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