Mechanochemistry assisted asymmetric organocatalysis: A sustainable approach

• Pankaj Chauhan and
• Swapandeep Singh Chimni

Beilstein J. Org. Chem. 2012, 8, 2132–2141, doi:10.3762/bjoc.8.240

• . Alkylation of imine Alkylation of glycine imine by using a chiral phase-transfer catalyst emerged as a very good strategy for the asymmetric synthesis of amino-acid derivatives [59][60][61]. Lamaty and co-workers prepared a series of glycine Schiff bases 22 in excellent yield (97–98%) in short reaction time

• by milling protected glycine hydrochloride 21 and diphenylmethanimine (20), under solvent-free conditions (Scheme 11) [62]. The asymmetric alkylation of glycine imine 22 was carried out by using a phase-transfer catalyst under basic conditions in a ball-mill. The Schiff base reacted rapidly with

• various halogenated derivatives 23 in a ball-mill in the presence of KOH and the chiral ammonium salt derived from cinchonidine (XIII) as phase-transfer catalyst, to provide excellent yield (91–97%) and good enantioselectivity (35–75% ee) of the corresponding amino esters 24. Purification of the product

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

• -pyrazolines starting from α,β-unsaturated ketones and phenylhydrazine or N-tert-butyloxycarbonylhydrazine in the presence of a chiral Brønsted acid or a phase-transfer catalyst [70][71]. Compared with monosubstituted hydrazines in organocatalytic asymmetric synthesis, disubstituted hydrazines were also

Synthesis and oxidation of some azole-containing thioethers

• Andrei S. Potapov,
• Nina P. Chernova,
• Vladimir D. Ogorodnikov,
• Tatiana V. Petrenko and
• Andrei I. Khlebnikov

Beilstein J. Org. Chem. 2011, 7, 1526–1532, doi:10.3762/bjoc.7.179

• -ethanedithiol. The benzotriazole-containing thioether 5 was prepared following a similar strategy to that for product 3 by using 1-chloromethylbenzotriazole as electrophile (Scheme 1). The mixture of aqueous Na2S and NaOH solution with acetonitrile is biphasic, and we found that the addition of a phase-transfer

• catalyst expedites the nucleophilic substitution and reduces the formation of side-products. The formation of compound 5 was observed earlier by Katritzky et al. as a side product (11% yield) during the preparation of 1-mercaptomethylbenzotriazole [14]. The product yield achieved by the procedure proposed

Novel carbazole–pyridine copolymers by an economical method: synthesis, spectroscopic and thermochemical studies

• Aamer Saeed,
• Madiha Irfan and
• Shahid Ameen Samra

Beilstein J. Org. Chem. 2011, 7, 638–647, doi:10.3762/bjoc.7.75

• of tetrabutylammonium bromide (TBAB) as a phase-transfer catalyst [23][24][25]. The N-alkylcarbazoles were subsequently subjected to Friedel–Crafts acetylation at the active positions of carbazole, viz. 3 and 6, with anhydrous aluminum chloride in dry chloroform [26]. The monomers were purified by

A rapid and efficient synthetic route to terminal arylacetylenes by tetrabutylammonium hydroxide- and methanol-catalyzed cleavage of 4-aryl-2-methyl-3-butyn-2-ols

• Jie Li and
• Pengcheng Huang

Beilstein J. Org. Chem. 2011, 7, 426–431, doi:10.3762/bjoc.7.55

• , NaOAc, KOH and K2CO3, no deprotected product 2a was detected by TLC even after 24 h (Table 1, entries 1–5). Terminal arylacetylene 2a was obtained in good yield only by NaOH with the addition of water and Bu4NI (the phase transfer catalyst), but this reaction required 23 h (Table 1, entry 6). Using

The preparation of 3-substituted-1,5-dibromopentanes as precursors to heteracyclohexanes

• Bryan Ringstrand,
• Martin Oltmanns,
• Jeffrey A. Batt,
• Aleksandra Jankowiak,
• Richard P. Denicola and
• Piotr Kaszynski

Beilstein J. Org. Chem. 2011, 7, 386–393, doi:10.3762/bjoc.7.49

• olefination (Method 2B, Scheme 2). These routes promise the minimal number of steps and ease of chemical transformations. Results Preparation of dibromides Dibromides 1a–1d were prepared in 67–91% yield in refluxing 48% HBr with tributylhexadecylphosphonium bromide [44] as a phase-transfer catalyst either

• preparation of diols VII and tetrahydropyrans VIII. Both VII and VIII can be efficiently converted to dibromides I using HBr and tributylhexadecylphosphonium bromide as the phase-transfer catalyst. Diols VII are obtained by reduction of a 3-substituted glutaric acid or ester. The latter can be prepared in a

A two step synthesis of a key unit B precursor of cryptophycins by asymmetric hydrogenation

• Benedikt Sammet,
• Mathilde Brax and
• Norbert Sewald

Beilstein J. Org. Chem. 2011, 7, 243–245, doi:10.3762/bjoc.7.32

• sulfate. A completely different route to unit B precursor 8 (Scheme 2) is based on a phase transfer catalyst (PTC) mediated asymmetric alkylation. However, the required cinchonine derived chiral catalyst is not commercially available [9]. Results and Discussion We envisaged a two step synthesis for the

• %). Unit B synthesis by a chiral PTC approach. Reagents and conditions [9]: a) N-(Diphenylmethylene)glycine tert-butyl ester, 50% KOH, toluene/CHCl3, chiral phase transfer catalyst (0.01 equiv), 0 °C, 20 h (87%; 96% ee); b) 15% citric acid, THF, rt, 16 h; c) FmocCl, Na2CO3, THF, rt, 14 h, (72% over two

Synthesis of cationic dibenzosemibullvalene-based phase-transfer catalysts by di-π-methane rearrangements of pyrrolinium-annelated dibenzobarrelene derivatives

• Heiko Ihmels and
• Jia Luo

Beilstein J. Org. Chem. 2011, 7, 119–126, doi:10.3762/bjoc.7.17

• pyrrolidinium-annelated dibenzosemibullvalene may act as phase-transfer catalyst in alkylation reactions. Keywords: di-π-methane rearrangement; dibenzobarrelenes; dibenzosemibullvalenes; phase-transfer catalysis; photochemistry; polycylic compounds; Introduction The di-π-methane (DPM) rearrangement is among

• of an enantiopure pyDBS derivative to act as phase-transfer catalyst in stereoselective reactions may be considered in long-term studies. Herein we demonstrate that the pyrrolidinium-annelated dibenzosemibullvalene structure is indeed available via the DPM rearrangement of appropriately substituted

• dibenzobarrelene derivatives and that such a compound may act as phase-transfer catalyst in alkylation reactions. Note that according to the IUPAC rules the compounds presented in the following should be classified as pyrrolidinium or pyrrolinium derivatives because of the higher priority of the cationic

Shelf-stable electrophilic trifluoromethylating reagents: A brief historical perspective

• Norio Shibata,
• Andrej Matsnev and
• Dominique Cahard

Beilstein J. Org. Chem. 2010, 6, No. 65, doi:10.3762/bjoc.6.65

• one of us (D.C.) with reagents of type 5 in order to provide milder conditions. Indeed, cyclic and acyclic β-ketoesters were efficiently trifluoromethylated with S-(trifluoromethyl)dibenzothiophenium tetrafluoroborate in the presence of a phase-transfer catalyst to afford the corresponding α

Synthesis of oxa-bridged derivatives from Diels–Alder bis-adducts of butadiene and 1,2,3,4-tetrahalo-5,5-dimethoxycyclopentadiene

• Faiz Ahmed Khan and
• Karuppasamy Parasuraman

Beilstein J. Org. Chem. 2010, 6, No. 64, doi:10.3762/bjoc.6.64

• reaction was carried out in presence of the phase transfer catalyst TBHSO4 the bis-oxa-bridged compounds 8 and 10 were obtained (after esterification with diazomethane) in 31 and 37%, respectively (Scheme 3). The relative stereochemistry in 8 was unambiguously established by the single crystal X-ray

Enantioselective nucleophilic difluoromethylation of aromatic aldehydes with Me₃SiCF₂SO₂Ph and PhSO₂CF₂H reagents catalyzed by chiral quaternary ammonium salts

• Chuanfa Ni,
• Fang Wang and
• Jinbo Hu

Beilstein J. Org. Chem. 2008, 4, No. 21, doi:10.3762/bjoc.4.21

• reported the nucleophilic difluoromethylation of aromatic aldehydes with PhSO2CF2H under the phase transfer condition using Aliquat 336 (a commercially available quaternary ammonium salt) as the phase transfer catalyst [35]. With most aldehydes, the reaction affords moderate to excellent yields of products

• the ability of known chiral ammonium salts to promote the enantioselective difluoromethylation of aldehydes with PhSO2CF2H. In a preliminary study, we examined the reaction with the chiral quaternary ammonium salt 6a (see Scheme 1) as the phase transfer catalyst at room temperature, with 30% NaOH as

Asymmetric aza-Diels- Alder reaction of Danishefsky's diene with imines in a chiral reaction medium

• Bruce Pégot,
• Olivier Nguyen Van Buu,
• Didier Gori and
• Giang Vo-Thanh

Beilstein J. Org. Chem. 2006, 2, No. 18, doi:10.1186/1860-5397-2-18

• very important for chirality transfer. This had already been reported by Colonna and co-workers[34][35] in the borohydride asymmetric reduction of carbonyl compounds using a chiral phase transfer catalyst. This observation is supported by our studies in the asymmetric Baylis-Hillman reaction.[25] Thus