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

Enantioselective PCCP Brønsted acid-catalyzed aza-Piancatelli rearrangement

• Gabrielle R. Hammersley,
• Meghan F. Nichol,
• Helena C. Steffens,
• Jose M. Delgado,
• Gesine K. Veits and
• Javier Read de Alaniz

Beilstein J. Org. Chem. 2019, 15, 1569–1574, doi:10.3762/bjoc.15.160

• rearrangement has been developed using a chiral Brønsted acid based on pentacarboxycyclopentadiene (PCCP). This reaction provides rapid access to valuable chiral 4-amino-2-cyclopentenone building blocks from readily available starting material and is operationally simple. Keywords: aza-Piancatelli; Brønsted

• aza-Piancatelli reaction, we sought to identify other asymmetric catalytic systems capable of controlling the absolute stereochemistry. To this end, we envisioned that the chiral pentacarboxycyclopentadiene (PCCP) Brønsted acid catalyst (8) recently developed by the Lambert lab might be suitable

• (Figure 1) [38]. First, the pKa values measured in acetonitrile (MeCN) are lower than chiral phosphoric acids (Brønsted acid pKa = 8.85 vs chiral phosphoric acids pKa = 12–14) [38]. Given the enhanced acidity, we reasoned that this type of chiral Brønsted acid catalyst could facilitate the dehydration

Superelectrophilic carbocations: preparation and reactions of a substrate with six ionizable groups

• Sean H. Kennedy,
• Makafui Gasonoo and
• Douglas A. Klumpp

Beilstein J. Org. Chem. 2019, 15, 1515–1520, doi:10.3762/bjoc.15.153

• example, alcohol 18 provides a mixture of products 19 and 20 in a 32:68 ratio, presumably through the pentacation 5 (Scheme 5). Even with the use of the stronger Brønsted acid, CF3SO3H–SbF5, significant quantities of the cyclization product 20 are observed. This raises an obvious question: why does

Different reactivity of phosphorylallenes under the action of Brønsted or Lewis acids: a crucial role of involvement of the P=O group in intra- or intermolecular interactions at the formation of cationic intermediates

• Stanislav V. Lozovskiy,
• Alexander Yu. Ivanov and
• Aleksander V. Vasilyev

Beilstein J. Org. Chem. 2019, 15, 1491–1504, doi:10.3762/bjoc.15.151

• [22][23][24][25], Ir [26], Rh [27][28], and Co [29]. However, only electron-rich allenes, bearing electron-donating substituents, take part in the metal-catalyzed reactions. There are just a few examples of Brønsted acid catalyzed intermolecular hydroarylations of allenes by electron-rich arenes

• carried out a large-scale one-pot solvent-free synthesis of amides 6a,b starting from propargyl alcohols 7a,b at room temperature (Scheme 4). At the first step, alcohols 7a,b in the reaction with PCl3 were transformed into the corresponding allenes 1a,h. Then, the addition of Brønsted acid (TfOH or H2SO4

Robust perfluorophenylboronic acid-catalyzed stereoselective synthesis of 2,3-unsaturated O-, C-, N- and S-linked glycosides

• Madhu Babu Tatina,
• Xia Mengxin,
• Rao Peilin and
• Zaher M. A. Judeh

Beilstein J. Org. Chem. 2019, 15, 1275–1280, doi:10.3762/bjoc.15.125

• activation of alcohols [24], epoxide opening [25][26], Friedel–Crafts alkylations [27], dehydrative glycosylation [28] and many other reactions [29][30][31]. The robustness and mildness of organoboronic acid catalysts in comparison to traditional strong Lewis and Brønsted acid catalysts inspired us to

Mechanochemical synthesis of hyper-crosslinked polymers: influences on their pore structure and adsorption behaviour for organic vapors

• Sven Grätz,
• Sebastian Zink,
• Hanna Kraffczyk,
• Marcus Rose and
• Lars Borchardt

Beilstein J. Org. Chem. 2019, 15, 1154–1161, doi:10.3762/bjoc.15.112

• –Teller) surface areas of up to 2000 m2g−1 for fully amorphous materials [10]. Typically, chloromethyl or methoxy groups are used to crosslink aromatic building blocks by using stoichiometric or even excess amounts of FeCl3 as catalyst. Recently, also a metal-free Brønsted acid-catalyzed reaction using

The LANCA three-component reaction to highly substituted β-ketoenamides – versatile intermediates for the synthesis of functionalized pyridine, pyrimidine, oxazole and quinoxaline derivatives

• Tilman Lechel,
• Roopender Kumar,
• Mrinal K. Bera,
• Reinhold Zimmer and
• Hans-Ulrich Reissig

Beilstein J. Org. Chem. 2019, 15, 655–678, doi:10.3762/bjoc.15.61

• , KE30, KE31 and KE35 (Table 1, entries 2, 23, 30, 31, and 35). The present approach does not allow the synthesis of β-ketoenamides with substituents R2 = H or R3 = H. The reaction of lithiated methoxyallene with hydrogen cyanide as second component was not examined due to the assumed Brønsted acid

Aqueous olefin metathesis: recent developments and applications

• Valerio Sabatino and
• Thomas R. Ward

Beilstein J. Org. Chem. 2019, 15, 445–468, doi:10.3762/bjoc.15.39

• , the presence of a Brønsted acid led to the protonation of one phosphine ligand rather than reacting with the ruthenium alkylidene moiety. Scavenging of the trialkylphosphine moiety resulted in a more active complex capable of initiating the ROMP of 2,3-difunctionalized norbornadienes and 7-oxo

A tutorial review of stereoretentive olefin metathesis based on ruthenium dithiolate catalysts

• Daniel S. Müller,
• Olivier Baslé and
• Marc Mauduit

Beilstein J. Org. Chem. 2018, 14, 2999–3010, doi:10.3762/bjoc.14.279

• styrene 32 the protonation and loss of the catechothiolate ligand by Brønsted acid 34 is a faster process leading to catalyst degradation. It should be noted that stereoretentive CM and RCM with (E)-2-butene (E-25) as capping reagent were also reported, however, these reactions required a significantly

MoO₃ on zeolites MCM-22, MCM-56 and 2D-MFI as catalysts for 1-octene metathesis

• Hynek Balcar,
• Martin Kubů,
• Naděžda Žilková and
• Mariya Shamzhy

Beilstein J. Org. Chem. 2018, 14, 2931–2939, doi:10.3762/bjoc.14.272

• various strength. MCM-22(28) and MCM-56(13) exhibited the highest concentrations of acid sites (both Brønsted and Lewis) in accord with their highest Al concentrations. The acid sites concentrations of MCM-22(70) and 2D-MFI(26) were lower and close to each other. The Brønsted acid site concentration of

• HZSM-5(25) was as high as that of MCM-22(28), however, its Lewis acid site concentration was significantly lower. After supporting Mo compounds the concentrations of Brønsted acid sites decreased significantly which may indicate that MoOx species reacted predominantly with Brønsted acid sites of the

• been already described [6] and it assumed that most of Mo active species in zeolite-based catalysts are formed by reacting molybdenum oxide with Si-O(H)-Al groups [12][27]. Similarly, Lim et al. showed recently [28], that Brønsted acid sites improve dispersion of molybdenum oxide on the surface

An overview on recent advances in the synthesis of sulfonated organic materials, sulfonated silica materials, and sulfonated carbon materials and their catalytic applications in chemical processes

• Hashem Sharghi,
• Pezhman Shiri and
• Mahdi Aberi

Beilstein J. Org. Chem. 2018, 14, 2745–2770, doi:10.3762/bjoc.14.253

• adsorption showed some peaks at 1646, 1626, 1549 and 1476 cm−1. The peaks at 1646, 1626, and 1549 cm−1 could be due to the vibration of pyridinium (PyH+) species, corresponding to the existence of Brønsted acid sites on the s-MWCNTs 137. The peak at 1476 cm−1 was also labeled to the coordination of electron

• pair in the nitrogen orbital of pyridine to Brønsted acid sites. No IR signal relating to the Lewis acid sites was detected at 1455 cm−1, as well. Finally, the s-MWCNTs 137 were used in the esterification of palm fatty acid distillate (PFAD) with methanol. The esterification of palm fatty acid with

β-Hydroxy sulfides and their syntheses

• Mokgethwa B. Marakalala,
• Edwin M. Mmutlane and
• Henok H. Kinfe

Beilstein J. Org. Chem. 2018, 14, 1668–1692, doi:10.3762/bjoc.14.143

• their corresponding analogs (Brønsted acid p-TsOH and Lewis base n-Bu3P, respectively) as shown in Scheme 2 [21]. In comparison to the reaction carried out under the same reaction conditions but in aqueous or organic media, it was found that the solvent-free reactions provided the β-hydroxy sulfides in

• atmosphere [50]. Product yields ranged from 18 to 96%, but in moderate enantioselectivities of 20–64% ee (Scheme 16). Sun and co-workers reported the use of BINOL-based Brønsted acid catalysts such as TRIP 60 for the asymmetric thiolysis of meso-epoxides with benzothiazoles 62 as nucleophiles as shown in

• sulfide syntheses by ring opening of epoxides under different Lewis and Brønsted acid and base catalysts. n-Bu3P-catalyzed thiolysis of epoxides and aziridines to provide the corresponding β-hydroxy and β-amino sulfides. Zinc(II) chloride-mediated thiolysis of epoxides. Thiolysis of epoxides and one-pot

Glycosylation reactions mediated by hypervalent iodine: application to the synthesis of nucleosides and carbohydrates

• Yuichi Yoshimura,
• Hideaki Wakamatsu,
• Yoshihiro Natori,
• Yukako Saito and
• Noriaki Minakawa

Beilstein J. Org. Chem. 2018, 14, 1595–1618, doi:10.3762/bjoc.14.137

• (OTf)2, Yb(OTf)3) and a Brønsted acid (TfOH) were proven useful as activators, by which the reaction finished in a short time and gave the products with high stereoselectivity [79]. Recently, the reaction was revisited by Kajimoto et al., who sought a glycosylation reaction that could be applied to

Recent applications of chiral calixarenes in asymmetric catalysis

• Mustafa Durmaz,
• Erkan Halay and
• Selahattin Bozkurt

Beilstein J. Org. Chem. 2018, 14, 1389–1412, doi:10.3762/bjoc.14.117

• methyl ester 120 in four steps (Scheme 39) [73]. The organocatalytic properties of this inherently chiral calixarene Brønsted acid was firstly examined in the aza-Diels–Alder reaction of imines bearing electron-withdrawing or electron-donating substituents 122 with Danishefsky’s diene (123, Scheme 40

Fluorocyclisation via I(I)/I(III) catalysis: a concise route to fluorinated oxazolines

• Felix Scheidt,
• Christian Thiehoff,
• Gülay Yilmaz,
• Stephanie Meyer,
• Constantin G. Daniliuc,
• Gerald Kehr and
• Ryan Gilmour

Beilstein J. Org. Chem. 2018, 14, 1021–1027, doi:10.3762/bjoc.14.88

• fluorooxygenation of readily accessible N-allylcarboxamides via an I(I)/I(III) manifold to generate 2-oxazolines containing a fluoromethyl group. Catalysis is conditional on the oxidation competence of Selectfluor®, whilst HF serves as both a fluoride source and Brønsted acid activator. The C(sp3)–F bond of the

• likelihood that HF also functions as a Brønsted acid activator in catalysis. Employing an amine/HF ratio of 1:4.5, product formation was observed (Table 1, entry 1, 46%). Reducing this ratio to 1:3 had a detrimental effect on catalysis efficiency, generating the product in <5% yield (Table 1, entry 2

AuBr₃-catalyzed azidation of per-O-acetylated and per-O-benzoylated sugars

• Jayashree Rajput,
• Srinivas Hotha and
• Madhuri Vangala

Beilstein J. Org. Chem. 2018, 14, 682–687, doi:10.3762/bjoc.14.56

• of the anomeric acetate carbonyl oxygen, probably the Brønsted acid, HBr, generated from AuBr3 and water present in the reaction medium is also participating in the catalytic cycle. Also no reaction was observed when peracetylated galactose and 3 equiv of trimethylsilyl azide were stirred at room

Mannich base-connected syntheses mediated by ortho-quinone methides

• Petra Barta,
• Ferenc Fülöp and
• István Szatmári

Beilstein J. Org. Chem. 2018, 14, 560–575, doi:10.3762/bjoc.14.43

• -amidoalkyl-2-naphthols carried out in the presence of Lewis and Brønsted acid catalysts. As depicted in Table 1, entries 13–38, the applicability of p-toluenesulfonic acid (p-TSA) [27], montmorillonite K10 [30], Indion-130 [31], iodine (I2) [32], potassium dodecatungstocobaltate (K5CoW12O40·3H2O) [33

• indole polyheterocycles 49 and 50 in good yields with 90–99% ee. One of the latest publications around this topic has been reported by Deb et al. [86][87]. Various 2-(aminoalkyl)phenols or 1-(aminoalkyl)naphthols 51 were reacted with indoles under Brønsted acid catalysis resulting in 3-(α,α-diarylmethyl

• aminonaphthols and cyclic amines. Brønsted acid-catalysed reaction between aza-o-QMs and 2- or 3-substituted indoles. Formation of 3-(α,α-diarylmethyl)indoles 52 in different synthetic pathways. Alkylation of o-QMs with N-, O- or S-nucleophiles. Formation of DNA linkers and o-QM mediated polymers. Comparison of

Synthesis and stability of strongly acidic benzamide derivatives

• Frederik Diness,
• Niels J. Bjerrum and
• Mikael Begtrup

Beilstein J. Org. Chem. 2018, 14, 523–530, doi:10.3762/bjoc.14.38

• p-toluenesulfonic acid (5) which is commonly used as a soluble organic acid catalyst in chemical reactions. Remarkably, neither the N-triflylbenzamides nor the N,N’-bis(triflyl)benzimidamides have been studied as Brønsted acid catalysts. Their chemical stability including compatibility with

One-pot preparation of 4-aryl-3-bromocoumarins from 4-aryl-2-propynoic acids with diaryliodonium salts, TBAB, and Na₂S₂O₈

• Teppei Sasaki,
• Katsuhiko Moriyama and
• Hideo Togo

Beilstein J. Org. Chem. 2018, 14, 345–353, doi:10.3762/bjoc.14.22

• -vinylphenols at 110 °C [19], the FeCl3-catalyzed areneselenyl-cyclization of aryl 2-alkynoates with ArSeSeAr at rt [20], and the Rh-catalyzed annulation of arylthiocarbamates with alkynes/AgOTf/Cu(OAc)2 at 120 °C [21]. As examples of the transition-metal-free construction of the coumarin skeleton, the Brønsted

• acid-catalyzed reaction of phenols and propynoic acids [22] and the (−)-riboflavin-catalyzed photochemical reaction of cinnamic acids [23] were reported recently. Moreover, the use of radical cyclization for the construction of the coumarin skeleton has become widespread. Examples include the radical

Novel amide-functionalized chloramphenicol base bifunctional organocatalysts for enantioselective alcoholysis of meso-cyclic anhydrides

• Lingjun Xu,
• Shuwen Han,
• Linjie Yan,
• Haifeng Wang,
• Haihui Peng and
• Fener Chen

Beilstein J. Org. Chem. 2018, 14, 309–317, doi:10.3762/bjoc.14.19

• ][20] and valine [21] etc. [22][23][24][25][26][27][28][29], while fully synthetic catalysts are rare, only involving Brønsted acid/base catalysts [30], cyclohexanediamine catalysts [31][32], and diphenylethylenediamine catalysts [33][34]. Significant progress in our laboratory has been made in the

• desired bifunctional Brønsted acid/base reactivity for enantioselective desymmetrization of anhydrides [48][49]. Results and Discussion A series of new chloramphenicol based-amide bifunctional catalysts 7a–q (Scheme 1) were synthesized from the appropriate optically pure (1R,2R)-diamine 6, prepared via

• anhydride 8a was carried out in various conditions with 10 equivalents of methanol. Resultingly, this novel Brønsted acid/base catalysts 7a–q proved to be sufficiently active and gave the monoester product with high yield and superior enantioselectivity, suggesting the unique reactivity of this amide-based

Regiodivergent condensation of 5-alkoxycarbonyl-1H-pyrrol-2,3-diones with cyclic ketazinones en route to spirocyclic scaffolds

• Alexey Yu. Dubovtsev,
• Maksim V. Dmitriev,
• Аndrey N. Maslivets and
• Michael Rubin

Beilstein J. Org. Chem. 2017, 13, 2179–2185, doi:10.3762/bjoc.13.218

• tertiary amines), while the use of stronger bases (hydroxides or alkoxides) caused decomposition of the base-sensitive 1H-pyrrole-2,3-dione moiety 9. An attempt to perform the reaction in the presence of catalytic amounts of Brønsted acid (TsOH) also did not provide the spirocyclic products. Instead

New bio-nanocomposites based on iron oxides and polysaccharides applied to oxidation and alkylation reactions

• Daily Rodríguez-Padrón,
• Alina M. Balu,
• Antonio A. Romero and
• Rafael Luque

Beilstein J. Org. Chem. 2017, 13, 1982–1993, doi:10.3762/bjoc.13.194

• pulses. Pyridine, due to low steric hindrance, adsorbs nonspecifically in both types of centers, while dimethylpyridine adsorbs specifically on Brønsted acid centers, due to the high steric hindrance of the methyl groups [54]. It is noticeable that the TiO2-Fe2O3-PS4 catalyst possesses both Lewis and

• Brønsted acid sites with a more marked Lewis acidity. The Fe2O3-PS4-MNP material presents instead only Lewis acid sites, while the Fe2O3-PS4 does not show appreciable acidity to be quantized (Table 2). Acidity measurements from both methodologies (PY DRIFT, PY and DMPY pulse chromatography titration data

• ) were generally in good agreement, supporting the validity of our assumption on DMPY adsorbing selectively on Brønsted acid sites. Inductively coupled plasma–mass spectrometry (ICP–MS) The elemental composition of the TiO2-Fe2O3-PS4 material was determined by ICP–MS. The content of iron and titanium was

Synthesis of new pyrrolidine-based organocatalysts and study of their use in the asymmetric Michael addition of aldehydes to nitroolefins

• Alejandro Castán,
• Ramón Badorrey,
• José A. Gálvez and
• María D. Díaz-de-Villegas

Beilstein J. Org. Chem. 2017, 13, 612–619, doi:10.3762/bjoc.13.59

• reaction conditions [21]. For example, in secondary amine-catalysed asymmetric reactions a Brønsted acid additive was found to accelerate the formation of the enamine intermediate and thus to improve not only the reactivity but also the diastereoselectivity and enantioselectivity [22][23]. On the other

• hand, the presence of thiourea additives could activate nitroalkenes when used as substrates by double hydrogen bonding, which lead to improved reactivities [24]. Based on these findings, we decided to explore the effect of a Brønsted acid or an achiral thiourea as additive on the reaction between

• possibility of accelerating the formation of the enamine intermediate and simultaneously activating the nitroalkene by using a combination of organocatalyst OC4, a Brønsted acid and an achiral thiourea. Thus the reaction was repeated in the presence of a combination of benzoic acid and N,N'-diphenylthiourea

Brønsted acid-mediated cyclization–dehydrosulfonylation/reduction sequences: An easy access to pyrazinoisoquinolines and pyridopyrazines

• Ramana Sreenivasa Rao and
• Chinnasamy Ramaraj Ramanathan

Beilstein J. Org. Chem. 2017, 13, 428–440, doi:10.3762/bjoc.13.46

• strategy using a Brønsted acid. Subsequent dehydrosulfonylation reactions of the ene-diamides, in a one pot manner, smoothly transformed them to substituted pyrazinones. A concise synthesis of praziquantel (1) has also been achieved through this method. Keywords: Brønsted acid; piperazine-2,6-diones

• corresponding products. Further, these 4-benzenesulfonylpiperazine-2,6-diones were subjected to an imide carbonyl group activation strategy, to develop a practical approach to synthesize pyrazinoisoquinoline and pyridopyrazines via Brønsted acid assisted 6-exo-trig cyclization of arylethylpiperazine-2,6-diones

• Brønsted acid [32]. Sulfonamides also participate in amide hydrolysis with external nucleophiles such as phosphide anions [33] or phenyldimethylsilyllithium [34]. The combinations of thiophenol/K2CO3 [35] or NaOH/MeOH [36] are also known to hydrolyse sulfonamides. These methods lead to the formation of the

Synthesis of structurally diverse 3,4-dihydropyrimidin-2(1H)-ones via sequential Biginelli and Passerini reactions

• Andreas C. Boukis,
• Baptiste Monney and
• Michael A. R. Meier

Beilstein J. Org. Chem. 2017, 13, 54–62, doi:10.3762/bjoc.13.7

• , aldehyde 1 is activated by a Lewis- or a Brønsted acid. In the next step, urea/thiourea 2 can serve as a nucleophile and react with the activated carbonyl carbon to form a heminal species. However, under acidic conditions heminals can eliminate water and form an N-acyliminium cation 3. This reactive cation