolIF2 (Figure 5). From these preliminary mechanistic investigations, conclusions

Heterogeneous photocatalysis in flow chemical reactors

• Christopher G. Thomson,
• Ai-Lan Lee and
• Filipe Vilela

Beilstein J. Org. Chem. 2020, 16, 1495–1549, doi:10.3762/bjoc.16.125

• and heterogeneous Brønsted acid catalyst, which are both required for independent steps in the synthesis [132]. They suggested that the porphyrin was protonated by the amberlyst-15 sulphonate groups and immobilised to the solid surface by electrostatic forces, rationalising the observed change in

The charge-assisted hydrogen-bonded organic framework (CAHOF) self-assembled from the conjugated acid of tetrakis(4-aminophenyl)methane and 2,6-naphthalenedisulfonate as a new class of recyclable Brønsted acid catalysts

• Svetlana A. Kuznetsova,
• Alexander S. Gak,
• Yulia V. Nelyubina,
• Vladimir A. Larionov,
• Han Li,
• Michael North,
• Vladimir P. Zhereb,
• Alexander F. Smol'yakov,
• Artem O. Dmitrienko,
• Michael G. Medvedev,
• Igor S. Gerasimov,
• Ashot S. Saghyan and
• Yuri N. Belokon

Beilstein J. Org. Chem. 2020, 16, 1124–1134, doi:10.3762/bjoc.16.99

• , elemental analysis, and 1H NMR spectroscopy. The framework was supported by hydrogen bonds between the sulfonate anions and the ammonium cations of NDS and protonated TAPM moieties, respectively. The CAHOF material functioned as a new type of catalytically active Brønsted acid in a series of reactions

• , providing some dissolved F-1 as the real catalyst. In all cases, the catalyst could easily be recovered and recycled. Keywords: Brønsted acid catalyst; charge-assisted hydrogen-bonded framework; Diels–Alder; epoxide ring opening; heterogeneous catalyst; Introduction Tremendous successes in homogeneous

• catalyst for the Brønsted acid-catalyzed ring opening reactions of epoxides with alcohols and water, with the latter reaction occurring in a three-phase medium. In addition, a Diels–Alder reaction was promoted by F-1 in heptane. Results and Discussion Mixing together aqueous solutions of two equivalents of

Asymmetric synthesis of CF₂-functionalized aziridines by combined strong Brønsted acid catalysis

• Xing-Fa Tan,
• Fa-Guang Zhang and
• Jun-An Ma

Beilstein J. Org. Chem. 2020, 16, 638–644, doi:10.3762/bjoc.16.60

• combined strong Brønsted acid catalytic platform consisting of a chiral disulfonimide and 2-carboxyphenylboronic acid. The optical purity of the obtained CF2-substituted aziridines could be further improved by a practical dissolution–filtration procedure. Keywords: aziridines; chiral disulfonimides

• hinges upon the discovery of a combined strong Brønsted acid system comprised of a chiral disulfonimide and 2-carboxyphenylboronic acid. Results and Discussion We commenced the desired one-pot transformation by conducting the model reaction between phenylglyoxal monohydrate (1a), 4-methoxyaniline (2a

• phenyl sulfone under mild conditions by a combined strong Brønsted acid system consisting of chiral disulfonimide and 2-carboxyphenylboronic acid. The optical purity of the obtained CF2-substituted aziridines could be further improved by a practical dissolution–filtration procedure. Substrate expansion

Recent advances in photocatalyzed reactions using well-defined copper(I) complexes

• Mingbing Zhong,
• Xavier Pannecoucke,
• Philippe Jubault and
• Thomas Poisson

Beilstein J. Org. Chem. 2020, 16, 451–481, doi:10.3762/bjoc.16.42

• from the oxidation of the amine with the formed [Cu(II)] complex, followed by a deprotonation by DABCO. The resulting alkoxide is finally converted into the alcohol by the protonated DABCO. During this study, the authors found that replacement of the base by the Brønsted acid (R)-1,1-binaphtyl-2,2-diyl

• avoided the presence of a Brønsted acid in the reaction media. As such, Collins and co-workers developed the catalyst [Cu(I)(pypzs)(BINAP)]BF4 where the ligand (5-(4-fluorosulfonyl)amino-3-(2pyridyl)pyrazole)) (pypzs) had an acidic proton prone to activate the carbonyl group during the pinacol coupling

Synthesis of 3-alkenylindoles through regioselective C–H alkenylation of indoles by a ruthenium nanocatalyst

• Abhijit Paul,
• Debnath Chatterjee,
• Srirupa Banerjee and
• Somnath Yadav

Beilstein J. Org. Chem. 2020, 16, 140–148, doi:10.3762/bjoc.16.16

• oxidative dehydrogenation [19]. Recently, Maji and co-workers reported the synthesis of 3-alkenylindoles from indoles and α-hydrogen-containing alkyl-/arylaldehydes by successive Brønsted acid/base catalysis (Scheme 1) [20]. The third category, which is also the most explored and popular one, involves the

[1,3]/[1,4]-Sulfur atom migration in β-hydroxyalkylphosphine sulfides

• Katarzyna Włodarczyk,
• Piotr Borowski and
• Marek Stankevič

Beilstein J. Org. Chem. 2020, 16, 88–105, doi:10.3762/bjoc.16.11

• reaction. In the presence of a Brønsted acid, mainly [1,3]-rearrangement is observed, whereas a Lewis acid catalyzes the [1,4]-sulfur migration. To gain insight into the mechanism of these transformations, the stereochemistry of these rearrangements have been tested, along with the conduction of some

• cationic intermediates underwent hydrolysis during aqueous workup, leading to the observed products. It was assumed that a similar sulfur atom migration should occur in the presence of Brønsted acid under dry conditions. Therefore, a set of phosphine sulfides prepared above was subjected to the reaction

• cyclopentyl and cycloheptyl fragments, which afforded the β-mercaptophosphine oxides in low yields. In order to gain insight into the reaction mechanism when using Brønsted acid, it was decided to perform the reactions at different temperatures and to monitor the reaction mixture at different times (Table 4

Microwave-assisted synthesis of 2-substituted 4,5,6,7-tetrahydro-1,3-thiazepines from 4-aminobutanol

• María C. Mollo,
• Natalia B. Kilimciler,
• Juan A. Bisceglia and
• Liliana R. Orelli

Beilstein J. Org. Chem. 2020, 16, 32–38, doi:10.3762/bjoc.16.5

• -toxic and environmentally safe. These reagents play a dual role, as they enhance the electrophilicity of nitriles, carbonyls and carbinols, and can also react irreversibly with water. Unlike PPA, a Brønsted acid incompatible with some acid-sensitive functionalities, PPE and PPSE are aprotic and mild

Extension of the 5-alkynyluridine side chain via C–C-bond formation in modified organometallic nucleosides using the Nicholas reaction

• Renata Kaczmarek,
• Dariusz Korczyński,
• James R. Green and
• Roman Dembinski

Beilstein J. Org. Chem. 2020, 16, 1–8, doi:10.3762/bjoc.16.1

• -acetate hexacarbonyl dicobalt complexes and a Lewis or Brønsted acid. A range of heteroatom nucleophiles have been incorporated into alkyne dicobalt complexes by this chemistry [34][35][36][37][38][39][40]. However, reactions with carbon-based nucleophiles provide an opportunity to access the structurally

Bacterial terpene biosynthesis: challenges and opportunities for pathway engineering

• Eric J. N. Helfrich,
• Geng-Min Lin,
• Christopher A. Voigt and
• Jon Clardy

Beilstein J. Org. Chem. 2019, 15, 2889–2906, doi:10.3762/bjoc.15.283

• sesquiterpenes (type I TCs), diterpenes can either be generated by type I TCs or type II TCs [11][55]. Type II TCs initiate carbocation formation by Brønsted acid catalysis to protonate a terminal isoprene double bond or an epoxide ring (Figure 5b) [56]. Thus, cyclization mediated by type II TCs leads to an

Recent advances in transition-metal-catalyzed incorporation of fluorine-containing groups

• Xiaowei Li,
• Xiaolin Shi,
• Xiangqian Li and
• Dayong Shi

Beilstein J. Org. Chem. 2019, 15, 2213–2270, doi:10.3762/bjoc.15.218

• Sodeoka and co-workers in 2012 [131]. The reaction was carried out with Togni’s reagent as the CF3 source and TsOH as a Brønsted acid in CH2Cl2 at 40 °C (Scheme 71). Notably, trifluoromethylstyrenes were formed through further transformations of the oxytrifluoromethylated products with high efficiency. In

Friedel–Crafts approach to the one-pot synthesis of methoxy-substituted thioxanthylium salts

• Kenta Tanaka,
• Yuta Tanaka,
• Mami Kishimoto,
• Yujiro Hoshino and
• Kiyoshi Honda

Beilstein J. Org. Chem. 2019, 15, 2105–2112, doi:10.3762/bjoc.15.208

• ) sulfide (1a) with benzoyl chloride (2a) in the presence of Brønsted acids in chlorobenzene at several temperatures (Table 1). When we used a strong Brønsted acid such as trifluoromethanesulfonic acid (TfOH) at room temperature, the desired thioxanthylium salt 3a was obtained with 21% yield while other

A review of the total syntheses of triptolide

• Xiang Zhang,
• Zaozao Xiao and
• Hongtao Xu

Beilstein J. Org. Chem. 2019, 15, 1984–1995, doi:10.3762/bjoc.15.194

• , the trans-decalin scaffold is the ideal objective of numerous methodology studies. Inspired by nature’s highly efficient and stereochemically controlled syntheses of terpenes, up to now, various synthetic strategies to trans-decalin have been developed, e.g., Brønsted acid or Lewis acid-mediated