The interplay of configuration and conformation in helical perylenequinones: Insights from chirality induction in liquid crystals and calculations

• Elisa Frezza,
• Silvia Pieraccini,
• Stefania Mazzini,
• Alberta Ferrarini and
• Gian Piero Spada

Beilstein J. Org. Chem. 2012, 8, 155–163, doi:10.3762/bjoc.8.16

• as where p is the helical pitch (in μm) of the cholesteric phase, and c and r are the concentration (molar fraction) and the enantiomeric excess of the dopant, respectively. The sign of HTP is taken as positive or negative if the induced cholesteric is right- or left-handed, respectively. The HTP of

Chimeric self-sufficient P450cam-RhFRed biocatalysts with broad substrate scope

• Aélig Robin,
• Valentin Köhler,
• Alison Jones,
• Afruja Ali,
• Paul P. Kelly,
• Elaine O'Reilly,
• Nicholas J. Turner and
• Sabine L. Flitsch

Beilstein J. Org. Chem. 2011, 7, 1494–1498, doi:10.3762/bjoc.7.173

• turned out to be the most efficient for the epoxidation of the tetrahydropyridine ring, with substrate 7b epoxidised to 8b in 73% conversion. Racemic epoxide 8b was synthesised to confirm the structure of the biotransformation product and to determine the enantiomeric excess. This was found to be 18% for

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

• the determination of the enantiomeric excess of the products. Acknowledgements This work was funded by MICINN (Grant CTQ2008-00947/BQU and Consolider Ingenio 2010 Grant CSD2006-0003), DIUE (Grant 2009SGR623), and ICIQ Foundation. P.O.M. and R. M.-R. thank MICINN for Juan de la Cierva fellowships.

Coupled chemo(enzymatic) reactions in continuous flow

• Ruslan Yuryev,
• Simon Strompen and
• Andreas Liese

Beilstein J. Org. Chem. 2011, 7, 1449–1467, doi:10.3762/bjoc.7.169

• lipase in the presence of the acidic chemocatalysts and for reduction of zeolite-catalyzed side reactions [30][31]. The substrates and supercritical CO2 were fed continuously, thus omitting the use of organic solvents for product extraction. An enantiomeric excess of 97% was achieved for the product (R

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

• up to 91% enantiomeric excess, with either F–TEDA (1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate)) in acetonitrile solution or NFSI (N-fluorobenzenesulfonimide) in dichloromethane solution as fluorinating reagents. The effects of various reaction parameters and of the

• prepared in situ from Me2AlCl and several chiral diols, were not catalytically active. However, the well-established Lewis acid [(R,R-TADDOLato)TiCl2] [82] (Scheme 1) displayed good catalytic activity (5 mol %, 1→1-F in 4 h) and gave (S)-1-F with an enantiomeric excess of 28% (for a complex incorporating

• fluorination by choosing NFSI (N-fluorobenzenesulfonimide) as the reagent with better solubility. Reaction with NFSI in dichloroethane gave 1-F with 24% ee (5 mol % K1, 40 °C, 3 d, 66% conversion), which is close to the enantiomeric excess observed with F–TEDA in acetonitrile at the same temperature (Table 2

Efficient and selective chemical transformations under flow conditions: The combination of supported catalysts and supercritical fluids

• M. Isabel Burguete,
• Eduardo García-Verdugo and
• Santiago V. Luis

Beilstein J. Org. Chem. 2011, 7, 1347–1359, doi:10.3762/bjoc.7.159

• conversion nor enantiomeric excess were observed to degrade over time, under steady-state conditions. Furthermore, with the initial ligand tested (Skewphos) the enantioselectivity achieved was similar to that obtained in conventional solvents (66% ee). Nevertheless, a further screening of a battery of

Bromine–lithium exchange: An efficient tool in the modular construction of biaryl ligands

• Laurence Bonnafoux,
• Frédéric R. Leroux and
• Françoise Colobert

Beilstein J. Org. Chem. 2011, 7, 1278–1287, doi:10.3762/bjoc.7.148

• stoichiometric amount of chiral diamines. An enantiomeric excess of up to 63% was obtained [67]. H. Kagan et al. reported the desymmetrization of prochiral aromatic or vinylic dihalide substrates by halogen–metal exchange in the presence of a stoichiometric amount of diamines, with enantiomeric excess up to 26

Chiral gold(I) vs chiral silver complexes as catalysts for the enantioselective synthesis of the second generation GSK-hepatitis C virus inhibitor

• María Martín-Rodríguez,
• Carmen Nájera,
• José M. Sansano,
• Abel de Cózar and
• Fernando P. Cossío

Beilstein J. Org. Chem. 2011, 7, 988–996, doi:10.3762/bjoc.7.111

• enantiomerically pure cycloadduct 5b (Table 1, entry 6). Attempts to increase the enantioselectivity, in the example run with AgTFA, by replacing triethylamine by diisopropylethylamine (DIPEA) were not successful, and only a slight increment of enantiomeric excess was observed (Table 1, entry 8). Again, the gold

• 0 °C resulting in excellent enantiodiscrimination (99% ee) to the detriment of the reaction time, which had to be increased to 3 days (Table 2, entry 8). The result obtained in this last example was excellent but the enantiomeric excess achieved at room temperature in the reaction performed with (Sa

Cationic gold(I) axially chiral biaryl bisphosphine complex-catalyzed atropselective synthesis of heterobiaryls

• Tetsuro Shibuya,
• Kyosuke Nakamura and
• Ken Tanaka

Beilstein J. Org. Chem. 2011, 7, 944–950, doi:10.3762/bjoc.7.105

• furnished 2a with the highest enantiomeric excess (Table 1, entry 3). An increase in the steric bulk of the aryl group on the phosphorus atom of H8-BINAP lead to a decrease in the ee (Table 1, entry 4). The use of sterically more demanding (R)-DTBM-Segphos as a ligand furnished 2a in high yield with the

Complete transfer of chirality in an intramolecular, thermal [2 + 2] cycloaddition of allene-ynes to form non-racemic spirooxindoles

• Kay M. Brummond and
• Joshua M. Osbourn

Beilstein J. Org. Chem. 2011, 7, 601–605, doi:10.3762/bjoc.7.70

• contains two doublets at δ 7.54 and 7.52, while the spectrum of enantiomerically enriched 7 shows only a single doublet at δ 7.54. Based on this result, the enantiomeric excess of enantiomerically enriched acetate 7 is greater than 95%. Next, the focus turned to the conversion of enantiomerically enriched

• 1.23 ppm and a second singlet at δ 1.20 ppm. The enantiomerically enriched compound 8 shows one singlet, corresponding to the resonance for the tert-butyl group at δ 1.22 ppm; thus the enantiomeric excess of allenyloxindole 8 is greater than 95%. With the enantiomerically enriched allene-yne 8 in hand

• . Based on this analysis, the product was formed with greater than 95% enantiomeric excess (Figure 3). Our working hypothesis for the mechanism for transfer of chiral information from the allene to the spirooxindole-containing cyclobutene is that the reaction still proceeds through the thermally generated

Oxidative allylic rearrangement of cycloalkenols: Formal total synthesis of enantiomerically pure trisporic acid B

• Silke Dubberke,
• Muhammad Abbas and
• Bernhard Westermann

Beilstein J. Org. Chem. 2011, 7, 421–425, doi:10.3762/bjoc.7.54

• (+)-β-ketoacids decarboxylate and racemize to cyclohexenones 2 during the enzymatic reaction and workup (Scheme 1) [12][14]. Treatment of (±)-1c in phosphate buffer at pH 7.0 led to selective saponification of the (S)-isomer (+)-1c to give rac-2c. The enantiomeric excess of (−)-1c was monitored during

• the reaction, and after completion of the reaction the enantiomeric excess was determined to be >99% by GLC using a cyclodextrin modified stationary phase (LIPODEX E). The R-configuration at the stereocenter was established by chemical correlation [10][14]. The first step towards the synthesis of

Supramolecular FRET photocyclodimerization of anthracenecarboxylate with naphthalene-capped γ-cyclodextrin

• Qian Wang,
• Cheng Yang,
• Gaku Fukuhara,
• Tadashi Mori,
• Yu Liu and
• Yoshihisa Inoue

Beilstein J. Org. Chem. 2011, 7, 290–297, doi:10.3762/bjoc.7.38

• modification of γ-CD causes a dramatic switching of stereoselectivity in AC photodimerization [12][22]. Thus, by using native γ-CD the chiral cyclodimer 2 is obtained in 41% enantiomeric excess (ee), whereas biphenyl-capped γ-CD 5 yields antipodal 2 in −57% ee (Scheme 1) [22]: Where the positive/negative sign

Effects of anion complexation on the photoreactivity of bisureido- and bisthioureido-substituted dibenzobarrelene derivatives

• Heiko Ihmels and
• Jia Luo

Beilstein J. Org. Chem. 2011, 7, 278–289, doi:10.3762/bjoc.7.37

• . For example, the achiral derivative 1b crystallizes in the chiral space group P212121, and irradiation of the chiral crystals gives dibenzosemibullvalene 2b with a high enantiomeric excess, >95% ee [33]. Since achiral molecules crystallize only very rarely in chiral space groups, the ionic chiral

• dibenzobarrelene derivative 1c which forms the chiral ammonium carboxylate 1c-P with (S)-proline (Scheme 2). After irradiation, acidic workup and subsequent esterification with diazomethane, the dibenzosemibullvalene 2c was obtained with high enantiomeric excess (>95% ee) [35][36]. Interestingly, several

Achiral bis-imine in combination with CoCl₂: A remarkable effect on enantioselectivity of lipase-mediated acetylation of racemic secondary alcohol

• K. Arunkumar,
• M. Appi Reddy,
• T. Sravan Kumar,
• B. Vijaya Kumar,
• K. B. Chandrasekhar,
• P. Rajender Kumar and
• Manojit Pal

Beilstein J. Org. Chem. 2010, 6, 1174–1179, doi:10.3762/bjoc.6.134

• achieve better selectivity, we assessed the use of achiral bis-imines in combination with CoCl2 (Table 2). The reactions were complete within 10 h when diarylidene-ethane-1,2-diamines were used (entries 1–8, Table 2). While 35% enantiomeric excess was achieved in some of these cases (entries 2, 3 and 5

• , Table 2), the best results, however, were obtained with bis(heteroarylmethylene)ethane-1,2-diamines (entries 9 and 10, Table 2), especially 3i. The bis-imine 3i facilitated enantioselective acetylation of the (R)-isomer over the (S)-antipode with high enantiomeric excess (91% ees) and yield (80%). The

Kinetics and mechanism of vanadium catalysed asymmetric cyanohydrin synthesis in propylene carbonate

• Michael North and
• Marta Omedes-Pujol

Beilstein J. Org. Chem. 2010, 6, 1043–1055, doi:10.3762/bjoc.6.119

• –90% enantiomeric excess at room temperature with just 0.1 mol % of catalyst [11]. Complex 1 also catalyses the asymmetric addition of other cyanide sources including potassium cyanide [12][13][14][15], cyanoformates [15][16][17][18][19][20] and acyl cyanides [17][19][21] to aldehydes, and will accept

• concentration of 0.56 M. In each case, the enantiomeric excess of the cyanohydrin product was determined by chiral GC after conversion of the trimethylsilyl ether into the corresponding acetate by the method of Kagan [80], a process which is known to cause no racemization (Scheme 3). The results of this study

• are presented in Table 1. It is apparent from Table 1, that for reactions catalysed by titanium based catalyst 1, changing the solvent to propylene carbonate had a severely detrimental effect on the enantioselectivity of the reactions. In some cases, the enantiomeric excess of the cyanohydrin was more

Development of dynamic kinetic resolution on large scale for (±)-1-phenylethylamine

• Lisa K. Thalén and
• Jan-E. Bäckvall

Beilstein J. Org. Chem. 2010, 6, 823–829, doi:10.3762/bjoc.6.97

• % isolated yield with 97% ee (Table 2, entry 8). Further attempts to find appropriate conditions at high concentrations (1–3 M) resulted in significant byproduct formation. Since it proved difficult to obtain both a high enantiomeric excess value and a high yield at a reduced catalytic loading, and that the

• be observed. The concentration of the reaction had a large impact on the rate of uncatalyzed acylation. This could be seen in the decrease of enantiomeric excess observed when going from a concentration of 0.2 M to 0.5 M (Table 3, entries 2 and 4). Also, a decrease in the amount of enzyme to

• compensate for the decrease in loading of racemization catalyst gave both a better enantiomeric excess value and a better yield (Table 3, entries 2–3). Addition of 1 equiv acyl donor 7, followed by addition of 0.5 equiv after 24 hours led to unsatisfactory ee values (Table 3, entry 2–4). By only adding 0.1

Synthesis of the fluorescent amino acid rac-(7-hydroxycoumarin-4-yl)ethylglycine

• Manfred Braun and
• Torsten Dittrich

Beilstein J. Org. Chem. 2010, 6, No. 69, doi:10.3762/bjoc.6.69

• transfer catalysis [16]. However, only an insignificant enantiomeric excess was observed in the alkylation product 7 when a representative protocol was applied [17]. Finally, the three protecting groups, the tert-butyl ester, the imine, and the silyl ether, were removed in a single step by hydrolysis with

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

• observed with this reagent. Umemoto was first to report, in 1994, an enantioselective electrophilic trifluoromethylation of a ketone enolate mediated by a chiral borepin derived from a binaphthol with S-(trifluoromethyl)dibenzothiophenium tetrafluoroborate 5b. The best enantiomeric excess was 45% for 20

Molecular recognition of organic ammonium ions in solution using synthetic receptors

• Andreas Späth and
• Burkhard König

Beilstein J. Org. Chem. 2010, 6, No. 32, doi:10.3762/bjoc.6.32

• separation problems several approaches are available. A general trend is observable: 18-crown-6-systems reveal higher binding constants then 15-crown-5-systems, due to the better size fit of the guest ion. Aromatic substituents lead to better recognition and enantiomeric excess (up to 70%) with aromatic

Concise methods for the synthesis of chiral polyoxazolines and their application in asymmetric hydrosilylation

• Wei Jie Li,
• Zun Le Xu and
• Sheng Xiang Qiu

Beilstein J. Org. Chem. 2010, 6, No. 29, doi:10.3762/bjoc.6.29

• spectrometer. Mass spectra were taken on a LCQ DECA XP LC/MS system or a VG ZAB-HS mass spectrometer. Elemental analyses were carried out on a Perkin-Elmer 240C elemental analyzer. Optical rotation values were measured on a Polartronic HNQW 5 polarimeter. Enantiomeric excess (ee) was determined by HPLC

• determined by optical rotation and its enantiomeric excess was determined by HPLC analysis with chiral stationary phases. Chemical structures of polyoxazolines. The conditions and results of the reaction of polycarboxylic acids or their esters with chiral β-amino alcohols. Enantioselective Rh(I)-catalyzed

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

• , while the diastereoselectivity was higher when TFA was used. The enantiomeric excess did not significantly diminish during this procedure, which makes this reaction a convenient and efficient route to optically pure 1,1,2-triarylalkanes. A drawback of this first diastereoselective FC alkylation of

Can we measure catalyst efficiency in asymmetric chemical reactions? A theoretical approach

• Shaimaa El-Fayyoumy,
• Matthew H. Todd and
• Christopher J. Richards

Beilstein J. Org. Chem. 2009, 5, No. 67, doi:10.3762/bjoc.5.67

• such a comparison. We propose that a catalyst is more efficient if fewer atoms are utilised to give a product in a required enantiomeric excess. We illustrate this concept by analysing several well-known asymmetric catalytic chemical reactions carried out in academic laboratories, and compare small

• applied to an asymmetric catalyst. Discussion Definition of Asymmetric Catalyst Efficiency (ACE) The enantiomeric excess (ee) of a product and the yield of the corresponding reaction are crucial factors in defining “success”, and essentially describe the amount of major enantiomer produced. Clearly also

• catalyst is that used in a hydrogenation reaction, which tallies well with this method’s extensive industrial usage. An industrial example is shown in entry 4. This is interesting since it illustrates that a high enantiomeric excess need not be the only criterion by which a catalyst is judged: this

Continuous flow enantioselective arylation of aldehydes with ArZnEt using triarylboroxins as the ultimate source of aryl groups

• Julien Rolland,
• Xacobe C. Cambeiro,
• Carles Rodríguez-Escrich and
• Miquel A. Pericàs

Beilstein J. Org. Chem. 2009, 5, No. 56, doi:10.3762/bjoc.5.56

• , with a HP-5 column. Enantiomeric excess was determined by HPLC analysis of the pure product with an AD-H column. Exact conditions for this and other products are given in Supporting Information File 1. General procedure for the phenylation of tolualdehyde in continuous flow conditions The continuous

Chiral amplification in a cyanobiphenyl nematic liquid crystal doped with helicene-like derivatives

• Alberta Ferrarini,
• Silvia Pieraccini,
• Stefano Masiero and
• Gian Piero Spada

Beilstein J. Org. Chem. 2009, 5, No. 50, doi:10.3762/bjoc.5.50

• propensity of a dopant to induce a helical organization in the LC matrix is then quantified by its helical twisting power (HTP), which is defined as [2][3]: where p is the helical pitch of the cholesteric phase and c and r are the concentration (molar fraction) and the enantiomeric excess of the dopant

2-Phenyl- tetrahydropyrimidine- 4(1H)-ones – cyclic benzaldehyde aminals as precursors for functionalised β²-amino acids

• Markus Nahrwold,
• Arvydas Stončius,
• Anna Penner,
• Beate Neumann,
• Hans-Georg Stammler and
• Norbert Sewald

Beilstein J. Org. Chem. 2009, 5, No. 43, doi:10.3762/bjoc.5.43

• , the determined optical activity of 16 corresponds well to its expected enantiomeric excess of 89% and doubtlessly confirms the anticipated stereochemistry of the alkylation reaction. Conclusion Within this article, novel procedures are presented to efficiently synthesise 2-phenyl-substituted