Synthesis of constrained analogues of tryptophan

• Elisabetta Rossi,
• Valentina Pirovano,
• Marco Negrato,
• Giorgio Abbiati and
• Monica Dell’Acqua

Beilstein J. Org. Chem. 2015, 11, 1997–2006, doi:10.3762/bjoc.11.216

• results are summarized in Table 1. The reaction was ineffective under thermal conditions (Table 1, entry 1). As a consequence we tested simple Lewis acids as potential promoters for the transformation. Magnesium perchlorate, scandium and copper triflate and boron trifluoride failed to give the desired

• Lewis acid, only EtAlCl2 is able to trigger the reaction toward the formation of the desired cycloadducts 3/3'. Similarly, Piersanti and co-workers reported the unique capability of EtAlCl2, with respect to related hard Lewis acids, to activate 2 toward the nucleophilic addition of indoles [29]. They

Aerobic addition of secondary phosphine oxides to vinyl sulfides: a shortcut to 1-hydroxy-2-(organosulfanyl)ethyl(diorganyl)phosphine oxides

• Svetlana F. Malysheva,
• Alexander V. Artem’ev,
• Nina K. Gusarova,
• Nataliya A. Belogorlova,
• Alexander I. Albanov,
• C. W. Liu and
• Boris A. Trofimov

Beilstein J. Org. Chem. 2015, 11, 1985–1990, doi:10.3762/bjoc.11.214

• approaches to construct new C–P bonds, that provide straightforward access to tertiary phosphines and their chalcogenides [8][9][10][11][12]. Conventionally, the activation of the P–H bonds in this reaction is achieved by using radical initiators [13][14][15], Brønsted/Lewis acids [16][17] and bases [18][19

Lewis acid-promoted hydrofluorination of alkynyl sulfides to generate α-fluorovinyl thioethers

• Davide Bello and
• David O'Hagan

Beilstein J. Org. Chem. 2015, 11, 1902–1909, doi:10.3762/bjoc.11.205

• thioester enols and enolates, important intermediates in enzymatic C–C bond forming reactions. The method opens access to appropriate analogues for investigations in this direction. Keywords: alkynyl sulfides; α-fluorovinyl thioethers; hydrofluorination; Lewis acids; organofluorine; Introduction

• obtained as a 4:1 mixture of Z/E-isomers, and they could only be isolated in a modest yield (35%) as shown in Scheme 2 and Table 1 (entry 7). Encouraged by this result, a number of Lewis acids were tested, including SnCl2, ZnCl2, Sc(OTf)3, AuCl·SMe2 and B(C6F5)3 (Table 1). The Lewis acids (1.5 equivalents

• group, possibly with its participation in the reaction process. 19F NMR was used to probe changes in the Lewis acids in the reaction. Ratios of 1:2 Lewis acid:3HF·Et3N mixtures in CD2Cl2 were stirred at room temperature for 2 hours, and the aliquots (0.7 mL) were assayed in Teflon NMR tubes. 19F NMR

Tandem cross enyne metathesis (CEYM)–intramolecular Diels–Alder reaction (IMDAR). An easy entry to linear bicyclic scaffolds

• Javier Miró,
• María Sánchez-Roselló,
• Álvaro Sanz,
• Fernando Rabasa,
• Carlos del Pozo and
• Santos Fustero

Beilstein J. Org. Chem. 2015, 11, 1486–1493, doi:10.3762/bjoc.11.161

• tandem process (Table 1, entries 9 and 10). The use of Lewis acids as co-catalysts was also tested although the efficiency of the process did not improve neither with Ti(OiPr)4 nor with BF3·OEt2 (Table 1, entries 11 and 12). Alternatively, thiourea derivatives have proven to be very effective hydrogen

Diastereoselective and enantioselective conjugate addition reactions utilizing α,β-unsaturated amides and lactams

• Katherine M. Byrd

Beilstein J. Org. Chem. 2015, 11, 530–562, doi:10.3762/bjoc.11.60

• This section will focus on Lewis acid-catalyzed ECA reactions, which employ stabilized nucleophiles such as silyl enol ethers and amines. Chiral Lewis acids play a dual role in these reactions. They activate the Michael acceptors by coordinating with the carbonyl oxygen and they provide a chiral

Selective methylation of kaempferol via benzylation and deacetylation of kaempferol acetates

• Qinggang Mei,
• Chun Wang,
• Weicheng Yuan and
• Guolin Zhang

Beilstein J. Org. Chem. 2015, 11, 288–293, doi:10.3762/bjoc.11.33

• and then hydrolysis in 1 M HCl (aq) at room temperature was an excellent method for selective removal of the 3- and 5-O-acetyl groups from triacetylated kaempferol (2). No side products were detected by TLC analysis. With other Lewis acids such as ZnCl2, the result was very different. The desired

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

• in a basic medium [173]. The oxidative esterification of aldehydes was performed using peroxides in the presence of Lewis acids or in the absence of the latter; for example, using oxone in the presence of In(OTf)3 [174][175], oxone [174][176], Caro’s acid [177], H2O2aq (30%) in the presence of CaCl2

Efficient deprotection of F-BODIPY derivatives: removal of BF₂ using Brønsted acids

• Mingfeng Yu,
• Joseph K.-H. Wong,
• Cyril Tang,
• Peter Turner,
• Matthew H. Todd and
• Peter J. Rutledge

Beilstein J. Org. Chem. 2015, 11, 37–41, doi:10.3762/bjoc.11.6

• recent reports have deployed Lewis acids to achieve this transformation: Thompson and co-workers used boron trihalides in dichloromethane under anhydrous conditions, followed by treatment with acetone/water (10:1) to achieve quantitative removal of the BF2 moiety [18]; Ravikanth and co-workers screened a

• range of metal-based Lewis acids (ZrCl4, TiCl4, AlCl3, Sc(OTf)3, SnCl4) and reported yields up to 96% using ZrCl4 in refluxing methanol/acetonitrile [19]. Related efforts have wrought substitution at boron in F-BODIPY analogues without removing it from the dipyrrin. For example, Lundrigan et al. have

• organic acid TFA or the inorganic acid HCl. These conditions are complementary to those previously reported for converting F-BODIPYs to the parent dipyrrins using either strong bases or Lewis acids. Compared to existing methods, the Brønsted acid conditions are relatively mild and operationally simple

Lewis acid-catalyzed redox-neutral amination of 2-(3-pyrroline-1-yl)benzaldehydes via intramolecular [1,5]-hydride shift/isomerization reaction

• Chun-Huan Jiang,
• Xiantao Lei,
• Le Zhen,
• Hong-Jin Du,
• Xiaoan Wen,
• Qing-Long Xu and
• Hongbin Sun

Beilstein J. Org. Chem. 2014, 10, 2892–2896, doi:10.3762/bjoc.10.306

• for 24 h gave the trisubstituted amine 3a in 50% yield (entry 1, Table 1). Encouraged by this result, we screened readily available Brønsted and Lewis acids (Table 1). Except the Lewis acid AlCl3, other strong Brønsted acids and common Lewis acids could be used as the catalyst in this reaction

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

• these methods use priorly activated terminal alkynes, e.g., silylacetylene, activated with various Lewis acids such as SnCl4, BF3·OEt2, TiCl4, I2, InBr3, and ZrCl4 [24][25][26][27][28][29][30], followed by a Ferrier type rearrangement [31][32][33][34] (Scheme 1). A consequence of the prior activation of

• challenge. We reasoned that the development of a strategy which in situ activates the terminal alkyne and further catalyzes the reaction without the aid of other Lewis acids might be a solution to this problem. Thus, in continuation of our efforts [36][37][38], we describe a highly stereoselective TMSOTf

Regio- and stereoselective synthesis of new diaminocyclopentanols

• Evgeni A. Larin,
• Valeri S. Kochubei and
• Yuri M. Atroshchenko

Beilstein J. Org. Chem. 2014, 10, 2513–2520, doi:10.3762/bjoc.10.262

• with hydrogen bond donors [25]. Additionally, the synthesis of aminocyclitols from cyclitol epoxides has been described [26][27]. It has been shown that the reaction of cyclitol epoxides with nitrogen-containing nucleophiles in the presence of Lewis acids gave a mixture of C1 and C2 adducts. Both

Facile synthesis of 1H-imidazo[1,2-b]pyrazoles via a sequential one-pot synthetic approach

• András Demjén,
• Márió Gyuris,
• János Wölfling,
• László G. Puskás and
• Iván Kanizsai

Beilstein J. Org. Chem. 2014, 10, 2338–2344, doi:10.3762/bjoc.10.243

• of either a Brønsted or a Lewis acid catalyst (Table 1, entry 1). However, the use of Lewis acids, such as indium(III) salts or TMSCl, improved the reaction rate, with yields up to 67% (Table 1, entries 2–4). The GBB-3CR catalysed by Brønsted acids, including PTSA or HClO4, led to similar yields as

Indium-mediated allylation in carbohydrate synthesis: A short and efficient approach towards higher 2-acetamido-2-deoxy sugars

• Christopher Albler,
• Ralph Hollaus,
• Hanspeter Kählig and
• Walther Schmid

Beilstein J. Org. Chem. 2014, 10, 2230–2234, doi:10.3762/bjoc.10.231

• (vide infra), which adopt rigid pyranoid conformations. Unfortunately, we were not able to directly achieve the opening of the obtained 2,3-epoxyaldehydes with azide nucleophiles. A variety of different Lewis acids, solvent systems and azide sources were screened but all attempts led to the

A new approach for the synthesis of bisindoles through AgOTf as catalyst

• Jorge Beltrá,
• M. Concepción Gimeno and
• Raquel P. Herrera

Beilstein J. Org. Chem. 2014, 10, 2206–2214, doi:10.3762/bjoc.10.228

• interesting biological properties, the development of new efficient, general and eco-friendly protocols using benign catalytic systems with structurally different aldehydes and easy accessible catalysts still remains a continuous demand. Among the variety of Lewis acids employed in this reaction such as In

Visible light photoredox-catalyzed deoxygenation of alcohols

• Daniel Rackl,
• Viktor Kais,
• Peter Kreitmeier and
• Oliver Reiser

Beilstein J. Org. Chem. 2014, 10, 2157–2165, doi:10.3762/bjoc.10.223

• -hydroxycarbonyl compounds 6b–e turned out to be amenable for the desoxygenation process (Table 4). Especially interesting from a preparative point of view, the monodeoxygenation of diethyl tartrate to maleate could be achieved from 6e in excellent yields (Table 4, entry 5). Notably, after screening various Lewis

• acids (Supporting Information File 1) it was found that 6e could be selectively prepared by copper(II) chloride-catalyzed benzoylation of diol 8 with anhydride 9, which in turn can be generated from its acid 5 by treatment with acetic anhydride (Scheme 6). Since acetic anhydride is technically being

Synthesis of 2-substituted tryptophans via a C3- to C2-alkyl migration

• Michele Mari,
• Simone Lucarini,
• Francesca Bartoccini,
• Giovanni Piersanti and
• Gilberto Spadoni

Beilstein J. Org. Chem. 2014, 10, 1991–1998, doi:10.3762/bjoc.10.207

• such as trifluoroethanol (TFE) did not accelerate the reaction and still gave low yields after 24 h (Table 1, entry 5). Next, we tested the effect of different acids on the reaction. Although some Lewis acids such as TiCl4, SnCl4, and Sc(OTf)3 did not show beneficial effects (Table 1, entries 6–8), a

Synthesis of a bifunctional cytidine derivative and its conjugation to RNA for in vitro selection of a cytidine deaminase ribozyme

• Nico Rublack and
• Sabine Müller

Beilstein J. Org. Chem. 2014, 10, 1906–1913, doi:10.3762/bjoc.10.198

• of Boc-protected amines can be selectively cleaved with Lewis acids [40]. Thus, stirring of 9 in anhydrous DCM with ZnBr2 delivered the free amine 10 with a yield of 82% (Figure 4). To this, biotin was coupled by in situ activation of the carboxylic acid as an NHS-ester using 1-ethyl-3-(3

Expedient synthesis of 1,6-anhydro-α-D-galactofuranose, a useful intermediate for glycobiological tools

• Luciana Baldoni and
• Carla Marino

Beilstein J. Org. Chem. 2014, 10, 1651–1656, doi:10.3762/bjoc.10.172

• leaving group at C-1, and the electron-donating nature of the TBS groups. Thus, while compounds 13 [38] and 14 [39] were prepared by treatment with Lewis acids of fully protected precursors and proved to be stable and therefore useful as synthetic intermediates, attempts to prepare compound 15 by

Homogeneous and heterogeneous photoredox-catalyzed hydroxymethylation of ketones and keto esters: catalyst screening, chemoselectivity and dilution effects

• Axel G. Griesbeck and
• Melissa Reckenthäler

Beilstein J. Org. Chem. 2014, 10, 1143–1150, doi:10.3762/bjoc.10.114

• hydroxymethylation in the presence of additional strong Lewis acids such as AlCl3 or BF3. These compounds alone did not show catalytic activity in methanol, only in combination with Ti(OiPr)4. With the optimal homogenous reaction conditions in hand, we applied several other aromatic and aliphatic open chain and

cis–trans Isomerization of silybins A and B

• Michaela Novotná,
• Radek Gažák,
• David Biedermann,
• Florent Di Meo,
• Petr Marhol,
• Marek Kuzma,
• Lucie Bednárová,
• Kateřina Fuksová,
• Patrick Trouillas and
• Vladimír Křen

Beilstein J. Org. Chem. 2014, 10, 1047–1063, doi:10.3762/bjoc.10.105

• to be the most suitable Lewis acid for silybin isomerization (Table 1). Other Lewis acids either gave lower isomerization yields (SnCl4), did not work at all, or even caused decomposition. Toluene-4-sulfonic acid, as a representative protic acid, gave no reaction under the same conditions. In this

A convenient enantioselective decarboxylative aldol reaction to access chiral α-hydroxy esters using β-keto acids

• Zhiqiang Duan,
• Jianlin Han,
• Ping Qian,
• Zirui Zhang,
• Yi Wang and
• Yi Pan

Beilstein J. Org. Chem. 2014, 10, 969–974, doi:10.3762/bjoc.10.95

• one point-binding aldehydes and two-point binding β-keto acids under mild reaction conditions. The lack of strong Lewis acids or very basic intermediates enabled it to tolerate functionalities that would normally be incompatible with ester enolates, for instance, hydroxy groups, phenols, enolizable

• investigating different Lewis acids with various chiral PyBox ligands 4–8 (Table 1), we discovered that Sc(OTf)3 and tridentate PyBox ligand 6a could promote the decarboxylative aldol reaction of β-keto acid 1a with α-keto ester 2a in excellent yield with high enantioselectivity in toluene (Table 1, entry 9

Domino reactions of 2H-azirines with acylketenes from furan-2,3-diones: Competition between the formation of ortho-fused and bridged heterocyclic systems

• Alexander F. Khlebnikov,
• Mikhail S. Novikov,
• Viktoriia V. Pakalnis,
• Roman O. Iakovenko and
• Dmitry S. Yufit

Beilstein J. Org. Chem. 2014, 10, 784–793, doi:10.3762/bjoc.10.74

• [41][43][46]. It was found that water [37], Brønsted [44][48] and Lewis acids [40][41][43] facilitate the formation of pyrazine derivatives. 2H-Azirines undergo ring opening on electronic excitation to give nitrile ylides [50]. Nitrile ylide formation under thermal conditions even from such strained

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

• carbophilic Lewis acids even if their use as σ-activators has been rarely reported. Recently, transformations involving Au(I)/Au(III)-redox catalytic systems have been reported in the literature. In this review we highlight all these aspects of silver and gold-mediated processes and their application in

An oxidative amidation and heterocyclization approach for the synthesis of β-carbolines and dihydroeudistomin Y

• Suresh Babu Meruva,
• Akula Raghunadh,
• Raghavendra Rao Kamaraju,
• U. K. Syam Kumar and
• P. K. Dubey

Beilstein J. Org. Chem. 2014, 10, 471–480, doi:10.3762/bjoc.10.45

• the yield of 7 under Bischler–Napieralski cyclodehydration reaction conditions using POCl3 as the Lewis acid were proved to be futile. This prompted us to test the efficiency of other Lewis acids in this cyclodehydration reaction. The Bischler–Napieralski cyclodehydration reaction was then carried out

• with different Lewis acids such as BF3·Et2O, SnCl4, TiCl4 etc., in mutiple solvents under various reaction conditions. The best conversion was obtained when the reaction was conducted in BF3·Et2O in ether at 25–30 °C and the product dihydro-β-carboline 7 was isolated in 55% of yield. The formation of

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

• synthesis of C-glycosides [12][13][14]. Among these, the Ferrier rearrangement [15] of glycals with protic acids [5][16][17] or Lewis acids [18][19][20][21][22] and carbon nucleophiles such as allylsilanes [23], silylacetylenes [24], silyl enol ethers [25], olefins [26], and organozinc reagents [27] has

• glycosyl amino acids [52] to name but a few. Amongst the various reported methods to prepare these compounds [53][54][55], the most common method is via C-glycosylation of 2-aminosugars [56][57][58], which is challenging owing to the incompatibility of protic or Lewis acids with amino and amido