Synthesis and antiviral activities of spacer-linked 1-thioglucuronide analogues of glycyrrhizin

• Christian Stanetty,
• Andrea Wolkerstorfer,
• Hassan Amer,
• Andreas Hofinger,
• Ulrich Jordis,
• Dirk Claßen-Houben and
• Paul Kosma

Beilstein J. Org. Chem. 2012, 8, 705–711, doi:10.3762/bjoc.8.79

• diphenylmethyl ester group with TFA/anisole (as carbocation scavenger), giving the monoacid derivatives 10 and 11 in 75% and 89% yield, respectively. Subsequent transesterification of 10 and 11 with methanolic NaOMe afforded the deacetylated methyl ester glucuronide derivatives 12 and 14, respectively, which

Efficient oxidation of oleanolic acid derivatives using magnesium bis(monoperoxyphthalate) hexahydrate (MMPP): A convenient 2-step procedure towards 12-oxo-28-carboxylic acid derivatives

• Jorge A. R. Salvador,
• Vânia M. Moreira,
• Rui M. A. Pinto,
• Ana S. Leal and
• José A. Paixão

Beilstein J. Org. Chem. 2012, 8, 164–169, doi:10.3762/bjoc.8.17

• group, creating a tertiary carbocation at C-13. Then, a concerted stereoselective 1,2-migration of the 12β-H to the 13β-position with the rearrangement of the 12α-hydroxy group affords the final 12-oxo-28-carboxylic acid structure [18]. The molecular structure of compound 11, determined by single

Structural conditions required for the bridge lithiation and substitution of a basic calix[4]arene

• Conrad Fischer,
• Wilhelm Seichter and
• Edwin Weber

Beilstein J. Org. Chem. 2011, 7, 1602–1608, doi:10.3762/bjoc.7.188

• decade two main preparative routes for the methylene-bridge substitution of p-tert-butyltetramethoxycalix[4]arene have been established: A protocol described by Biali et al. yields a stabilized methylene carbocation through bromination that is ready for electrophilic substitution under SN1 conditions [3

Koch–Haaf reaction of adamantanols in an acid-tolerant hastelloy-made microreactor

• Takahide Fukuyama,
• Yu Mukai and
• Ilhyong Ryu

Beilstein J. Org. Chem. 2011, 7, 1288–1293, doi:10.3762/bjoc.7.149

• organic molecules, and we also reported that Pd-catalyzed carbonylation [13] and radical carbonylation [16] could be successfully carried out in a continuous microflow system with higher efficiency than in a batch autoclave system. In this study, we focused on the carbonylation of carbocation

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

• [52] and entry 15 in Table 1). Regeneration of the active cationic gold species or assistance in the formation of the reactive allylic carbocation intermediate II are key steps in which the Brønsted cocatalysis could be exerted [52]. Finally, the mandatory role of enol tautomer (or gold–enolate

A practical microreactor for electrochemistry in flow

• Kevin Watts,
• William Gattrell and
• Thomas Wirth

Beilstein J. Org. Chem. 2011, 7, 1108–1114, doi:10.3762/bjoc.7.127

• trapped immediately after generation. In the "cation flow" method, a carbocation is generated continuously in a flow system by low temperature electrolysis. The generation of the cation can be monitored by a FTIR spectrometer in the flow system. The electrochemically generated N-acyliminium ions can then

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

• give a stabilized carbocation XXVI, usually a benzylic cation (Scheme 24). A subsequent ring closure through the carbon adjacent to the gold atom provides the final cycloadduct 46, featuring a four membered ring carbocycle. Importantly, these reactions could be rendered enantioselective with catalysts

• ]. The authors propose stepwise mechanisms proceeding through a common cationic intermediate XXVI, which can evolve into the cyclopentyl cycloadducts via the species XXXI. Alternatively, when the benzylic carbocation in XXVI is intercepted by the carbon atom bearing the gold atom, the (2 + 2) adduct 46

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

• alcohols has been developed by utilizing NaAuCl4. The benzylic and secondary alcohols (55 and 58) worked well under mild conditions with low catalyst loading (Scheme 11). The chiral benzyl alcohol 60 gave racemic ether 61, which suggested the intermediacy of a carbocation. Ye et al. reported an expedient

Gold-catalyzed propargylic substitutions: Scope and synthetic developments

• Olivier Debleds,
• Eric Gayon,
• Emmanuel Vrancken and
• Jean-Marc Campagne

Beilstein J. Org. Chem. 2011, 7, 866–877, doi:10.3762/bjoc.7.99

• , alcohols, thiols, electron rich aromatic compounds), and showed that gold probably acts as a Lewis acid to promote the formation of a stabilized propargylic carbocation intermediate [23][24]. A related reaction was subsequently reported by Dyker [25] in 2006, using azulene and 1,3-dimethoxybenzene in

• allenyl(cyano)cuprate on butyraldehyde [71][72][73], was reacted with allyltrimethylsilane. The reaction was totally regioselective for the propargylic site. Different reaction paths can be expected: i) The formation of a propargylic carbocation that can be attacked by allyltrimethylsilane or vicinal

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

• proceed over indene formation, since it is required to stabilize the intermediate carbocation. As before, the only difference in the product structure is whether H or I has been incorporated. Sanz and co-workers reported in 2010 that indenes were formed from o-alkynylstyrenes in the presence of AuCl(PPh3

• ]. This reaction proceeds via an exceptional 5-endo halocyclization, which likely results from the stability of the tertiary carbocation, since only H at the C1-position rendered the enyne unreactive under the conditions used (excess NIS, CH2Cl2 at reflux). From a synthetic point of view, the development

• , the reaction yield was considerably reduced. Again in this case, product formation is best understood by assuming the intermediate formation of the most stabilized carbocation. Surprisingly, the analogous gold-catalyzed process has not been described up to now; instead, 3-silyloxy-1,5-enynes with a

Isotopic labelling studies for a gold-catalysed skeletal rearrangement of alkynyl aziridines

• Paul W. Davies,
• Nicolas Martin and
• Neil Spencer

Beilstein J. Org. Chem. 2011, 7, 839–846, doi:10.3762/bjoc.7.96

• , either by solvent stabilisation or the formation of a closer contact ion pair, and/or by more basic solvents/counter ions assisting proton elimination [18][26][27]. However, for the proposed skeletal rearrangement mechanism to be valid, gold bound carbocation B must be prone to undergo a relatively rare

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

• and trapping of the carbocation by the sulfonamide. Subsequent intramolecular hydroamination gave the pyrrolidine products. 1.3 Oxiranes As an oxophilic Lewis acid, gold can activate epoxides towards the attack of nucleophiles. A good example is the AuCl3 catalyzed ring opening of aryl alkyl epoxide

• cyclopropyl ring opening to give carbocation 45, which is then trapped in the presence of an external nucleophile to give, after protonolysis, furans 43 (Scheme 14, path a). Alternatively, gold can complex both of the unsaturated moieties as in 46, triggering the cyclopropyl ring opening through an

• . Cyclopropanes 85a are generated in situ by intermolecular cyclopropanation of enyne 84 and a carbene resulting from the rearrangement of propargyl ester 83. When tertiary propargyl esters are used, the 5-endo-dig cyclization generates the carbocation 89. Migration of the pivaloyloxy group affords the allylic

Alkene selenenylation: A comprehensive analysis of relative reactivities, stereochemistry and asymmetric induction, and their comparisons with sulfenylation

• Vadim A. Soloshonok and
• Donna J. Nelson

Beilstein J. Org. Chem. 2011, 7, 744–758, doi:10.3762/bjoc.7.85

• anti stereospecificity [58][59]. The nucleophile attacks the more substituted carbon in the seleniranium ion (Scheme 1), unless that carbon bears bulky groups, such as tert-butyl or cyclohexyl [55][56] or unless the open carbocation is stabilized, e.g., by an aryl group or a heteroatom [60][61][62][63

When cyclopropenes meet gold catalysts

• Frédéric Miege,
• Christophe Meyer and
• Janine Cossy

Beilstein J. Org. Chem. 2011, 7, 717–734, doi:10.3762/bjoc.7.82

• undergo subsequent ring-opening to produce an organogold species that can be viewed as a hybrid between a gold-stabilized allylic carbocation B and a gold carbene C. The organogold carbenoid species generated by the ring-opening of cyclopropenes can participate in a variety of reaction types such as

• indicate that a secondary gold-substituted carbocation (at C1) is as stable as a tertiary dimethyl-substituted carbocation (at C3) and that the magnitude of stabilization from the gold moiety increases with increasing electrophilicity of the allylic cation. Toste et al. investigated the effect of the

• ligand on the structure of gold-substituted 3,3-dimethyl allyl cations of type D. Increasing trans σ-donation from the ligand and strongly π-acidic ligands such as phosphites (decreasing back π-donation from gold to C1) led to a longer C1–Au bond and hence a more carbocation-like character for the

Asymmetric synthesis of tertiary thiols and thioethers

• Jonathan Clayden and
• Paul MacLellan

Beilstein J. Org. Chem. 2011, 7, 582–595, doi:10.3762/bjoc.7.68

• -withdrawing nature of the substituent inhibits SN1 dissociation and carbocation formation, and the planar ester group poses minimal steric hindrance towards approach of the nucleophile. Yields of substitution in α-aryl-α-hydroxy esters 5 are poor with a significantly decreased enantiomeric ratio in the

Kinetic evaluation of the solvolysis of isobutyl chloro- and chlorothioformate esters

• Malcolm J. D’Souza,
• Matthew J. McAneny,
• Dennis N. Kevill,
• Jin Burm Kyong and
• Song Hee Choi

Beilstein J. Org. Chem. 2011, 7, 543–552, doi:10.3762/bjoc.7.62

• –solvolysis of 4, points to strong rear-side nucleophilic solvation of the developing resonance-stabilized carbocation. Another useful tool for mechanistic studies is the l/m ratio. We have found [17] that values >2.7 are typical of solvolytic mechanisms proceeding by an addition–elimination pathway with the

• resonance-stabilized carbocation intermediate is more efficient for isopropyl chlorothioformate (5) when compared to 2, as the presence of the additional carbon pushes the isopropyl group out of the plane of the thioether atom in 2' (Figure 1). This opinion is supported by an increase seen in the l/m ratio

• 1 solvolyzes by a bimolecular addition–elimination (AN + DN) process due to the inductive ability of the isobutoxy group, whereas in the four aqueous TFE mixtures a predominant unimolecular SN1 mechanism with rear-side solvation of the developing carbocation is suggested. For 2, due to a more

Molecular rearrangements of superelectrophiles

• Douglas A. Klumpp

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

• observed. It is proposed that the adjacent ammonium group decreases the basicity of the C1–C2 bond in 24, leading to protonation at the more distant C2–C3 bond. The final product is formed by reaction of benzene at the carbocation site. A number of superelectrophilic ring opening reactions are followed by

• dehydrogenation. For example, 2-decalone (51) (primarily trans) was reacted with HF-SbF5-CCl4 at 0 °C to give products 52 and 53 in 25% and 12% yields, respectively (Scheme 12). The proposed mechanism involves protonation and hydride abstraction to give superelectrophile 54. It is notable that the carbocation

• forms in the 6- or 7-position on the decalone ring, as this provides maximum separation of the cationic centers. In accord with other carbocation rearrangements, the superelectrophile 54 (a 2° carbocation) isomerizes to 56 (a 3° carbocation). This likely occurs through the ring-fused, protonated

Anion–π interactions influence pK_a values

• Christopher J. Cadman and
• Anna K. Croft

Beilstein J. Org. Chem. 2011, 7, 320–328, doi:10.3762/bjoc.7.42

• indicated that aromatics in close proximity to an incipient carbocation could accelerate tosylation reactions by up to 1800-fold [21]. Similarly, neighbouring aromatics have been shown to have an effect on radical reactions proceeding through a polarised transition state [22]. Clearly there is excellent

β-Hydroxy carbocation intermediates in solvolyses of di- and tetra-hydronaphthalene substrates

• Jaya S. Kudavalli and
• Rory A. More O'Ferrall

Beilstein J. Org. Chem. 2010, 6, 1035–1042, doi:10.3762/bjoc.6.118

• formation of a β-hydroxynaphthalenium carbocation with a conformation in which a C–OH occupies an axial position β to the carbocation centre preventing stabilisation of the carbocation by C–H hyperconjugation, which would occur in the conformation initially formed from the cis isomer. It is suggested that C

• product (naphthol) is expected to be too fast [6][7]. This conclusion is confirmed by the lack of saturation of the effect and the normal salt effects observed for sodium acetate and sodium azide, which would be expected to trap a carbocation intermediate more effectively than the trichloroacetate anion

• . The normal salt effect exerted by the sodium azide also confirms that the solvolysis proceeds via the formation of a carbocation intermediate rather than by an SN2 mechanism. 1-Chloro- and 1-chloro-2-hydroxy-1,2-tetrahydronaphthalenes Rate constants for solvolysis of cis- and trans-1-chloro-2-hydroxy

Redox-active tetrathiafulvalene and dithiolene compounds derived from allylic 1,4-diol rearrangement products of disubstituted 1,3-dithiole derivatives

• Filipe Vilela,
• Peter J. Skabara,
• Christopher R. Mason,
• Thomas D. J. Westgate,
• Asun Luquin,
• Simon J. Coles and
• Michael B. Hursthouse

Beilstein J. Org. Chem. 2010, 6, 1002–1014, doi:10.3762/bjoc.6.113

• group, the nature of the solvent and the acid used (e.g., 3). The extent to which the aryl groups and their substituents are able to stabilise the carbocation, formed by the loss of a protonated hydroxy group, is key in determining the reaction product. The diverse range of π-rich structures that can be

• affords a carbocation that can be stabilised by resonance of π-electrons from either the 1,3-dithiole-2-thione or thienyl groups. This intermediate can then undergo intramolecular nucleophilic ring-closure by the remaining hydroxy group with the formation of the dihydrofuan 4 shown in Scheme 1. Dominating

• this process is the competing electrophilic attack of the allylic carbocation at the 3-position of the thiophene ring bearing the carbinol group, forming a new C–C bond and producing a 6-membered ring. Concomitantly, the cationic centre migrates to produce a C=S+ form, where it is stabilised by π

Acid catalyzed cyclodimerization of 2,2-bis(trifluoromethyl)-4-alkoxy-oxetanes and -thietanes. Synthesis of 2,2,6,6-tetrakis(trifluoromethyl)-4,8-dialkoxy-1,5-dioxocanes and 3,3,7,7-tetrakis(trifluoromethyl)-9-oxa-2,6-dithia-bicyclo[3.3.1]nonane

• Viacheslav A. Petrov and
• Will Marshall

Beilstein J. Org. Chem. 2010, 6, No. 46, doi:10.3762/bjoc.6.46

• possible mechanism describing the formation of compound 5 is depicted by Scheme 8. Protonation of the thietane sulfur of 4, followed by ring opening, results in the formation of an oxygen stabilized carbocation D. Electrophilic attack of D at the sulfur of the second thietane molecule and loss of H+ would

A review of new developments in the Friedel–Crafts alkylation – From green chemistry to asymmetric catalysis

• Magnus Rueping and
• Boris J. Nachtsheim

Beilstein J. Org. Chem. 2010, 6, No. 6, doi:10.3762/bjoc.6.6

• . In addition both, syn- and anti diastereoisomer of the starting material gave the syn-configured products as the major diastereoisomer. This strongly indicates a carbocation as reaction intermediate and rules out an SN1-type reaction mechanism. Additionally, low temperature NMR studies in superacidic

Solvent-free phase-vanishing reactions with PTFE (Teflon^®) as a phase screen

• Kevin Pels and
• Veljko Dragojlovic

Beilstein J. Org. Chem. 2009, 5, No. 75, doi:10.3762/bjoc.5.75

• stereoselective reaction, that usually is not the case with cis-stilbene and the reaction product is a mixture of D,L- and meso-dibromostilbenes [23][24][25]. Besides the common mechanism involving a bromonium ion as an intermediate, bromination of cis-stilbene may involve the corresponding carbocation [24][25

Diastereoselective functionalisation of benzo-annulated bicyclic sultams: Application for the synthesis of cis-2,4-diarylpyrrolidines

• Susan Kelleher,
• Pierre-Yves Quesne and
• Paul Evans

Beilstein J. Org. Chem. 2009, 5, No. 69, doi:10.3762/bjoc.5.69

• designed to understand the cis-stereochemical outcome of these reactions, are described. In the case of 5b, a novel solvent dependent carbocation rearrangement occurs with the formation of 18b. cis-Dibromides 13a and 13b undergo regioselective dehydrobromination, and the participation of the resultant

• hoped for alcohol 8. A similar outcome was observed under both Sakurai-type conditions and treatment with potassium cyanide. It seems probable that these failures reflect a combination of the inability of the N-lone pair to stabilise the developing α-carbocation (the resultant N-sulfonyl iminium ion

• demonstrates the pronounced fold of this structure. Consequently, since the SN2 process is retarded the bromide may then intercept either carbocation 21, or 22 in an SN1 sense, from the less hindered, top face, forming 13a and 13b. Using methanol as the solvent the cis-1,2-difunctionalised compounds 16a and

End game strategies towards the total synthesis of vibsanin E, 3-hydroxyvibsanin E, furanovibsanin A, and 3-O-methylfuranovibsanin A

• Brett D. Schwartz,
• Craig M. Williams and
• Paul V. Bernhardt

Beilstein J. Org. Chem. 2008, 4, No. 34, doi:10.3762/bjoc.4.34

• obtained as the minor component. Furan 41 is an interesting molecule in that it contains a bridgehead double bond, presumably formed due to the ease of carbocation formation at the benzylic (tertiary) centre. Unfortunately, the bridgehead double bond contained within 41 could not be hydrated. Conversion of