Non-noble metal-catalyzed cross-dehydrogenation coupling (CDC) involving ether α-C(sp³)–H to construct C–C bonds

• Hui Yu and
• Feng Xu

Beilstein J. Org. Chem. 2023, 19, 1259–1288, doi:10.3762/bjoc.19.94

• oxidant to generate an oxygen-radical cationic intermediate, which undergoes abstraction of a hydrogen radical (or loses a proton first, followed by an electron) to afford an oxonium ion intermediate. Finally, the oxonium ion is attacked by various nucleophiles to obtain the target functionalized product

• extract a hydrogen from the ether C (sp3)–H bond to form radicals. Subsequently, a single electron transfer (SET) leads to the oxonium species. Then, the enamine generated in situ from methyl aryl ketone and pyrrolidine undergoes a nucleophilic reaction with the oxonium species followed by hydrolysis to

• metal-triggered oxidation of the ether substrate to obtain the corresponding radical or oxonium ion as the key intermediate to obtain the final coupling product. Subsequently, some novel Co-catalyzed coupling mechanisms have been proposed. In 2016, Lu et al. reported that the Co/TBHP catalyst oxidation

Efficient and regioselective synthesis of dihydroxy-substituted 2-aminocyclooctane-1-carboxylic acid and its bicyclic derivatives

• İlknur Polat,
• Selçuk Eşsiz,
• Uğur Bozkaya and
• Emine Salamci

Beilstein J. Org. Chem. 2022, 18, 77–85, doi:10.3762/bjoc.18.7

• with NaHSO4 in a mixture of methylene chloride/MeOH proceeded as described in Scheme 5. First, the C=O group of the ester prefers to attack the protonated epoxide to give intermediate 15. Then, water, which is available in methanol as an impurity, attacks the oxonium ion to give dealkylation product 10

A photochemical C=C cleavage process: toward access to backbone N-formyl peptides

• Haopei Wang and
• Zachary T. Ball

Beilstein J. Org. Chem. 2021, 17, 2932–2938, doi:10.3762/bjoc.17.202

• . C–O cleavage, Figure 1A) through H-atom abstraction from a photoexcited intermediate, which produces an oxonium-type intermediate (in brackets). Hydrolysis of this intermediate then affords an alcohol product. Recently [16][17], we demonstrated that vinylogous analogues of this mechanism (Figure 1B

• nitroso 3, and related compounds 4 and 5 are all consistent with the classical C–X cleavage mechanism and with hydrolysis of the presumed oxonium intermediate 6’, but are inconsistent with the production of formyl products. In contrast, photoirradiation of the same alkenyl ether 6 under acidic conditions

Iron-catalyzed domino coupling reactions of π-systems

• Austin Pounder and
• William Tam

Beilstein J. Org. Chem. 2021, 17, 2848–2893, doi:10.3762/bjoc.17.196

• the furan framework. Following Fe-catalyzed oxidation, the resulting oxonium cation can be deprotonated to afford the final dihydrofuran. Further, this inter-/intramolecular radical addition/cyclization methodology has been applied for the synthesis of various substituted dihydropyrans [100][101] and

Synthetic strategies toward 1,3-oxathiolane nucleoside analogues

• Umesh P. Aher,
• Dhananjai Srivastava,
• Girij P. Singh and
• Jayashree B. S

Beilstein J. Org. Chem. 2021, 17, 2680–2715, doi:10.3762/bjoc.17.182

• -based lactol. This is a necessary requirement to produce the desired stereochemistry in the product. It was extensively reported that the β-selectivity could be due to the formation of an oxonium ion, which is stabilized through anchimeric assistance of the ʟ-menthyl ester function. The method requires

• stereoselectivity during β-selective glycosidic bond formation. The general pathway for glycosidic bond formation (Figure 4) shows that the glycoside donor moiety has to be activated using an appropriate activator to form an oxonium ion. The attack of a nucleobase (glycosyl acceptor) may occur on either side of the

• oxonium ion, which can result in two anomers, i.e., an α- and a β-anomer. The factors affecting such stereocontrolled glycoside bond formations are also discussed in this review. The preparation of the racemate 1c was reported by Belleau et al. in 1989 (Scheme 26) [38]. The method involved the coupling of

Advances in mercury(II)-salt-mediated cyclization reactions of unsaturated bonds

• Sumana Mandal,
• Raju D. Chaudhari and
• Goutam Biswas

Beilstein J. Org. Chem. 2021, 17, 2348–2376, doi:10.3762/bjoc.17.153

• cyclization of secondary and tertiary l-alkynyl-2,3-epoxyalcohols 131a [94]. This is an example of a Hg(II)-salt-catalyzed rearrangement of 1-alkynyl-2,3-epoxy alcohols to substituted furans. The furan 133a was formed by dehydration of intermediate 132a through the corresponding oxonium cation. When R3 is an

On the application of 3d metals for C–H activation toward bioactive compounds: The key step for the synthesis of silver bullets

• Renato L. Carvalho,
• Amanda S. de Miranda,
• Mateus P. Nunes,
• Roberto S. Gomes,
• Guilherme A. M. Jardim and
• Eufrânio N. da Silva Júnior

Beilstein J. Org. Chem. 2021, 17, 1849–1938, doi:10.3762/bjoc.17.126

• oxonium species in the methylation step. Abiraterone acetate (53) is a drug used in cancer treatment [138]. The Mn-catalyzed methylation of an abiraterone analogue was achieved by replacing the fluorination step by mesylation in 15% of overall yield and only a single diastereoisomer was observed (Scheme

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

• carbenium ions. Calculations of the isodesmic reactions (1), (2), and (3) demonstrate the overall destabilizing effect of CF3 compared to H or CH3 when directly attached to a carbenium ion (i.e., α position, Scheme 1) [5][29]. Even an oxonium ion appears to be significantly destabilized by the presence of

• ). This dication reacts with benzene to provide pyridinium–oxonium dication 159 in solution. Further arylation does not occur spontaneously, which was evident because alcohol 157 was isolated at the end of the reaction. Upon heating at 60 °C, the second arylation takes place, presumably via the formation

Semiautomated glycoproteomics data analysis workflow for maximized glycopeptide identification and reliable quantification

• Steffen Lippold,
• Arnoud H. de Ru,
• Jan Nouta,
• Peter A. van Veelen,
• Magnus Palmblad,
• Manfred Wuhrer and
• Noortje de Haan

Beilstein J. Org. Chem. 2020, 16, 3038–3051, doi:10.3762/bjoc.16.253

• relying only on oxonium ions and precursor mass, using a score above 30 were also described recently [15]. A suitable cut-off score should always be carefully evaluated for each (glyco)peptide moiety with respect to the glycoform coverage and accuracy [11]. Byonic identified the relevant N-glycosylation

All-carbon [3 + 2] cycloaddition in natural product synthesis

• Zhuo Wang and
• Junyang Liu

Beilstein J. Org. Chem. 2020, 16, 3015–3031, doi:10.3762/bjoc.16.251

• complex intermediate C. C is subjected to the nucleophilic attack of n-butyl vinyl ether (140) and generates alkenyl metallic intermediate D. Intramolecular nucleophilic attack onto the oxonium carbon of D affords the [3 + 2] cycloaddition product 141 with regeneration of the catalyst A. In 2020, Ye and co

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

• conversions, computational and experimental data indicated that the protonated hydroxy groups (oxonium ions) are not persistent intermediates, but rather cleavage of the carbon–oxygen bond is almost instantaneous [12]. It is assumed that ionization to the carbocations occurs in a stepwise process, first

Tandem copper and photoredox catalysis in photocatalytic alkene difunctionalization reactions

• Nicholas L. Reed,
• Madeline I. Herman,
• Vladimir P. Miltchev and
• Tehshik P. Yoon

Beilstein J. Org. Chem. 2019, 15, 351–356, doi:10.3762/bjoc.15.30

• ]. Subsequent oxidation of radical 7 by Cu(II) affords a formally cationic intermediate that is trapped by the carbamoyl oxygen to afford oxonium 8. The loss of the tert-butyl cation provides the oxyamination product 2, which can be isolated in good yields with excellent diastereoselectivity. Turnover of the

Oxidative radical ring-opening/cyclization of cyclopropane derivatives

• Yu Liu,
• Qiao-Lin Wang,
• Zan Chen,
• Cong-Shan Zhou,
• Bi-Quan Xiong,
• Pan-Liang Zhang,
• Chang-An Yang and
• Quan Zhou

Beilstein J. Org. Chem. 2019, 15, 256–278, doi:10.3762/bjoc.15.23

• formed. An ipso-cyclization with aromatic ring occurres and gives the intermediate 61 when R1 is a para-methoxy group. The oxonium ion 62 is produced by the oxidation of the intermediate 61 under the action of Fe3+ [81]. Lastly, the oxonium ion 62 is transformed into the desired product 56 in the

Silanediol versus chlorosilanol: hydrolyses and hydrogen-bonding catalyses with fenchole-based silanes

• Falco Fox,
• Jörg M. Neudörfl and
• Bernd Goldfuss

Beilstein J. Org. Chem. 2019, 15, 167–186, doi:10.3762/bjoc.15.17

• the yields and the enantiomeric excess. In a third reaction, the 1,4 addition of silyl keten acetals 11 to chromone 20 is investigated (Table 11, Scheme 8). Chromone 20 is first transformed to the oxonium ion pair 21. Catalyst BIFOXSi(OH)2 (9) binds the triflate anion via hydrogen bonding and leaves

Carbonylonium ions: the onium ions of the carbonyl group

• Daniel Blanco-Ania and
• Floris P. J. T. Rutjes

Beilstein J. Org. Chem. 2018, 14, 2568–2571, doi:10.3762/bjoc.14.233

• replacement of carbon atoms by the heteroatoms oxygen, nitrogen and sulfur, respectively. Thus, “oxacarbenium ion” would denote a carbenium ion whose carbon atom is replaced by an oxygen atom, that is, an oxonium ion (3; Figure 2). Although if a coherent structure by formal subtraction of hydride from the

• term “carboxonium ion” to describe intermediates 1. The issue with this term is that it is used for many different oxonium ions independently of their structure, mainly intermediates with a variable number of oxygen atoms bound to the central carbon atom. This name is used to describe protonated

• carboxylic acids (carboxylic acidium ions), protonated esters and protonated aldehydes and ketones amongst others [31]. The name thought to be originated from the combination of the terms “carbenium” and “oxonium”, but not correctly applied to such a broad scope of intermediates. Clearly the intermediates

Syn-selective silicon Mukaiyama-type aldol reactions of (pentafluoro-λ⁶-sulfanyl)acetic acid esters with aldehydes

• Anna-Lena Dreier,
• Andrej V. Matsnev,
• Joseph S. Thrasher and
• Günter Haufe

Beilstein J. Org. Chem. 2018, 14, 373–380, doi:10.3762/bjoc.14.25

• under liberation of a chloride. For the formed oxonium ion, two conformers A and B are possible due to free rotation around the single bond neighboring the SF5 group and the former benzylic carbon atom (Scheme 4). Due to the possible repulsive interaction of the SF5 group with the quinoid ring in

Rh(II)-mediated domino [4 + 1]-annulation of α-cyanothioacetamides using diazoesters: A new entry for the synthesis of multisubstituted thiophenes

• Jury J. Medvedev,
• Ilya V. Efimov,
• Yuri M. Shafran,
• Vitaliy V. Suslonov,
• Vasiliy A. Bakulev and
• Valerij A. Nikolaev

Beilstein J. Org. Chem. 2017, 13, 2569–2576, doi:10.3762/bjoc.13.253

• domino reactions of diazo compounds with intermediate formation of ylides [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. Thus, it was for example shown that ammonium or oxonium ylides generated in the course of intermolecular processes can be easily trapped by ketones, imines, α,β-unsaturated

• examples of such reactions are for instance syntheses of multisubstituted indolines [23][24][25], pyrrolidines [26][27][28][29], dihydropyrroles [29], tetrahydrofurans [27][28], and 2,5-dihydrofurans [31][32], which proceed as intramolecular interaction of generated ammonium or oxonium ylides with carbonyl

Photocatalyzed synthesis of isochromanones and isobenzofuranones under batch and flow conditions

• Manuel Anselmo,
• Lisa Moni,
• Hossny Ismail,
• Davide Comoretto,
• Renata Riva and
• Andrea Basso

Beilstein J. Org. Chem. 2017, 13, 1456–1462, doi:10.3762/bjoc.13.143

• oxonium ion 7 evolves to isochromanone 4c, while the 4-methoxybenzyl carbocation is sufficiently long lived to react with acetonitrile and afford acetamide 5 upon reaction with traces of water. Interestingly enough, when the reaction was performed in acetone, compound 4c was isolated in lower yield (60

Unusual reactions of diazocarbonyl compounds with α,β-unsaturated δ-amino esters: Rh(II)-catalyzed Wolff rearrangement and oxidative cleavage of N–H-insertion products

• Valerij A. Nikolaev,
• Jury J. Medvedev,
• Olesia S. Galkina,
• Ksenia V. Azarova and
• Christoph Schneider

Beilstein J. Org. Chem. 2016, 12, 1904–1910, doi:10.3762/bjoc.12.180

• intermediates generated from diazo compounds (ammonium, oxonium, C=X-ylides and others) to react with a variety of electrophiles/nucleophiles yielding complex and challenging organic molecules from relatively straightforward initial compounds [9][10]. The research group by Hu and co-workers elaborated recently

Three-component synthesis of highly functionalized aziridines containing a peptide side chain and their one-step transformation into β-functionalized α-ketoamides

• Lena Huck,
• Juan F. González,
• Elena de la Cuesta and
• J. Carlos Menéndez

Beilstein J. Org. Chem. 2016, 12, 1772–1777, doi:10.3762/bjoc.12.166

• ether function existent in aziridine derivatives 2 because of the presence of the 2-methoxy substituent would furnish oxonium species 7 via a Neber reaction. Its O-demethylation by the trifluoroacetate anion would yield compound 8, whose amino group would finally be trifluoroacetylated by the methyl

• trifluoroacetate liberated in the previous step, to give 6. Alternatively, loss of a molecule of methanol from starting compound 2 would lead to intermediate 5 (see Scheme 3), whose reaction with trifluoroacetic acid would furnish 9. Neber-type chemistry would afford the oxonium species 10, which would finally be

The hydrolysis of geminal ethers: a kinetic appraisal of orthoesters and ketals

• Sonia L. Repetto,
• James F. Costello,
• Craig P. Butts,
• Joseph K. W. Lam and
• Norman M. Ratcliffe

Beilstein J. Org. Chem. 2016, 12, 1467–1475, doi:10.3762/bjoc.12.143

• geometry elements to optimise. In all cases, the five-membered ring moved towards the final planar oxonium ion, but no enthalpic barrier was found for the C(2)–OMe bond cleavage. This supports entropic control of this elimination reaction, and it is therefore not surprising that the more planar ring for

• to elimination of the OMe group and formation of the intermediate oxonium ion. The dual performance of cyclic geminal ethers as FDII for jet fuels will be reported shortly. Experimental All preparative operations were performed at the synthetic laboratories of the School of Chemistry, University of

The chemical behavior of terminally tert-butylated polyolefins

• Dagmar Klein,
• Henning Hopf,
• Peter G. Jones,
• Ina Dix and
• Ralf Hänel

Beilstein J. Org. Chem. 2015, 11, 1246–1258, doi:10.3762/bjoc.11.139

• group is readily seen in the IR spectrum (νmax = 1702 cm−1) and the carbonyl carbon signal in the 13C NMR spectrum at δ = 217 ppm is also of particular diagnostic value. We propose that 31 is produced from epoxide 29 by initial protonation to the oxonium ion 30, which then undergoes a Wagner–Meerwein

Novel carbocationic rearrangements of 1-styrylpropargyl alcohols

• Christine Basmadjian,
• Fan Zhang and
• Laurent Désaubry

Beilstein J. Org. Chem. 2015, 11, 1017–1022, doi:10.3762/bjoc.11.114

• intermediate 12 that undergoes an oxo-cyclization to afford an oxonium en route to furan 13 (pathway B). Alternatively, intermediate 12 may result from the allylic [1,3]-transposition of a perrhenate ester [10]. Replacement of the highly electron-donating 2,4,6-trimethoxyphenyl group by a 4-chlorophenyl

C-5’-Triazolyl-2’-oxa-3’-aza-4’a-carbanucleosides: Synthesis and biological evaluation

• Roberto Romeo,
• Caterina Carnovale,
• Salvatore V. Giofrè,
• Maria A. Chiacchio,
• Adriana Garozzo,
• Emanuele Amata,
• Giovanni Romeo and
• Ugo Chiacchio

Beilstein J. Org. Chem. 2015, 11, 328–334, doi:10.3762/bjoc.11.38

• presence of NH4Cl, promotes an equilibrium process which starts from 11 and leads to a mixture of α- and β-anomers, via the intermediate oxonium ion 15 (path a) or 16 (path b) (Figure 3). As reported in similar systems [45], in the equilibrium mixture the β-anomer 11, thermodynamically more stable

3α,5α-Cyclocholestan-6β-yl ethers as donors of the cholesterol moiety for the electrochemical synthesis of cholesterol glycoconjugates

• Aneta M. Tomkiel,
• Adam Biedrzycki,
• Jolanta Płoszyńska,
• Dorota Naróg,
• Andrzej Sobkowiak and
• Jacek W. Morzycki

Beilstein J. Org. Chem. 2015, 11, 162–168, doi:10.3762/bjoc.11.16

• in Scheme 3. The proposed formation of disteroidal oxonium ions accounts for an alkyl (aryl) group transfer from C-6 to C-3. Interestingly, the isomerization itself is not an electrochemical reaction. Electrooxidation is needed only to initiate the whole process, i.e., to generate the homoallylic