True and masked three-coordinate T-shaped platinum(II) intermediates

• Manuel A. Ortuño,
• Salvador Conejero and
• Agustí Lledós

Beilstein J. Org. Chem. 2013, 9, 1352–1382, doi:10.3762/bjoc.9.153

• * and IMes hampering the approach of the CH bond to the metal center. Once again, Lewis acids as acetonitrile can access the empty site, which means that a potential reactant molecule can coordinate to the metal for further reactions. Masked T-shaped via agostic interaction The agostic interaction [29

• generated [41] by methide abstraction from the neutral derivative cis-[Pt(Me)2(PiPr3)2] by using highly electrophilic Lewis acids such as B(C6F5)3 or [CPh3][1-H-closo-CB11Me11] (with the concomitant formation of MeB(C6F5)3− or MeCPh3, respectively). This synthetic route uses the same procedure described by

Mild and efficient cyanuric chloride catalyzed Pictet–Spengler reaction

• Ashish Sharma,
• Mrityunjay Singh,
• Nitya Nand Rai and
• Devesh Sawant

Beilstein J. Org. Chem. 2013, 9, 1235–1242, doi:10.3762/bjoc.9.140

• Brønsted acids. However, recently several other aprotic or Lewis acids, such as Yb(OTf)3 [23][24][25], AuCl3/AgOTf [26], Me3SiCl [27][28], BF3·Et2O [29], iodine [30], zeolite [31] and enzymes [32][33][34], have been used for carrying out the Pictet–Spengler reaction. Though the Pictet–Spengler reaction has

Conformational analysis and intramolecular interactions in monosubstituted phenylboranes and phenylboronic acids

• Josué M. Silla,
• Rodrigo A. Cormanich,
• Roberto Rittner and
• Matheus P. Freitas

Beilstein J. Org. Chem. 2013, 9, 1127–1134, doi:10.3762/bjoc.9.125

• , in addition to typical intramolecular hydrogen bonds [7]. Niedenzu [8] presented studies in organic synthesis with evidence of intramolecular interactions between boron and electronegative atoms such as F, Cl, O, N and S. Indeed, boron-containing compounds are Lewis acids, because of the empty p

Reductions of aldehydes and ketones with a readily available N-heterocyclic carbene borane and acetic acid

• Vladimir Lamm,
• Xiangcheng Pan,
• Tsuyoshi Taniguchi and
• Dennis P. Curran

Beilstein J. Org. Chem. 2013, 9, 675–680, doi:10.3762/bjoc.9.76

• easily to release reactive borane (BH3) to solution under typical thermal reaction conditions. Perhaps because of their stability, the potential use of carbene-boranes as hydride sources has been largely overlooked. Lindsay and McArthur reported that strong Lewis acids such as BF3 and Sc(OTf)3 activate

Asymmetric synthesis of a highly functionalized bicyclo[3.2.2]nonene derivative

• Toshiki Tabuchi,
• Daisuke Urabe and
• Masayuki Inoue

Beilstein J. Org. Chem. 2013, 9, 655–663, doi:10.3762/bjoc.9.74

• selective activation of acrolein in the presence of the Lewis-basic allylic oxygen was a prerequisite to the successful formation of 8. We anticipated that bulky trialkylsilyl triflates would function as such chemoselective Lewis acids, because the activation of the TBS-connected oxygen of 7 by the silyl

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

• Lewis acids are employed to catalyze the reaction, and Prins-type reactions have found numerous synthetic applications [6][19][20][21][22][23][24][25]. In spite of the extensive experimental studies, there have been no computational studies of the classic Prins reaction. The reaction is also important

Metal-mediated aminocatalysis provides mild conditions: Enantioselective Michael addition mediated by primary amino catalysts and alkali-metal ions

• Matthias Leven,
• Jörg M. Neudörfl and
• Bernd Goldfuss

Beilstein J. Org. Chem. 2013, 9, 155–165, doi:10.3762/bjoc.9.18

• catalysis provides an effective alternative to conventional primary amino catalysis were strong acid additives are essential components. Keywords: alkali metals; aminocatalysis; DFT-calculations; Lewis acids; Michael addition; Introduction Organocatalysis based on primary or secondary amines enables a

• ][13]. In most cases, transition or rare-earth metals are employed as Lewis acids in order to activate electrophilic compounds to react with substrates that are directly bonded to an organic catalyst [9][10][11][12][13]. An example is the enamine–metal Lewis acid bifunctional catalysis for asymmetric

• -hydroxycoumarin (1) to prochiral α,β-unsaturated ketones. Instead of acid additives, which sometimes lead to decomposition of sensitive components [18], alkali-metal ions are employed as very mild Lewis acids (Scheme 1). The activation of Michael acceptors by iminium ions enables asymmetric additions of 4

Removal of benzylidene acetal and benzyl ether in carbohydrate derivatives using triethylsilane and Pd/C

• Abhishek Santra,
• Tamashree Ghosh and
• Anup Kumar Misra

Beilstein J. Org. Chem. 2013, 9, 74–78, doi:10.3762/bjoc.9.9

• reported for the removal of benzylidene acetals by using strong protic and Lewis acids [10][11][15] as well as some heterogeneous acidic catalysts [16][17]. Removal of benzylidene acetal under nonacidic conditions includes hydrogenolysis using hydrogen gas over Pd/C [18], or treatment with hydrazine [19

A new approach toward the total synthesis of (+)-batzellaside B

• Jolanta Wierzejska,
• Shin-ichi Motogoe,
• Yuto Makino,
• Tetsuya Sengoku,
• Masaki Takahashi and
• Hidemi Yoda

Beilstein J. Org. Chem. 2012, 8, 1831–1838, doi:10.3762/bjoc.8.210

• exemplified by performing the procedures with AllylTMS and two types of Lewis acids, which allows for simpler synthetic operation due to the ease of purification of the products. The present study clearly demonstrates that L-pyroglutamic acid can be used as a versatile chiral source for synthesizing this

Asymmetric Brønsted acid-catalyzed aza-Diels–Alder reaction of cyclic C-acylimines with cyclopentadiene

• Magnus Rueping and
• Sadiya Raja

Beilstein J. Org. Chem. 2012, 8, 1819–1824, doi:10.3762/bjoc.8.208

• chiral Lewis acids [5][6][7][8][9][10][11][12][13][14][15][16]. Recently, chiral Brønsted acids have attracted interest as effective catalysts for a variety of asymmetric transformations involving imine electrophiles [17][18][19][20][21][22][23]. Among others, the aza-Diels–Alder reaction of imino

Imidazolinium and amidinium salts as Lewis acid organocatalysts

• Oksana Sereda,
• Nicole Clemens,
• Tatjana Heckel and
• René Wilhelm

Beilstein J. Org. Chem. 2012, 8, 1798–1803, doi:10.3762/bjoc.8.205

• desulfurization of the products containing a thiol or thiocarbonyl group. Keywords: Diels–Alder; ionic liquids; organocatalysis; soft Lewis acids; thiiranes; thioesters; Introduction Salts with melting points below 100 °C are known as ionic liquids and are often used as novel solvents for reactions and

• hydrogen-bond activation catalysts [18]. The utilization of the soft Lewis basic sulfur groups in this reaction is rare. Often mercury Lewis acids are applied [19][20][21][22] to activate the sulfur group and there is also the risk of desulfurization [23]. The only known asymmetric Diels–Alder reaction

• ) was performed in different solvents, at varied temperatures, by using a broad range of catalysts, which are summarized in Table 1. First a few metal-based Lewis acids were applied in the reaction. BF3•Et2O is known to activate also sulfur carbonyl groups [24] and gave the product in 90% yield, while

Metal–ligand multiple bonds as frustrated Lewis pairs for C–H functionalization

• Matthew T. Whited

Beilstein J. Org. Chem. 2012, 8, 1554–1563, doi:10.3762/bjoc.8.177

• number of unsaturated main-group compounds can exhibit electronic properties and reactivity reminiscent of transition metals [3]. A different approach was pioneered by Stephan, who demonstrated that appropriately encumbered (i.e., "frustrated") Lewis acids and bases could achieve synergistic heterolytic

• have extended the approach to include transition metals as Lewis acids and bases [7][8]. In this review, we show that the FLP concept may be extended to encompass certain metal–ligand multiply bonded species, provided that the metal retains an open coordination site to facilitate cooperative reactivity

• analogues of the recently developed frustrated Lewis pairs involving main-group Lewis acids and bases. Although the manner in which "frustration" occurs is somewhat different (i.e., it is primarily electronic and not steric in origin), the types of reactivity observed are remarkably similar. One example in

Highly enantioselective access to cannabinoid-type tricyles by organocatalytic Diels–Alder reactions

• Stefan Bräse,
• Nicole Volz,
• Franziska Gläser and
• Martin Nieger

Beilstein J. Org. Chem. 2012, 8, 1385–1392, doi:10.3762/bjoc.8.160

• former experiments of our group [29][30][31] it became apparent that without a catalyst no conversion towards the desired product occurred. Only the homodimer of the diene 10 could be isolated. Metal-based Lewis acids (e.g., catechol boronates) proved to be inefficient or too reactive. Due to the fact

An intramolecular inverse electron demand Diels–Alder approach to annulated α-carbolines

• Zhiyuan Ma,
• Feng Ni,
• Grace H. C. Woo,
• Sie-Mun Lo,
• Philip M. Roveto,
• Scott E. Schaus and
• John K. Snyder

Beilstein J. Org. Chem. 2012, 8, 829–840, doi:10.3762/bjoc.8.93

• avoid ketonization of the ester [86]. Other Lewis acids, MgCl2, Mg(OTf)2, Zn(OTf)2, Yb(OTf)3 and Sc(OTf)3, were not successful in catalyzing the amidation. Stoichiometric amounts of catalyst were required for the amidation, presumably due to the product itself sequestering the catalyst and preventing

N-Heterocyclic carbene/Brønsted acid cooperative catalysis as a powerful tool in organic synthesis

• Rob De Vreese and
• Matthias D’hooghe

Beilstein J. Org. Chem. 2012, 8, 398–402, doi:10.3762/bjoc.8.43

• combinations of metals (Lewis acids) and carbenes can be used as a novel catalytic system [15][16][17]. Moreover, this type of cooperation has been demonstrated to feature a unique reactivity that is difficult to achieve by using one of both catalysts individually. The main challenge in that respect involves

Organic synthesis using (diacetoxyiodo)benzene (DIB): Unexpected and novel oxidation of 3-oxo-butanamides to 2,2-dihalo-N-phenylacetamides

• Wei-Bing Liu,
• Cui Chen,
• Qing Zhang and
• Zhi-Bo Zhu

Beilstein J. Org. Chem. 2012, 8, 344–348, doi:10.3762/bjoc.8.38

• obtained, but 2,2-dichloro-N-phenylacetamide was provided as the major product, upon addition of Lewis acids such as FeCl3, ZnCl2 and CuCl2 in the reaction system. Based on this result, we developed a simple and efficient approach to the synthesis of 2,2-dihalo-N-phenylacetamides, on which we report herein

• ][36][37][38], but this report is the first to describe the synthesis of 2,2-dihalo-N-phenylacetamides through an oxidative process with PhI(OAc)2 and Lewis acids as the halogen source. Results and Discussion Initially, we used 3-oxo-N-phenylbutanamide (1a) as the model substrate to optimize the

• Lewis acids (Table 1, entry 2 and entry 3), and dioxane and DMF were practical solvents among the various solvents examined (Table 1, entry 3 and entry 8). It is noteworthy that no product 2a was obtained when the reaction was carried out without the addition of Lewis acids (Table 1, entry 1) or without

Continuous-flow enantioselective α-aminoxylation of aldehydes catalyzed by a polystyrene-immobilized hydroxyproline

• Xacobe C. Cambeiro,
• Rafael Martín-Rapún,
• Pedro O. Miranda,
• Sonia Sayalero,
• Esther Alza,
• Patricia Llanes and
• Miquel A. Pericàs

Beilstein J. Org. Chem. 2011, 7, 1486–1493, doi:10.3762/bjoc.7.172

• chiral enantiopure Lewis acids as catalysts [6][7][8][9][10][11]. Worth noting is the contribution made by Yamamoto et al., who introduced the use of nitrosobenzene as an electrophilic source of oxygen in the aminoxylation of preformed tin enolates catalyzed by a chiral, silver-based Lewis acid [12

Hypervalent iodine(III)-induced methylene acetoxylation of 3-oxo-N-substituted butanamides

• Wei-Bing Liu,
• Cui Chen,
• Qing Zhang and
• Zhi-Bo Zhu

Beilstein J. Org. Chem. 2011, 7, 1436–1440, doi:10.3762/bjoc.7.167

• the reaction afforded the desired product 1-(phenylcarbamoyl)-2-oxopropyl acetate (2a) by using DIB as the additive, and the optimum reaction time was 2 hours (Table 1, entries 1–3), whereas almost no desired product was obtained when Lewis acids were added (Table 1, entries 4–6). Among the various

Development of the titanium–TADDOLate-catalyzed asymmetric fluorination of β-ketoesters

• Lukas Hintermann,
• Mauro Perseghini and
• Antonio Togni

Beilstein J. Org. Chem. 2011, 7, 1421–1435, doi:10.3762/bjoc.7.166

• Lewis acids catalyze the α-fluorination of β-ketoesters by electrophilic N–F-fluorinating reagents. Asymmetric catalysis with TADDOLato–titanium(IV) dichloride (TADDOL = α,α,α',α'-tetraaryl-(1,3-dioxolane-4,5-diyl)-dimethanol) Lewis acids produces enantiomerically enriched α-fluorinated β-ketoesters in

• ketones in the presence of antimony chloride have appeared [59], while Umemoto used Lewis acids in substoichiometric amounts (0.4 equiv of ZnCl2 or AlCl3) to accelerate fluorinations of 1,3-dicarbonyl compounds with N-fluoropyridinium salts [72]. In 1998, Chambers and Hutchinson reported the reaction of

• presence of Lewis acids As a starting point for our investigation, we chose to study the fluorinations of the readily enolizable β-ketoesters 1 and 2 and of α-cyanoester 3 as model compounds with the reagent F–TEDA under either “neutral” conditions [66][67][68][69][70][71] or in the presence of Lewis acids

Manganese dioxide mediated one-pot synthesis of methyl 9H-pyrido[3,4-b]indole-1-carboxylate: Concise synthesis of alangiobussinine

• Jessica Baiget,
• Sabin Llona-Minguez,
• Stuart Lang,
• Simon P. MacKay,
• Colin J. Suckling and
• Oliver B. Sutcliffe

Beilstein J. Org. Chem. 2011, 7, 1407–1411, doi:10.3762/bjoc.7.164

• energy barrier we looked at adding Lewis acids as additives [21] in the process. However, the use of catalytic amounts of Lewis acids did not lead to an increase in the yield of β-carboline 6 (Table 1, entries i and j) and the use of one equivalent of zinc(II) chloride (Table 1, entry h) inhibited all

• shows that although Lewis acids can be used to activate Pictet–Spengler reactions, the absence of a Lewis acid is optimum for this one-pot cascade process. β-Carboline 6 is a useful intermediate, used by Panosyan and Still [22] in their synthesis of xestomanzamine A. We wished to apply the preparation

Cyclization of 5-hexynoic acid to 3-alkoxy-2-cyclohexenones

• Anne T. Hylden,
• Eric J. Uzelac,
• Zeljko Ostojic,
• Ting-Ting Wu,
• Keely L. Sacry,
• Krista L. Sacry,
• Lin Xi and
• T. Nicholas Jones

Beilstein J. Org. Chem. 2011, 7, 1323–1326, doi:10.3762/bjoc.7.155

• -cyclohexenones. Keywords: acyl chlorides; alcohols; alkynes; cyclization; Lewis acids; Findings Synthetic applications of 3-alkoxy-2-cyclohexenones toward the pursuit of natural products have been well documented [1][2][3][4]. Traditionally, 3-alkoxy-2-cyclohexenones have been prepared from 1,3-cyclohexadiones

Directed ortho,ortho'-dimetalation of hydrobenzoin: Rapid access to hydrobenzoin derivatives useful for asymmetric synthesis

• Inhee Cho,
• Labros Meimetis,
• Lee Belding,
• Michael J. Katz,
• Travis Dudding and
• Robert Britton

Beilstein J. Org. Chem. 2011, 7, 1315–1322, doi:10.3762/bjoc.7.154

• (benzoxaborol) 20 are both prepared in good yield and are readily derivatized through cross-coupling reactions. We are currently exploring the utility of the bis(benzoxaborol) 20 and derivatives of this substance as chiral Lewis acids and will report our findings in due course. Chiral diols useful for

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

• alcohols compared to allyl halides/acetates/carbonates generally necessitates harsher reaction conditions and/or the need for activating agents (i.e., Brønsted or Lewis acids) [3][4]. Recently, late-transition metal (LTM) catalysis (i.e., Hg, Pd, Pt, Au, and Ru) has received growing attention in organic

A practical route to tertiary diarylmethylamides or -carbamates from imines, organozinc reagents and acyl chlorides or chloroformates

• Erwan Le Gall,
• Antoine Pignon and
• Thierry Martens

Beilstein J. Org. Chem. 2011, 7, 997–1002, doi:10.3762/bjoc.7.112

• , we assumed that under these conditions, the formation of N–AG (AG = activating group) bonds would be reversible, thus cleavage would be effective in situ and only relatively small amounts of the Lewis acid would be necessary. While several common Lewis acids (TiCl4, AlCl3, CeCl3 and BF3·Et2O) were

• , combined with the above reported observations with common Lewis acids, may indicate that acylating reagents are particularly reliable for the activation of aldimines toward arylzinc additions. However, the use of carbonyl-containing electrophiles obviously constitutes an important drawback of the procedure

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

• past decade, the diverse reactivity of carbophilic Lewis acids has attracted considerable interest in the development of domino reactions [1][2][3][4][5] that are initiated by the catalytic activation of π-systems [6][7][8][9][10][11][12][13]. In particular, the utilization of gold pre-catalysts has