Engineering of indole-based tethered biheterocyclic alkaloid meridianin into β-carboline-derived tetracyclic polyheterocycles via amino functionalization/6-endo cationic π-cyclization

• Piyush K. Agarwal,
• Meena D. Dathi,
• Mohammad Saifuddin and
• Bijoy Kundu

Beilstein J. Org. Chem. 2012, 8, 1901–1908, doi:10.3762/bjoc.8.220

• protocols involving 1% TFA in DCM at rt, with p-TsOH in toluene under reflux, and AcOH in ethanol under reflux. The conditions failed to favor cationic π-cyclization even after prolonged stirring and resulted in imines as the only isolated product (Table 1, entries 1–6). Since the role of Brønsted acids are

• considered to be an important factor [67] in promoting cationic π-cyclization by enhancing the electrophilicity of the imines, we envisaged that employing stronger acids may facilitate 6-endo cyclization. Accordingly, the amine 2a was treated with 4-chlorobenzaldehyde by using strong Brønsted acids, such as

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

• -dienophiles has been investigated and it was shown that the reaction between arylimines and dienes, catalyzed by chiral Brønsted acids, proceeds with high levels of enantioselectivity. However, these reactions are limited to electron-rich dienes including Brassard’s and Danishefsky dienes [24][25][26][27][28

• diastereoselectivities under mild reaction conditions. The results reported not only show that chiral BINOL derived phosphoric acid diesters can be efficient catalysts for [4 + 2] cycloadditions involving less-electron-rich dienes but additionally demonstrate the high potential of these acidic Brønsted acids in

Organocatalytic cascade aza-Michael/hemiacetal reaction between disubstituted hydrazines and α,β-unsaturated aldehydes: Highly diastereo- and enantioselective synthesis of pyrazolidine derivatives

• Zhi-Cong Geng,
• Jian Chen,
• Ning Li,
• Xiao-Fei Huang,
• Yong Zhang,
• Ya-Wen Zhang and
• Xing-Wang Wang

Beilstein J. Org. Chem. 2012, 8, 1710–1720, doi:10.3762/bjoc.8.195

• to four days, the yield of the product was increased to 80% and the enantioselectivity was retained (Table 4, entry 6). Thereafter, several Brønsted acids 5b, 5d–j were tested as additives for this transformation. Although enantioselectivity was somewhat improved from 92 to 95% ee, the reactivity

Organocatalytic C–H activation reactions

• Subhas Chandra Pan

Beilstein J. Org. Chem. 2012, 8, 1374–1384, doi:10.3762/bjoc.8.159

• combination of para-toluenesulfonic acid (PTSA) and benzene was the optimum system for Akiyama and co-workers (Scheme 2). After evaluating different Lewis and Brønsted acids with their equivalents as well as different solvents, Seidel and co-workers found that 0.2 equivalents of triflic acid in ethanol under

• Brønsted acids as catalysts was screened for this reaction, and the best reaction efficiency in terms of yield and reaction time was achieved with benzoic acid (10 mol %). The scope of the reaction was investigated and was found to tolerate a wide variety of functional groups including nitro, nitriles

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

• (NHCs) has provided a window of opportunities for the development of novel catalytic strategies within the past few years. The recent successful combination of Brønsted acids with NHCs has added a new dimension to the field of cooperative catalysis, enabling the stereoselective synthesis of

• functionalized pyrrolidin-2-ones as valuable scaffolds in heterocyclic chemistry. This Commentary will briefly highlight the concept of N-heterocyclic carbene/Brønsted acid cooperative catalysis as a new and powerful methodology in organic chemistry. Keywords: Brønsted acids; cooperative catalysis; γ-lactams; N

• been described before to account for improved stereoselectivities [24][25][26]. Although the concept of cooperative catalysis involving NHCs had emerged previously in organic chemistry and the compatibility of NHCs and Brønsted acids had been reported by Rovis in 2010 [27], the NHC/Brønsted acid

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

• . In order to confirm that the catalysis was indeed due to the presence of gold, a control experiment with AgOTf (5 mol %) was performed on compound 1a. Under comparable reaction conditions (80 °C, 16 h), lactone 2a was isolated in poor yield (35%). Finally, the hypothetical cocatalysis by Brønsted

• acids (BA) was verified by means of experimental controls with TsOH (entry 15), and also in the presence of an acid scavenger (entry 16). Here, the desired cyclic compound 2a was obtained in lower yield (64%) with concomitant substantial decomposition of the starting allylic alcohol. Such evidence

Unusual behavior in the reactivity of 5-substituted-1H-tetrazoles in a resistively heated microreactor

• Bernhard Gutmann,
• Toma N. Glasnov,
• Tahseen Razzaq,
• Walter Goessler,
• Dominique M. Roberge and
• C. Oliver Kappe

Beilstein J. Org. Chem. 2011, 7, 503–517, doi:10.3762/bjoc.7.59

• is required in the selection of the reactor materials when solutions containing even relatively dilute concentrations of HCl or other Brønsted acids are handled in a flow environment [36]. The corrosion results described earlier involving comparatively benign mixtures of NMP/AcOH/H2O (5:3:2) passed

An efficient and practical entry to 2-amido-dienes and 3-amido-trienes from allenamides through stereoselective 1,3-hydrogen shifts

• Ryuji Hayashi,
• John B. Feltenberger,
• Andrew G. Lohse,
• Mary C. Walton and
• Richard P. Hsung

Beilstein J. Org. Chem. 2011, 7, 410–420, doi:10.3762/bjoc.7.53

• -hydrogen shifts from allenamides are described. These 1,3-hydrogen shifts could be achieved thermally or they could be promoted by the use of Brønsted acids. Under either condition, these processes are highly regioselective in favour of the α-position, and highly stereoselective in favour of the E

• ). There appears to be some solvent effect on the E/Z selectivity with more polar solvents providing the best ratio (Table 1, entries 2–4). In addition to thermal conditions, we screened several Brønsted acids at room temperature in order to investigate a milder condition. While PTSA resulted in poor E/Z

• ratio (Table 1, entry 5), a range of Brønsted acids were quite effective in affording the desired 2-amido-diene 2 (Supporting Information File 1 and Supporting Information File 2) with excellent E/Z selectivity [Table 1, entries 6–9]. After having established the 1,3-hydrogen shift under thermal and

SbCl₃-catalyzed one-pot synthesis of 4,4′-diaminotriarylmethanes under solvent-free conditions: Synthesis, characterization, and DFT studies

• Ghasem Rezanejade Bardajee

Beilstein J. Org. Chem. 2011, 7, 135–144, doi:10.3762/bjoc.7.19

• ][30][31][32][33][34]. The reaction of arylaldehydes with N,N-dimethylaniline is one of the most efficient methods for the synthesis of DTMs. This reaction is usually carried out in the presence of Brønsted acids such as sulfuric acid, hydrochloric acid, methanesulfonic acid, or p-TSA, as well as Lewis

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

• many other Lewis acids including BF3, BeCl2, TiCl4, SbCl5 or SnCl4 have been described as catalysts for the FC alkylation. Furthermore, strong Brønsted-acids including sulfuric acid, hydrofluoric acid or super acids such as HF•SbF5 and HSO3F•SbF5 have also been shown to accelerate this transformation

• variety of Lewis- and Brønsted acids with decreasing catalyst loadings and in consequence increasing efficiencies (Figure 1). With regard to the electrophiles employed in the FC alkylation protected or activated alcohols and styrenes have been found to be suitable alkylating reagents giving access to many

• catalytic FC benzylations using benzyl alcohols have been developed. These utilize for instance Cl2Si(OTf)2, Hf(OTf)4 [8], Yb(OTf)3, La(OTf)3 [9], InCl3 [10][11], NbCl5 [12], heterobimetallic Ir-Sn-catalysts [13][14], H-mont [15], [CpMoCl(CO)3]/o-chloranil [16], strong Brønsted acids [17][18][19] calix[6

Catalytic synthesis of (E)-α,β-unsaturated esters from aldehydes and 1,1-diethoxyethylene

• Rubén Manzano,
• Lidia Ozores,
• Andreas Job,
• Lars Rodefeld and
• Benjamin List

Beilstein J. Org. Chem. 2009, 5, No. 3, doi:10.3762/bjoc.5.3

• ] cycloadditions [28][29][30], Diels-Alder processes [31] and reactions with alcohols [32] and N-formyl imines [33], to afford orthoesters and N-formyl-β-aminoesters respectively. Results and Discussion Initial experiments revealed that Brønsted acids such as acetic acid indeed catalyze the newly designed

• transformation. After screening various catalysts and different reaction conditions, we found that boronic acids [34], especially 2,4,5-trifluorophenylboronic acid, proved to be quite active catalysts of the condensation of benzaldehyde with ketene diethyl acetal (Table 1). Brønsted acids are also effective but