Chiral Recognition of N‐Phthaloyl,N‐Tetrachlorophthaloyl,and N‐Naphthaloyl α‐Amino Acids and Their Esters on Polysaccharide‐Derived Chiral Stationary Phases

Enantiomeric separations of N‐phthaloyl (N‐PHT), N‐tetrachlorophthaloyl (N‐TCPHT), and N‐naphthaloyl (N‐NPHT) α‐amino acids and their esters were examined on several kinds of polysaccharide‐derived chiral stationary phases (CSPs). Resolution capability of CSPs was greater Chiralcel OF than the others for N‐PHT and N‐NPHT α‐amino acids and their esters. In N‐TCPHT α‐amino acids and their esters, good enantioselectivities showed Chiralcel OG for N‐TCPHT α‐amino acids, Chiralpak AD for N‐TCPHT α‐amino acid methyl esters, and Chiralcel OD for N‐TCPHT α‐amino acid ethyl esters, respectively. From the results of liquid chromatography and computational chemistry, it is concluded that l ‐form is preferred and more retained with electrostatic interaction in case of interaction between N‐PHT α‐amino acid derivatives and Chiralcel OF, N‐TCPHT α‐amino acid derivatives and Chiralcel OD, and N‐NPHT α‐amino acid derivatives and Chiracel OF. On the other hand, d ‐form is preferred and more retained with van der Waals interaction in case of interaction between N‐TCPHT α‐amino acid ester derivatives and Chiralcel OG and Chiralpak AD. Chirality 24:1037–1046, 2012. © 2012 Wiley Periodicals, Inc.     

Determination of absolute configuration in 4‐aryl‐3,4‐dihydro‐2(1H)‐pyrimidones by high performance liquid chromatography and CD spectroscopy

The absolute configuration of three 4‐aryl‐3,4‐dihydro‐2(1H)‐pyrimidones (Biginelli compounds, DHPMs) was established by comparison of the typical circular dichroism (CD) spectra of individual enantiomers with reference samples of known absolute configuration. The enantiomers were obtained by semipreparative separation of racemic mixtures on a Chiralcel OD‐H chiral stationary phase. The method was used to establish the enantiopreference of various lipases in biocatalytic kinetic resolution experiments employing activated DHPM esters. Chirality 11:659–662, 1999. © 1999 Wiley‐Liss, Inc.     

Enantiomeric resolution of asymmetric glycol by a lipase‐catalyzed transesterification reaction

The lipase catalyzed enantiomeric resolution of syn‐glycol was carried out to confirm the sector method, which can determine the absolute configuration of anti‐ and syn‐glycol from the ¹H‐NMR spectra of bis‐2‐methoxy‐2‐trifluoromethyl‐2‐phenylacetic acid (MTPA) esters. The lipase catalyzed transesterification reaction was most reactive at the C2 position (C2–OH) of (2R;3R)‐2,3‐octanediol. Both (2S;3S)‐ and (2R;3R)‐2,3‐octanediol were prepared using lipase. The ¹H‐NMR spectra of their bis‐(R)‐MTPA esters agreed well with those prepared previously via mono‐(R)‐MTPA esters. The result suggests the retention of the Mosher plane in MTPA esters possessing a hydroxyl group at the β position. The reaction rate and the stereoselectivity decreased at C2–OH with the addition of 18‐crown‐6. Chirality 11:338–342, 1999. © 1999 Wiley‐Liss, Inc.     

Lewis acid catalysed methylation of N‐(9H‐fluoren‐9‐yl)methanesulfonyl (Fms) protected lipophilic α‐amino acid methyl esters

This work reports an efficient Lewis acid catalysed N‐methylation procedure of lipophilic α‐amino acid methyl esters in solution phase. The developed methodology involves the use of the reagent system AlCl₃/diazomethane as methylating agent and α‐amino acid methyl esters protected on the amino function with the (9H‐fluoren‐9‐yl)methanesulfonyl (Fms) group. The removal of Fms protecting group is achieved under the same conditions to those used for Fmoc removal. Thus the Fms group can be interchangeable with the Fmoc group in the synthesis of N‐methylated peptides using standard Fmoc‐based strategies. Finally, the absence of racemization during the methylation reaction and the removal of Fms group were demonstrated by synthesising a pair of diastereomeric dipeptides. Copyright © 2015 European Peptide Society and John Wiley & Sons, Ltd.     

Rapid ester biosynthesis screening reveals a high activity alcohol‐O‐acyltransferase (AATase) from tomato fruit

Ethyl and acetate esters are naturally produced in various yeasts, plants, and bacteria. The biosynthetic pathways that produce these esters share a common reaction step, the condensation of acetyl/acyl‐CoA with an alcohol by alcohol‐O‐acetyl/acyltransferase (AATase). Recent metabolic engineering efforts exploit AATase activity to produce fatty acid ethyl esters as potential diesel fuel replacements as well as short‐ and medium‐chain volatile esters as fragrance and flavor compounds. These efforts have been limited by the lack of a rapid screen to quantify ester biosynthesis. Enzyme engineering efforts have also been limited by the lack of a high throughput screen for AATase activity. Here, we developed a high throughput assay for AATase activity and used this assay to discover a high activity AATase from tomato fruit, Solanum lycopersicum (Atf‐S.l). Atf1‐S.l exhibited broad specificity towards acyl‐CoAs with chain length from C₄ to C₁₀ and was specific towards 1‐pentanol. The AATase screen also revealed new acyl‐CoA substrate specificities for Atf1, Atf2, Eht1, and Eeb1 from Saccharomyces cerevisiae, and Atf‐C.m from melon fruit, Cucumis melo, thus increasing the pool of characterized AATases that can be used in ester biosynthesis of ester‐based fragrance and flavor compounds as well as fatty acid ethyl ester biofuels.     

Practical synthesis of novel phosphonopeptides containing AibP

Practical and convenient method for the synthesis of novel phosphonopeptides 8a–h and 10a–f incorporating the quaternary α‐aminophosphonate 6 is reported. The target compounds were prepared in moderate to good yield based on the preparation of the quaternary α‐aminophosphonate 6 followed by the formation of the α‐bromoamide 7 and subsequent nucleophilic substitution reaction with several amines or by peptide bond formation with several amino acids and with the quaternary α‐aminophosphonate 6 , using isobutyl chloroformate as activating agent under racemization‐free condition.     

Application of NMR spectroscopy for assignment of the absolute configuration of 8‐tert‐butyl‐2‐hydroxy‐7‐methoxy‐8‐methyl‐9‐oxa‐6‐azaspiro[4.5]dec‐6‐en‐10‐one

In order to assign the absolute configurations of 8‐tert‐butyl‐2‐hydroxy‐7‐methoxy‐8‐methyl‐9‐oxa‐6‐azaspiro[4.5]dec‐6‐en‐10‐one ( 2a , 2b ), their esters ( 5a , 5b , 5c , 5d ) with (R)‐ or (S)‐2‐methoxyphenylacetic acid ( 4a , 4b ) have been synthesized. The absolute configurations of these compounds have been determined on the basis of NOESY correlations between the protons of the tert‐butyl group and the cyclopentane fragment of the molecules. The crucial part of this analysis was assignment of the absolute configuration at C‐5. Additionally, by calculation of the chemical shift anisotropy, δ^RS, for the relevant protons, it was also possible to confirm the absolute configurations at the C‐2 centres of compounds 2a , 2b and 5a , 5b , 5c , 5d . Chirality, 25:422–426, 2013.© 2013 Wiley Periodicals, Inc.     

Comparative studies between covalently immobilized and coated chiral stationary phases based on polysaccharide derivatives for enantiomer separation of N‐protected α‐amino acids and their ester derivatives

Liquid chromatographic enantiomer separation of several N‐benzyloxycarbonyl (CBZ) and N‐tert‐butoxycarbonyl (BOC) α‐amino acids and their corresponding ethyl esters was performed on covalently immobilized chiral stationary phases (CSPs) (Chiralpak IA and Chiralpak IB) and coated‐type CSPs (Chiralpak AD and Chiralcel OD) based on polysaccharide derivatives. The solvent versatility of the covalently immobilized CSPs in enantiomer separation of N‐CBZ and BOC‐α‐amino acids and their ester derivatives was shown and the chromatographic parameters of their enantioselectivities and resolution factors were greatly influenced by the nature of the mobile phase. In general, standard mobile phases using 2‐propanol and hexane on Chiralpak IA showed fairly good enantioselectivities for resolution of N‐CBZ and BOC‐α‐amino acids and their esters. However, 50% MTBE/hexane (v/v) for resolution of N‐CBZ‐α‐amino acids ethyl esters and 20% THF/hexane (v/v) for resolution of N‐BOC‐α‐amino acids ethyl esters afforded the greatest enantioselectivities on Chiralpak IA. Also, liquid chromatographic comparisons of the enantiomer resolution of these analytes were made on amylose tris(3,5‐dimethylphenylcarbamate)‐derived CSPs (Chiralpak IA and Chiralpak AD) and cellulose tris(3,5‐dimethylphenylcarbamate)‐derived CSPs (Chiralpak IB and Chiralcel OD). Chiralpak AD and/or Chiralcel OD showed the highest enantioselectivities for resolution of N‐CBZ‐α‐amino acids and esters, while Chiralpak AD or Chiralpak IA showed the highest resolution of N‐BOC‐α‐amino acids and esters. Chirality 2009. © 2008 Wiley‐Liss, Inc.     

Efficient one‐pot synthesis of N‐ethyl amino acids

Mono‐N‐ethylated α‐amino acid esters are obtained in high yields using reductive amination procedures. Formation of imine is achieved by excess of acetaldehyde, followed by removal of acetaldehyde and reduction by NaBH(OAc)₃. The elaborated one‐pot synthesis allows for the efficient synthesis of side‐chain protected amino acid derivatives. Copyright © 1999 European Peptide Society and John Wiley & Sons, Ltd.     

Identification and Synthesis of Branched Wax‐type Esters,Novel Surface Lipids from the Spider Argyrodes elevatus (Araneae: Theridiidae)

The analysis of cuticular extracts from the kleptoparasitic spider Argyrodes elevatus revealed the presence of unusual esters, new for arthropods. These novel compounds proved to be methyl‐branched long‐chain fatty acid esters with methyl branches located either close or remote from the internally located ester group. The GC/MS analysis of the prosoma lipid blend from the male cuticle contained one major component, undecyl 2‐methyltridecanoate ( 1 ). In contrast, four major wax‐type esters, 2‐methylundecyl 2,8‐dimethylundecanoate ( 2 ), 2,8‐dimethylundecyl 2,8‐dimethylundecanoate ( 3 ), heptadecyl 4‐methylheptanoate ( 4 ), and 14‐methylheptadecyl 4‐methylheptanoate ( 5 ), were identified in the lipid blend of female prosomata. Structure assignments were based on mass spectra, gas chromatographic retention indices, and microderivatization. Unambiguous proof of postulated structures was ensured by an independent synthesis of all five esters. Preferentially, odd‐numbered carbon chains pointed to a distinct biosynthetic pathway, different from that of common fatty acids, because one or two C₃ starter units are incorporated during the biosynthesis of all acid and alcohol building blocks present in the five esters. The striking sexual dimorphism together with the unique biosynthesis points to a function of the esters in chemical communication of the spiders, although no behavioral data are currently available to test this assumption.     

(S)‐6‐Bromo‐BINOL‐based phosphoramidite ligand with C1 symmetry for enantioselective hydrogenation and allylic substitution

(S)‐6‐Br‐BINOL‐derived phosphoramidite, a s imple monodentate ligand with a stereogenic center at the phosphorus atom, was synthesized for the first time. This stereoselector generated a high level of enantioselectivity (80–95% ee) in the rhodium‐catalyzed hydrogenation of α‐dehydrocarboxylic acid esters and was also successfully employed in the asymmetric palladium‐catalyzed allylic substitution of (E)‐1,3‐diphenylallyl acetate. The optical yield also showed significant dependence with reaction type: up to 70% ee for allylic amination, up to 75% ee for allylic sulfonylation, and up to 90% ee for allylic alkylation. Chirality, 2010. © 2010 Wiley‐Liss, Inc.     

Enantioseparation of Racemic Flurbiprofen by Aqueous Two‐Phase Extraction With Binary Chiral Selectors of L‐dioctyl Tartrate and L‐tryptophan

A novel method for chiral separation of flurbiprofen enantiomers was developed using aqueous two‐phase extraction (ATPE) coupled with biphasic recognition chiral extraction (BRCE). An aqueous two‐phase system (ATPS) was used as an extracting solvent which was composed of ethanol (35.0% w/w) and ammonium sulfate (18.0% w/w). The chiral selectors in ATPS for BRCE consideration were L‐dioctyl tartrate and L‐tryptophan, which were screened from amino acids, β‐cyclodextrin derivatives, and L‐tartrate esters. Factors such as the amounts of L‐dioctyl tartrate and L‐tryptophan, pH, flurbiprofen concentration, and the operation temperature were investigated in terms of chiral separation of flurbiprofen enantiomers. The optimum conditions were as follows: L‐dioctyl tartrate, 80 mg; L‐tryptophan, 40 mg; pH, 4.0; flurbiprofen concentration, 0.10 mmol/L; and temperature, 25 °C. The maximum separation factor α for flurbiprofen enantiomers could reach 2.34. The mechanism of chiral separation of flurbiprofen enantiomers is discussed and studied. The results showed that synergistic extraction has been established by L‐dioctyl tartrate and L‐tryptophan, which enantioselectively recognized R‐ and S‐enantiomers in top and bottom phases, respectively. Compared to conventional liquid–liquid extraction, ATPE coupled with BRCE possessed higher separation efficiency and enantioselectivity without the use of any other organic solvents. The proposed method is a potential and powerful alternative to conventional extraction for separation of various enantiomers. Chirality 27:650–657, 2015. © 2015 Wiley Periodicals, Inc.     

Occurrence of Fatty Acid Short‐Chain‐Alkyl Esters in Fruits of Celastraceae Plants

Small amounts of a mixture of fatty acid short‐chain‐alkyl esters (FASCAEs) were obtained from the fruits of twelve plant species of Celastraceae family, and in five of them the FASCAEs were present not only in the arils but also in the seeds. These mixtures contained 32 individual FASCAE species, which formed four separate fractions, viz. FA methyl, ethyl, isopropyl, and butyl esters (FAMEs, FAEEs, FAIPEs, and FABEs, resp.). The FASCAE acyl components included the residues of 16 individual C₁₄–C₂₄ saturated, mono‐, di‐, and trienoic FAs. Linoleic, oleic, and palmitic acids, and, in some cases, also α‐linolenic acid predominated in FAMEs and FAEEs, while myristic acid was predominant in FAIPEs. It can be suggested that, in the fruit arils of some plant species, FAMEs and FAEEs were formed at the expense of a same FA pool characteristic of a given species and were strongly different from FAIPEs and FABEs esters regarding the mechanism of their biosynthesis. However, as a whole, the qualitative and quantitative composition of various FASCAE fractions, as well as their FA composition, varied considerably depending on various factors. Therefore, separate FASCAE fractions seem to be synthesized from different FA pools other than those used for triacylglycerol formation.     

The conformational properties of α,β‐dehydroamino acids with a C‐terminal ester group

α,β‐Dehydroamino acid esters occur in nature. To investigate their conformational properties, a systematic theoretical analysis was performed on the model molecules Ac‐ΔXaa‐OMe [ΔXaa = ΔAla, (E)‐ΔAbu, (Z)‐ΔAbu, ΔVal] at the B3LYP/6‐311+ + G(d,p) level in the gas phase as well as in chloroform and water solutions with the self‐consistent reaction field‐polarisable continuum model method. The Fourier transform IR spectra in CCl₄ and CHCl₃ have been analysed as well as the analogous solid state conformations drawn from The Cambridge Structural Database. The ΔAla residue has a considerable tendency to adopt planar conformations C5 (?, ψ ≈ ? 180°, 180°) and β2 (?, ψ ≈ ? 180°, 0°), regardless of the environment. The ΔVal residue prefers the conformation β2 (?, ψ ≈ ? 120°, 0°) in a low polar environment, but the conformations α (?, ψ ≈ ? 55°, 35°) and β (?, ψ ≈ ? 55°, 145°) when the polarity increases. The ΔAbu residues reveal intermediate properties, but their conformational dispositions depend on configuration of the side chain of residue: (E)‐ΔAbu is similar to ΔAla, whereas (Z)‐ΔAbu to ΔVal. Results indicate that the low‐energy conformation β2 is the characteristic feature of dehydroamino acid esters. The studied molecules constitute conformational patterns for dehydroamino acid esters with various side chain substituents in either or both Z and E positions. Copyright © 2011 European Peptide Society and John Wiley & Sons, Ltd.     

Lipase‐Catalyzed Kinetic Resolution of (±)‐1‐(2‐Furyl) Ethanol in Nonaqueous Media

S‐1‐(2‐Furyl) ethanol serves as an important chiral building block for the preparation of various natural products, fine chemicals, and is widely used in the chemical and pharmaceutical industries. In this work, lipase‐catalyzed kinetic resolution of (R/S)‐1‐(2‐furyl) ethanol using different acyl donors was investigated. Vinyl esters are good acyl donors vis‐à‐vis alkyl esters for kinetic resolution. Among them, vinyl acetate was found to be the best acyl donor. Different immobilized lipases such as Rhizomucor miehei lipase, Thermomyces lanuginosus lipase, and Candida antarctica lipase B were evaluated for this reaction, among which C. antarctica lipase B, immobilized on acrylic resin (Novozym 435), was found to be the best catalyst in n‐heptane as solvent. The effect of various parameters was studied in a systematic manner. Maximum conversion of 47% and enantiomeric excess of the substrate (ee_s) of 89% were obtained in 2 h using 5 mg of enzyme loading with an equimolar ratio of alcohol to vinyl acetate at 60°C at a speed of 300 rpm in a batch reactor. From the analysis of progress curve and initial rate data, it was concluded that the reaction followed the ordered bi–bi mechanism with dead‐end ester inhibition. Kinetic parameters were obtained by using nonlinear regression. This process is more economical, green, and easily scalable than the chemical processes. Chirality 26:286–292, 2014. © 2014 Wiley Periodicals, Inc.     

Chiral syntheses of methyl (R)‐2‐Sulfanylcarboxylic esters and acids with optical purity determination using HPLC

Accessible chiral syntheses of 3 types of (R)‐2‐sulfanylcarboxylic esters and acids were performed: (R)‐2‐sulfanylpropanoic (thiolactic) ester (53%, 98%ee) and acid (39%, 96%ee), (R)‐2‐sulfanylsucciinic diester (59%, 96%ee), and (R)‐2‐mandelic ester (78%, 90%ee) and acid (59%, 96%ee). The present practical and robust method involves (i) clean S_N2 displacement of methanesulfonates of (S)‐2‐hydroxyesters by using commercially available AcSK with tris(2‐[2‐methoxyethoxy])ethylamine and (ii) sufficiently mild deacetylation. The optical purity was determined by the corresponding (2R,5R)‐trans‐thiazolidin‐4‐one and (2S,5R)‐cis‐thiazolidin‐4‐one derivatives based on accurate high‐performance liquid chromatography analysis with high‐resolution efficiency. Compared with the reported method utilizing AcSCs (generated from AcSH and CsCO₃), the present method has several advantages, that is, the use of odorless AcCOSK reagent, reasonable reaction velocity, isolation procedure, and accurate, reliable optical purity determination. The use of accessible AcSK has advantages because of easy‐to‐handle odorless and hygroscopic solid that can be used in a bench‐top procedure. The Ti(OiPr)₄ catalyst promoted smooth trans‐cyclo‐condensation to afford (2R,5R)‐trans‐thiazolidin‐4‐one formation of (R)‐2‐sulfanylcarboxylic esters with available N‐(benzylidene)methylamine under neutral conditions without any racemization, whereas (2S,5R)‐cis‐thiazollidin‐4‐ones were obtained via cis‐cyclo‐condensation and no catalysts. Direct high‐performance liquid chromatography analysis of methyl (R)‐mandelate was also performed; however, the resolution efficiency was inferior to that of the thaizolidin‐4‐one derivatizations.     

A simple quantitative chiral analysis of amino acid esters by fluorine‐19 nuclear magnetic resonance using the modified James–Bull method

A simple chiral analysis of amino acid esters by fluorine‐19 nuclear magnetic resonance (¹⁹F NMR) through the modified James–Bull method is described. Thus, amino acid ester acid salt was treated with 5‐fluoro‐2‐formylphenylboronic acid and (S)‐BINOL in the presence of triethylamine (TEA) and MS4A for 10 minutes. The reaction mixture was analysed by ¹⁹F NMR directly to afford good quantifications.     

Synthesis of enantiomers of exo‐2‐norbornyl‐N‐n‐butylcarbamate and endo‐2‐norbornyl‐N‐n‐butylcarbamate for stereoselective inhibition of acetylcholinesterase

The acetylcholinesterase inhibition by enantiomers of exo‐ and endo‐2‐norbornyl‐N‐n‐butylcarbamates shows high stereoselelectivity. For the acetylcholinesterase inhibitions by (R)‐(+)‐ and (S)‐(?)‐exo‐2‐norbornyl‐N‐n‐butylcarbamates, the R‐enantiomer is more potent than the S‐enantiomer. But, for the acetylcholinesterase inhibitions by (R)‐(+)‐ and (S)‐(?)‐endo‐2‐norbornyl‐N‐n‐butylcarbamates, the S‐enantiomer is more potent than the R‐enantiomer. Optically pure (R)‐(+)‐exo‐, (S)‐(?)‐exo‐, (R)‐(+)‐endo‐, and (S)‐(?)‐endo‐2‐norbornyl‐N‐n‐butylcarbamates are synthesized from condensations of optically pure (R)‐(+)‐exo‐, (S)‐(?)‐exo‐, (R)‐(+)‐endo‐, and (S)‐(?)‐endo‐2‐norborneols with n‐butyl isocyanate, respectively. Optically pure norborneols are obtained from kinetic resolutions of their racemic esters by lipase catalysis in organic solvent. Chirality 2010. © 2009 Wiley‐Liss, Inc.     

New chiral stationary phases based on (R)‐1‐naphthylethylamine bound to 2,4,5,6‐tetrachloro‐1,3‐dicyanobenzene

Chiral functionalization of 2,4,5,6‐tetrachloro‐1,3‐dicyanobenzene (1) by regioselective nucleophilic substitution of one or two chlorine atoms by optically pure (R)‐(+)‐1‐naphthylethylamine (NEA), or by a glycine unit as a spacer to (R)‐NEA, enables the preparation of brush‐type chiral selectors (2, 3, 9, 13). By the introduction of the 3‐aminopropyltriethoxysilyl (APTES) group, reactive intermediates 4a/b, 5, 10a/b, and 14a/b are obtained ( a/b indicate a mixture of regioisomers with APTES in 6‐ and 2‐position). Binding of these to silica gel afforded four novel chiral stationary phases (CSPs) 6, 7, 15, and 16. HPLC columns containing CSPs with (R)‐NEA directly linked to polysubstituted aromatic ring (6, 7) are not very effective in resolution of most of the 23 racemic analytes, whereas the columns with distant π‐basic subunits (15, 16) exhibited higher resolving efficacy, in particular towards the isopropyl esters of racemic N‐3,5‐dinitrobenzoyl‐α‐amino acids. Effective resolution of test racemates reveals the importance of the presence of the hydrogen bond donor amido group and the distance between the persubstituted benzene ring in 1 and the π‐basic naphthalene ring of (R)‐NEA. Chirality 11:722–730, 1999. © 1999 Wiley‐Liss, Inc.     

The potential of white‐rot fungi to degrade phorbol esters of Jatropha curcas L. seed cake

The potential of solid‐state cultivation, with three white‐rot fungi (Bjerkandera adusta, Ganoderma resinaceum and Phlebia rufa), to decrease phorbol esters concentration of Jatropha curcas L. was evaluated in this study. Incubation was conducted in 250 mL Erlenmeyer flasks without agitation at 28°C for 30 days. Phorbol esters were analyzed by reverse‐phase HPLC after an extraction procedure using dichloromethane. All fungi studied were able to decrease the concentration of phorbol esters, mainly B. adusta and P. rufa which significantly reduced (p<0.05) phorbol esters contents to non‐toxic levels. These results suggest that white‐rot fungi could be potentially used as a possible approach for the biological treatment of the oilseed cake.