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

• an intermediate radical cation occurs, thus leading to a mesomerically stabilized homoallylic carbocation and a hydroxyl radical (Scheme 1) [2]. However, the glycosylation reaction was not very efficient due to competition between the sugar alcohol and cholesterol for the carbocation [3]. If

• cyclosteroid (i-steroid) rearrangement is a well-known steroid reaction [6]. The solvolysis of cholesteryl p-tosylate proceeds via the SN1 mechanism with the formation of a mesomerically stabilized carbocation. The addition of a nucleophile (alcohol, water, etc.) may occur either to C-3 or C-6, depending on

• (the kinetic product is formed) since the mesomeric carbocation is more positively charged in this position than in C-3. However, under acidic conditions the reaction becomes reversible and the 3β-substituted product, which is more stable (the thermodynamic product), is exclusively formed. The i

Cross-dehydrogenative coupling for the intermolecular C–O bond formation

• Igor B. Krylov,
• Vera A. Vil’ and
• Alexander O. Terent’ev

Beilstein J. Org. Chem. 2015, 11, 92–146, doi:10.3762/bjoc.11.13

• that proposed in the study [141]. According to the latter mechanism, the benzylic carbocation rather than benzyl iodide is generated and this carbocation undergoes nucleophilic attack by carboxylic acid. This mechanism is confirmed by the fact that iodide is inert under the reaction conditions. The

• hydrogen atom from the benzylic position of the C-reagent to form the C-radical, which is oxidized to the carbocation; in turn, the aldehyde is oxidized to acid, which reacts with the carbocation to give the target coupling product 142 (Scheme 28). It was shown that under the reaction conditions, tert

Redox active dendronized polystyrenes equipped with peripheral triarylamines

• Toshiki Nokami,
• Naoki Musya,
• Tatsuya Morofuji,
• Keiji Takeda,
• Masahiro Takumi,
• Akihiro Shimizu and
• Jun-ichi Yoshida

Beilstein J. Org. Chem. 2014, 10, 3097–3103, doi:10.3762/bjoc.10.326

• polystyrene equipped with peripheral triarylamines, which exhibited two sets of reversible redox peaks in the cyclic voltammetry curves. Keywords: carbocation; cross-coupling; dendrimer; dendronized polymer; redox; Introduction Assembling small functional molecules using dendrimers [1] and dendronized

• dendronized polystyrenes: (a) the functionalization of dendronized polystyrene (the “graft from” approach, Figure 1a); and (b) dendronization of polystyrenes with the dendritic carbocation equipped with functional groups (the “graft to” approach, Figure 1b). Both approaches have advantages and disadvantages

• than the oxidation potential of dendrimer 4, indicating that the triarylamine moiety is oxidized before the benzylsilane moiety. Using the “graft to” approach to synthesize peripherally functionalized dendronized polystyrene (Figure 1b) employing 5 as a precursor of the dendritic carbocation was

Selenium halide-induced bridge formation in [2.2]paracyclophanes

• Laura G. Sarbu,
• Henning Hopf,
• Peter G. Jones and
• Lucian M. Birsa

Beilstein J. Org. Chem. 2014, 10, 2550–2555, doi:10.3762/bjoc.10.266

• episelenonium ion 8 (Scheme 3). The episelenonium ion 8 should equilibrate with the ring-opened form, a benzylic type carbocation; the interaction of this intermediate with the opposing ethynyl substituent provides adduct 9. For steric reasons the chlorine anion attack from "outside" leading to intermediate 10

Five-membered ring annelation in [2.2]paracyclophanes by aldol condensation

• Henning Hopf,
• Swaminathan Vijay Narayanan and
• Peter G. Jones

Beilstein J. Org. Chem. 2014, 10, 2021–2026, doi:10.3762/bjoc.10.210

• generated carbocation can proceed either by a Hofmann or a Saytseff mode, and – as the results show – both pathways are followed. Stereoelectronic reasons are presumably responsible for the slight preference of the Hofmann route. Not only does the methyl group appear to be sterically preferred for proton

Structure/affinity studies in the bicyclo-DNA series: Synthesis and properties of oligonucleotides containing bc^en-T and iso-tricyclo-T nucleosides

• Branislav Dugovic,
• Michael Wagner and
• Christian J. Leumann

Beilstein J. Org. Chem. 2014, 10, 1840–1847, doi:10.3762/bjoc.10.194

• the modified residues, most likely on the level of the iodinated phosphite intermediate [35], leading to the formation of an allylic carbocation in the bcen-T unit and 5’-phosphorylated DNA fragment (Scheme 3). The synthesis of oligonucleotides using building block 10 proved to be even more difficult

Synthesis of 1-[bis(trifluoromethyl)phosphine]-1’-oxazolinylferrocene ligands and their application in regio- and enantioselective Pd-catalyzed allylic alkylation of monosubstituted allyl substrates

• Zeng-Wei Lai,
• Rong-Fei Yang,
• Ke-Yin Ye,
• Hongbin Sun and
• Shu-Li You

Beilstein J. Org. Chem. 2014, 10, 1261–1266, doi:10.3762/bjoc.10.126

• the R group has the ability to stabilize the carbocation, the electronic factor would favor the formation of the branched product (path b). The phosphorus atom has a stronger trans effect comparing with the oxazoline nitrogen, indicating that the carbon trans to phosphorus atom bears more

Direct C–H trifluoromethylation of di- and trisubstituted alkenes by photoredox catalysis

• Ren Tomita,
• Yusuke Yasu,
• Takashi Koike and
• Munetaka Akita

Beilstein J. Org. Chem. 2014, 10, 1099–1106, doi:10.3762/bjoc.10.108

• trifluoromethylating reagents (+CF3) can serve as more efficient CF3 radical sources under mild photocatalytic reaction conditions. In addition, the putative β-CF3 carbocation intermediate formed through SET photoredox processes is playing a key role in our reaction systems (vide infra). Trifluoromethylated alkenes

• the benzyl radical-type intermediate 3' in a regioselective manner. Subsequent one-electron-oxidation by highly oxidizing Ru species, [RuIII(bpy)3]3+, produces β-CF3 carbocation intermediate 3+. Finally, smooth elimination of the olefinic proton, which is made acidic by the strongly electron

• -withdrawing CF3 substituent, provides trifluoromethylated alkene 3. Preferential formation of one isomer in the reaction of unsymmetrical substrates is attributed to the population of the rotational conformers of the β-CF3 carbocation intermediate 3+. Our experimental result is consistent with the previous

Continuous flow nitration in miniaturized devices

• Amol A. Kulkarni

Beilstein J. Org. Chem. 2014, 10, 405–424, doi:10.3762/bjoc.10.38

• step is much slower – and hence rate-controlling – than the reverse of the second step due of the presence of water and the poor solubility of the aromatic species in the mineral acid medium. In the rate-controlling step the nitronium ion attacks the aromatic ring to give an intermediate carbocation

Efficient carbon-Ferrier rearrangement on glycals mediated by ceric ammonium nitrate: Application to the synthesis of 2-deoxy-2-amino-C-glycoside

• Alafia A. Ansari,
• Y. Suman Reddy and
• Yashwant D. Vankar

Beilstein J. Org. Chem. 2014, 10, 300–306, doi:10.3762/bjoc.10.27

• delocalized carbocation B. The acetate radical could accept an electron from Ce2+ thereby forming an acetate anion, which could in turn attack the silyl moiety of allyltrimethylsilane leading to the formation of trimethylsilyl acetate and an allyl anion. The allyl anion could then attack at the anomeric

Recent applications of the divinylcyclopropane–cycloheptadiene rearrangement in organic synthesis

• Sebastian Krüger and
• Tanja Gaich

Beilstein J. Org. Chem. 2014, 10, 163–193, doi:10.3762/bjoc.10.14

• products containing a dihydrooxepine moiety. In the case of occidenol (25, see Scheme 5), which has been isolated from the wood of Thuja koraiensis, farnesyl pyrophosphate (22) is supposed to undergo ring closure and the intermediate carbocation is trapped by hydroxide to give hedycaryol (23) [33

Synthesis of five- and six-membered cyclic organic peroxides: Key transformations into peroxide ring-retaining products

• Alexander O. Terent'ev,
• Dmitry A. Borisov,
• Vera A. Vil’ and
• Valery M. Dembitsky

Beilstein J. Org. Chem. 2014, 10, 34–114, doi:10.3762/bjoc.10.6

• approach to the synthesis of 1,2-dioxane rings is based on the intramolecular cyclization of hydroperoxides containing a C=C group. In the first step, the addition of a halonium ion to the double bond results in the formation of a carbocation, which is subjected to the intramolecular attack of the

Silica: An efficient catalyst for one-pot regioselective synthesis of dithioethers

• Samir Kundu,
• Babli Roy and
• Basudeb Basu

Beilstein J. Org. Chem. 2014, 10, 26–33, doi:10.3762/bjoc.10.5

• activate the double bond and subsequent assistance by the neighbouring sulfur atom coupled with the stability of the secondary carbocation lead to the Markovnikov addition resulting in the exclusive formation of 1,2-dithioether (Scheme 2, conditions A). On the other hand, the moist silica consisting of a

New developments in gold-catalyzed manipulation of inactivated alkenes

• Michel Chiarucci and
• Marco Bandini

Beilstein J. Org. Chem. 2013, 9, 2586–2614, doi:10.3762/bjoc.9.294

• , cationic [Au(I)] species exhibited comparable activity to TfOH at 85 °C (Table 1). This complementary behavior can be ascribed to different activation modes of the double bond. Computational investigations on the [H+]-catalyzed reaction predicted a transition state with significant carbocation character

• the key role of intramolecular hydrogen bond (H-bond) interactions for both reactivity and stereoselectivity [65]. Theoretical calculations and experimental evidences ruled out the SN1 reaction mechanism via allylic carbocation. Experiments with stereodefined hydroxy allylic alcohols revealed that the

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

• of this intermediate and therefore facilitates the overall process. Finally, a ring-closing process through attack of the vinylgold species to the stabilized carbocation yields the cyclobutane system and regenerates the catalyst. In view of this mechanistic proposal, Mascareñas and López recently

Gold(I)-catalyzed 6-endo hydroxycyclization of 7-substituted-1,6-enynes

• Ana M. Sanjuán,
• Alberto Martínez,
• Patricia García-García,
• Manuel A. Fernández-Rodríguez and
• Roberto Sanz

Beilstein J. Org. Chem. 2013, 9, 2242–2249, doi:10.3762/bjoc.9.263

• represented as two resonance structures highlighting both the carbocation or carbenoid nature of this intermediate (Scheme 2). Results and Discussion As established in Scheme 2, we were intrigued by the possibility that o-(alkynyl)-(3-methylbut-2-enyl)benzenes 1 could undergo a 6-endo-dig cyclization in the

Sequential Diels–Alder/[3,3]-sigmatropic rearrangement reactions of β-nitrostyrene with 3-methyl-1,3-pentadiene

• Peter A. Wade,
• Alma Pipic,
• Matthias Zeller and
• Panagiota Tsetsakos

Beilstein J. Org. Chem. 2013, 9, 2137–2146, doi:10.3762/bjoc.9.251

• , but products derived from carbocation rearrangement of the intermediate have been isolated. Houk et al. favor a concerted mechanism with no intervening energy minimum for cycloaddition based on calculational results [7][8][9]. The current results are in better agreement with the stepwise mechanism

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

• decalin moiety has formed, two sigmatropic, stereospecific [1,2]-shifts of intermediate 170 provide the tertiary carbocation 171, which could be trapped by the phenolic alcohol to give stachyflin (156) [143]. In 2011, the first enantioselective total synthesis of (+)-stachyflin (156) via a Lewis acid

• the tertiary carbocation at the ring junction. This is then trapped by the phenol to give the pentacyclic compounds 180a and 180b. The former diastereoisomer could be converted to 180b via inversion of the secondary alcohol. Cleavage of the methyl ether and the 3,4-dimethoxybenzyl (3,4DMB) group of

A reductive coupling strategy towards ripostatin A

• Kristin D. Schleicher and
• Timothy F. Jamison

Beilstein J. Org. Chem. 2013, 9, 1533–1550, doi:10.3762/bjoc.9.175

• terminated by cyclopropylmethylsilane (Scheme 3, reaction 3), which may be explained by the participation of a carbocation that is stabilized by the adjacent cyclopropane ring in the bisected conformation where (CHOR)R′ is oriented anti to the cyclopropane [28][29][30]. Finally, Micalizio has described a

Copper(II)-salt-promoted oxidative ring-opening reactions of bicyclic cyclopropanol derivatives via radical pathways

• Eietsu Hasegawa,
• Minami Tateyama,
• Ryosuke Nagumo,
• Eiji Tayama and
• Hajime Iwamoto

Beilstein J. Org. Chem. 2013, 9, 1397–1406, doi:10.3762/bjoc.9.156

• )2 gives rise to tertiary carbocation 13 [51][52], which is then deprotonated to form enone 4. Studies of the effect of the counter ion on copper(II)-promoted reactions of 1b (Scheme 8) gave the results summarized in Table 2. While no reaction occurred when copper(II) acetylacetonate, Cu(acac)2, is

• fast oxidation of the more stable isomer 10 by stronger oxidants such as CuCl2 or Cu(OTf)2 occurs to give the stable tertiary carbocation 13, which is then captured by Cl− or MeCN. In order to explore the generality of the proposed counter-anion-dependent reactivity switch in the nature of copper(II

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

• 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

Metal-free aerobic oxidations mediated by N-hydroxyphthalimide. A concise review

• Lucio Melone and
• Carlo Punta

Beilstein J. Org. Chem. 2013, 9, 1296–1310, doi:10.3762/bjoc.9.146

• in good yields. Moreover, by simply moving into acetonitrile, they observed the selective oxygenation of phthalane to the corresponding phthalaldehyde in 80% yield [23]. In both cases the reaction proceeds via the formation of a carbocation intermediate (Scheme 7). The same research group also

Copper-catalyzed aerobic aliphatic C–H oxygenation with hydroperoxides

• Pei Chui Too,
• Ya Lin Tnay and
• Shunsuke Chiba

Beilstein J. Org. Chem. 2013, 9, 1217–1225, doi:10.3762/bjoc.9.138

• hydroperoxide deliver 1,4-diol 3a. In the absence of molecular O2 (under a N2 atmosphere, Table 1, entry 15), the resulting C-radical I is oxidized by the Cu(II) species to give carbocation VI [19], which is trapped by the intramolecular hydroxy group to give 4a. We next explored the substrate scope of the

Interplay of ortho- with spiro-cyclisation during iminyl radical closures onto arenes and heteroarenes

• Roy T. McBurney and
• John C. Walton

Beilstein J. Org. Chem. 2013, 9, 1083–1092, doi:10.3762/bjoc.9.120

• carbocation, followed by proton loss. Kinetic or other data to help predict which mode would be favoured for a novel iminyl radical is essentially nonexistent. We discovered recently that oxime carbonates ArC(R)=N–OC(O)OR’ are clean and convenient precursors for iminyl as well as O-centred radicals [26][35

A new intermediate in the Prins reaction

• Shinichi Yamabe,
• Takeshi Fukuda and
• Shoko Yamazaki

Beilstein J. Org. Chem. 2013, 9, 476–485, doi:10.3762/bjoc.9.51

• -dioxane product. The intermediate is of almost the same stability as the product, and two species were suggested to be in a state of equilibrium. While the geometry of TS1 appears to be forwarded to that of a carbocation intermediate, the cation disappeared through the enlargement of the water cluster

• ][11][12][13][14][15][16][17][18] to afford 1,3-dioxane, 1,3-diol and allylic alcohol, where the carbocation intermediate (X) is included (Scheme 5). In the second step, X is formed. From X, two routes, (i) and (ii), are possible. In the route (i), a water molecule is linked with the cation center

• ) bond is formed. After TS1(Me), not the carbonium ion but rather the butane-1,3-diol, diol(Me), is afforded. This result demonstrates that steps 1, 2 and 3 in Scheme 5 occur at the same time without intervention of the carbocation X. From diol(Me), two TSs were obtained. One is TS2(Me) leading to the