A general metal-free approach for the stereoselective synthesis of C-glycals from unactivated alkynes

• Shekaraiah Devari,
• Manjeet Kumar,
• Ramesh Deshidi,
• Masood Rizvi and
• Bhahwal Ali Shah

Beilstein J. Org. Chem. 2014, 10, 2649–2653, doi:10.3762/bjoc.10.277

• forming an oxonium ion intermediate, which is attacked by trimethylsilylacetylene to give the corresponding product (Scheme 3, reaction 2). The stereochemistry of the reaction products is possibly determined by the coordination between two π-electron orbitals of the oxocarbonium ion and the acetylene

Silver and gold-catalyzed multicomponent reactions

• Giorgio Abbiati and
• Elisabetta Rossi

Beilstein J. Org. Chem. 2014, 10, 481–513, doi:10.3762/bjoc.10.46

• diastereoisomeric 1,2-dihydroisoquinolines 40 (Scheme 22) [60]. Iminium intermediate 28, generated in situ from the aldimine 27 under silver triflate catalysis is the usual electrophilic intermediate, whereas the nucleophile, in this case, is the oxonium ylide 39. The reaction resulted in the synthesis of highly

• substituted 1,2-dihydroisoquinolines 40 characterized by the presence of an α-hydroxy/alkoxy-α-carboxylate carbon pendant. The oxonium ylide 39 was prepared by a well-known procedure involving a rhodium carbenoid intermediate, generated in situ from the corresponding diazoacetate 37 under Rh2(OAc)4 catalysis

Gold(I)-catalyzed enantioselective cycloaddition reactions

• Fernando López and
• José L. Mascareñas

Beilstein J. Org. Chem. 2013, 9, 2250–2264, doi:10.3762/bjoc.9.264

• intermediates of type X by an intramolecular carbonyl group, followed by the ring closing of the resulting oxonium intermediate XI. The reaction occurs at low temperatures by using only 1 mol % of a phosphite-based gold catalyst. The enantioselective version, which is catalyzed by the DTBM-Segphos complex Au1

The chemistry of isoindole natural products

• Klaus Speck and
• Thomas Magauer

Beilstein J. Org. Chem. 2013, 9, 2048–2078, doi:10.3762/bjoc.9.243

• using an intramolecular Diels–Alder reaction. The synthetic endeavor began with the conversion of dihydropyran 79 to the acetal 80 (Scheme 9). Activation of the anomeric position with tin(IV) chloride presumably led to the formation of the oxonium ion 81. This intermediate was envisioned to undergo a

Gold-catalyzed intermolecular coupling of sulfonylacetylene with allyl ethers: [3,3]- and [1,3]-rearrangements

• Jungho Jun,
• Hyu-Suk Yeom,
• Jun-Hyun An and
• Seunghoon Shin

Beilstein J. Org. Chem. 2013, 9, 1724–1729, doi:10.3762/bjoc.9.198

• groups underwent smooth reactions (Table 2, entries 2 and 3). It is reasonable to assume that a bulky group R1 would decelerate the initial O-attack on the alkyne. However, once the Au-bound oxonium ion (A in Scheme 1) is formed, the resulting rearrangement seems to be facilitated by the presence of a

Anionic cascade reactions. One-pot assembly of (Z)-chloro-exo-methylenetetrahydrofurans from β-hydroxyketones

• István E. Markó and
• Florian T. Schevenels

Beilstein J. Org. Chem. 2013, 9, 1319–1325, doi:10.3762/bjoc.9.148

• -exo-methylenetetrahydrofuran 42 is converted into the oxonium cation 43. Subsequent capture of this intermediate 43 by a second molecule of 42 can occur via two different pathways. The first one involves the addition of the enol ether function of 42 onto 43, leading to the creation of a new C–C bond

• with concomitant generation of another oxonium species 47. Intramolecular capture of this electrophile by the pendant hydroxy substituent then delivers the spirocyclic adduct 48. Alternatively, reaction of the tertiary alcohol of 42 with the oxonium cation 43 affords the ketal 44. Protonation of the

• may indeed be operational in these cases. Protonation of diene 56 leads to the oxonium ion 57, which undergoes a 1,4-addition of another 56 unit, affording the new cation 58. At this stage, two different pathways can be followed. Either carbocation 58 can lose a proton, generating the observed triene

Synthesis of a novel chemotype via sequential metal-catalyzed cycloisomerizations

• Bo Leng,
• Stephanie Chichetti,
• Shun Su,
• Aaron B. Beeler and
• John A. Porco Jr.

Beilstein J. Org. Chem. 2012, 8, 1338–1343, doi:10.3762/bjoc.8.153

• malonate substituent (Nu, 24a) to minimize steric interactions relative to 24b, leading to the observed diastereoselectivity (Scheme 3, inset) [23][24]. Subsequent cyclopropane formation through addition of the vinyl metal to the oxonium intermediate affords metallocarbenoid 26, which may then undergo a

A concise synthesis of 3-(1-alkenyl)isoindolin-1-ones and 5-(1-alkenyl)pyrrol-2-ones by the intermolecular coupling reactions of N-acyliminium ions with unactivated olefins

• Nianhong Lu,
• Lihong Wang,
• Zhanshan Li and
• Wei Zhang

Beilstein J. Org. Chem. 2012, 8, 192–200, doi:10.3762/bjoc.8.21

• products 4 were isolated (Table 2, entries 2 and 8 and Table 3, entries 2 and 9) much like the ene-type adducts of oxonium ion with olefins [37][38]. This means that these alkenes may be envisioned as surrogates of their corresponding, more expensive and less atom-economical, allylsilane derivatives, which

Dependency of the regio- and stereoselectivity of intramolecular, ring-closing glycosylations upon the ring size

• Patrick Claude,
• Christian Lehmann and
• Thomas Ziegler

Beilstein J. Org. Chem. 2011, 7, 1609–1619, doi:10.3762/bjoc.7.189

• , entropy-reducing intramolecular cyclization step. Within the frame of an overall SN1 reaction, the cyclization step is regarded as the rate-determining step; therefore, the transition state for the nucleophilic addition to the glycosyl cyclic oxonium intermediate may allow for a rationalization of the

• compared to its next highest diastereomer 6d, β(1→3), as documented in Figure 6. Nevertheless, a closer approximation to the real transition state involving the pyranose cyclic oxonium intermediate is possible within our calibrated force field approach: Figure 7 highlights the stereoelectronic course of

• ? Figure 11 illustrates what we intuitively anticipated at the beginning of the discussion, namely that the aromatic stacking interactions have a remarkable influence on the course of an intramolecular cyclization reaction: If the precyclized (seco) cyclic oxonium ion is modeled as an approximation for its

Amines as key building blocks in Pd-assisted multicomponent processes

• Didier Bouyssi,
• Nuno Monteiro and
• Geneviève Balme

Beilstein J. Org. Chem. 2011, 7, 1387–1406, doi:10.3762/bjoc.7.163

• attack of the hydroxyl group onto the triple bond activated by a Pd(II) cationic complex, followed by a protodemetalation, which afforded the methylidenefuran 21. This reacts with the imine activated by the Pd(II) species through a Mannich-type process. Finally, addition of the phenol to the oxonium can

Gold(I)-catalyzed synthesis of γ-vinylbutyrolactones by intramolecular oxaallylic alkylation with alcohols

• Michel Chiarucci,
• Mirko Locritani,
• Gianpiero Cera and
• Marco Bandini

Beilstein J. Org. Chem. 2011, 7, 1198–1204, doi:10.3762/bjoc.7.139

• ]. Subsequently, the direct nucleophilic attack by the carboxylate unit would lead to an oxonium intermediate III [50][51] that, after dealkylation, resulted in the final lactone 2. Control experiments have been performed to indentify the presence of a Brønsted acid cocatalysis in the ring-closing procedure (see

Recent developments in gold-catalyzed cycloaddition reactions

• Fernando López and
• José L. Mascareñas

Beilstein J. Org. Chem. 2011, 7, 1075–1094, doi:10.3762/bjoc.7.124

• on an initial (3 + 2) dipolar cycloaddition between the carbonyl ylide IV and the alkene, followed by a ring expansion (1,2-alkyl migration) that is assisted by the oxy group and generates the oxonium intermediate V (Scheme 2). A final elimination process regenerates the catalyst and affords the

• , followed by addition of the generated alkenyl metallic species to the α,β-unsaturated silyl oxonium moiety, to give a bicyclic carbene intermediate XVI [59]. These species, which do not incorporate an α-hydrogen atom that could participate in a 1,2-hydrogen shift process, evolve through a 1,2-alkyl

• , attacking the gold–cyclopropyl carbene intermediate XIX to generate an oxonium cation species of type XX. Finally, a Prins-like cyclization renders the oxatricyclic product 28 and regenerates the gold(I) catalyst. Importantly, this strategy was successfully applied as a key step in the synthesis of

Functionalization of anthracene: A selective route to brominated 1,4-anthraquinones

• Kiymet Berkil Akar,
• Osman Cakmak,
• Orhan Büyükgüngör and
• Ertan Sahin

Beilstein J. Org. Chem. 2011, 7, 1036–1045, doi:10.3762/bjoc.7.118

• in C2 and C3 induces the formation of a bridged-oxonium ion 20 as an intermediate of 17. Hydrolysis of hexabromide 6 can lead to three halohydrines (17, 21 and 22), two of which are symmetric (Scheme 5). Dihalohydrine 17 is expected to be a good precursor to corresponding arene oxides 23 as shown in

Recent advances in the gold-catalyzed additions to C–C multiple bonds

• He Huang,
• Yu Zhou and
• Hong Liu

Beilstein J. Org. Chem. 2011, 7, 897–936, doi:10.3762/bjoc.7.103

• atoms to form the intermediates 26 or 27, which then rearrange to yield the oxonium intermediates 28 or 29, respectively. Gold(I)-catalyzed intramolecular cyclization of monopropargylic triols 32 has been reported to be a novel and mild approach [29] for producing olefin-containing spiroketals 33 (and

When gold can do what iodine cannot do: A critical comparison

• Sara Hummel and
• Stefan F. Kirsch

Beilstein J. Org. Chem. 2011, 7, 847–859, doi:10.3762/bjoc.7.97

• majority of gold-catalyzed cyclization modes have involved initial carbon-heteroatom bond formation [7][10]. As a general rule, the nucleophilic attack of a carbonyl (or imine) group onto an alkyne activated by gold-complexation generates first an oxonium (iminium) ion species, the cationic character of

• cyclic oxonium ion is produced in the first stage of the cascade via both gold- and iodonium-triggered cyclization. In both cases, the heterocyclization is followed by a 1,2-migration onto the oxonium ion, where the only difference is whether the final product bears a H or an I atom at the C4-position

• such as 58 were treated with AuCl, the formation of cyclohexadienes was strongly favored, presumably due to the highly stabilized oxonium ion intermediates (path b). Essentially, these transformations are not viable with an iodonium source as they do not possess the ability to proceed through simple

Synthetic applications of gold-catalyzed ring expansions

• David Garayalde and
• Cristina Nevado

Beilstein J. Org. Chem. 2011, 7, 767–780, doi:10.3762/bjoc.7.87

• (Scheme 22) [54]. In most cases trans products were favored. The reaction is proposed to proceed via the cyclopropyl carbene 73, which undergoes ring expansion to form the alkenyl–gold intermediate 74. Reaction of the latter with the oxonium cation produces 75, which upon gold departure forms tricycles 71

Construction of cyclic enones via gold-catalyzed oxygen transfer reactions

• Leping Liu,
• Bo Xu and
• Gerald B. Hammond

Beilstein J. Org. Chem. 2011, 7, 606–614, doi:10.3762/bjoc.7.71

• ; gold-catalyzed; oxonium; oxygen transfer; Review α,β-Unsaturated carbonyl derivatives are not only important building blocks in synthetic organic chemistry, but are also a significant motif in natural products and biologically active compounds [1][2][3][4][5][6][7][8]. The construction of the

• produced β,γ-unsaturated bicyclic enones rather than their α,β-unsaturated counterparts. Yamamoto and co-workers proposed a [2 + 2] mechanism for their gold-catalyzed cyclization of alkynyl ketones (Scheme 3). In their mechanism, the carbonyl group attacks the gold activated triple bond to form an oxonium

• -membered ring oxonium intermediate C than for the seven-membered ring oxonium A. This energetic preference is also observed in the stabilities of the oxoniums themselves, with C considerably more stable by 16.1 kcal/mol. The subsequent transformations are all computed to be feasible, with the barrier to [4

Molecular rearrangements of superelectrophiles

• Douglas A. Klumpp

Beilstein J. Org. Chem. 2011, 7, 346–363, doi:10.3762/bjoc.7.45

• present in the substrate, dicationic species can be produced. For example, the pentan-1-ol derivative 74 reacts with ozone in magic acid to yield dication 75 quantitatively (Scheme 15). This conversion involves protonation of the hydroxy group to give the oxonium ion 76 and reaction of O3H+ at the methine

• thought to be involved. In superacidic media, substrates such as ethylene glycol (159) are diprotonated and form the bis-oxonium ions, i.e., 160 as a stable species at −80 °C. When the solution is warmed to 25 °C, protonated acetaldehyde (162) is formed (Scheme 32). The conversion may occur by one of

Shelf-stable electrophilic trifluoromethylating reagents: A brief historical perspective

• Norio Shibata,
• Andrej Matsnev and
• Dominique Cahard

Beilstein J. Org. Chem. 2010, 6, No. 65, doi:10.3762/bjoc.6.65

• ) [22]. O-(Trifluoromethyl)oxonium salts All the previously described reagents allow trifluoromethylation only of soft nucleophiles and there is no possibility of preparing N-CF3 or O-CF3 compounds by direct trifluoromethylation via these compounds. O-Trifluoromethyl oxonium salts were anticipated to

Prins fluorination cyclisations: Preparation of 4-fluoro-pyran and -piperidine heterocycles

• Guillaume G. Launay,
• Alexandra M. Z. Slawin and
• David O'Hagan

Beilstein J. Org. Chem. 2010, 6, No. 41, doi:10.3762/bjoc.6.41

• hydrogen fluoride salts (Et4NF·5HF) as the reaction medium, without the requirement for BF3·OEt2 [14][15]. These reactions with fluoride follow from the much more commonly observed Prins reactions of halides (Cl−, Br− and I−) other than fluoride in the quenching of the intermediate oxonium intermediate [16

A stable enol from a 6-substituted benzanthrone and its unexpected behaviour under acidic conditions

• Marc Debeaux,
• Kai Brandhorst,
• Peter G. Jones,
• Henning Hopf,
• Jörg Grunenberg,
• Wolfgang Kowalsky and
• Hans-Hermann Johannes

Beilstein J. Org. Chem. 2009, 5, No. 31, doi:10.3762/bjoc.5.31

• process as discussed above via the enolate 8 as an intermediate. From this, the enol 4 is generated under the influence of the added acid. Further protonation provides the oxonium ion 9 which is set up for a retro-[2+4]-cycloaddition (see transition state 10) to lose hydrogen and finally become

The use of silicon- based tethers for the Pauson- Khand reaction

• Adrian P. Dobbs,
• Ian J. Miller and
• Saša Martinović

Beilstein J. Org. Chem. 2007, 3, No. 21, doi:10.1186/1860-5397-3-21

• from the reaction of allyldimethylpent-4-ynyloxysilane with tungsten pentacarbonyl, possibly formed via oxidation of THF to give an oxonium ion followed by addition of the alcohol cleaved from the silyl ether (Figure 3). In order to investigate if the presence of the silicon linker was preventing the