Search

Abstract: Depending on the experimental conditions, aspartic and glutamic anhydrides can be opened regioselectively with Grignard reagents, thus giving access to different isomers of chiral amino-ketoesters. [Copyright &y& Elsevier]

Cell-membrane glycerophospholipids and glycolipids have a common asymmetric skeleton of 1,2-diacyl- sn -glycerols. The 1,2-diacyl moiety in solutions permits a rapid equilibrium among three helical conformers, namely gt(+), gg(−), and tg, to display diverse conformational properties. The conformational property changes variably not only by head groups at the sn -3 position, but also by the solvent conditions applied. Recently, we came across an empirical rule in the conformational diversity in the solution state when we assumed the term of ‘helical disparity’ for the equilibrium between gt(+) and gg(−) conformers with reversed helical signs for each other. The sign and magnitude of the helical disparity (%) governs the (+)- or (−)-chirality around the lipid tail and corresponds to the magnitude of the exciton couplet CD (circular dichroism) bands. The empirical rule expresses that the disparity (%) changes linearly by the function of gt(+) population (%). Herein, the rule was verified by 1 H NMR spectroscopy using different types of 1,2-diacyl- sn -glycerols as model compounds. The present paper describes that the rule is formulated with a general equation (Eq-1): ‘helical disparity (%)’ = [gt(+)−gg(−)] (%) = A[gt(+)−B], in which A and B are constants taking values around 1.3 and 38, respectively. This rule is maintained regardless of the 1,2-diacyl and sn -3 substituent groups as far as examined here, while affording several exceptions. With Eq-1 (A = 1.3, B = 38), a conformational diagram can be obtained. This allows us to overview the diverse helical conformational properties of the asymmetric 1,2-diacyl- sn -glycerols in the solutions state. [ABSTRACT FROM AUTHOR]

This review outlines the most recent papers describing the stereoselective synthesis of γ-amino acids. In this update, the γ-amino acids have been classified according to the type of compounds, position and number of substituents, and depending on the type of substrate, acyclic, carbocyclic or azacyclic derivatives, following in the first case an order related to the strategy used, whereas in the second and third cases the classification is based on the ring size. [ABSTRACT FROM AUTHOR]

In the present paper a straightforward and efficient method for the synthesis of new chiral aziridinylether systems has been described. All of the new compounds have been tested in two stereocontrolled addition reactions of organozinc species to carbonyl compounds. The addition of diethyl- and phenylethynylzinc to aryl and alkyl aldehydes afforded the corresponding chiral products in high chemical yields (up to 90%) and with excellent ee’s of approximately 93%. [ABSTRACT FROM AUTHOR]

A general method for synthesizing enantiopure (100% ee) aliphatic alcohols with established absolute configurations has been developed and applied to alcohols CH 3 (CH 2 ) n –CH(OH)–(CH 2 ) m CH 3 , the enantiomeric discrimination of which is the most difficult, if m = n + 1 and n is large. Racemic saturated alcohols with short chains could be directly enantioresolved as ( S )-(+)-2-methoxy-2-(1-naphthyl)propionic acid (MαNP acid) esters by HPLC on silica gel, and their absolute configurations were simultaneously determined by 1 H NMR diamagnetic anisotropy. However, the application of this powerful MαNP ester method to alcohols with long chains was difficult, because of smaller values of the separation factor α. In such cases, the use of the corresponding acetylene alcohol MαNP esters was crucial. Acetylene alcohol MαNP esters were largely separated by HPLC on silica gel, and their absolute configurations were unambiguously determined by 1 H NMR as reported in the Part 1 paper. The MαNP esters obtained with established absolute configurations were catalytically hydrogenated to yield saturated alcohol MαNP esters. It was evidenced that no racemization occurred at the stereogenic center of the alcohol moiety during catalytic hydrogenation, by the coinjection of MαNP esters in HPLC. From the MαNP esters obtained, enantiopure (100% ee) aliphatic chain alcohols with established absolute configurations were recovered. Although the [ α ] D values of these alcohols were too small for the identification of the enantiomers, it was clarified that the analytical HPLC of MαNP esters is useful for identification in most cases. [ABSTRACT FROM AUTHOR]

Both enantiomers of bufuralol are pharmaceutically important molecules. While the ( S )-isomer with a higher β-blocking activity is recommended for hypertension treatment, the ( R )-enantiomer can be used as marker of hepatic activity. In this paper two new alternative approaches are described for their chemo-enzymatic synthesis, providing both highly enantiomerically enriched stereoisomers of the target molecule (ee 96–98%). One route is based on the baker’s yeast mediated stereoselective biotransformation of α-substituted ketones, and the other one on the lipase mediated kinetic resolution of the racemic bromoethanol. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports the preparation and testing of three new pyridino-18-crown-6 ether-based chiral stationary phases (S,S)-CSP-12, (S,S)-CSP-17 and (S,S)-CSP-20. Secondary amine (S,S)-7 was first transformed to triethoxysilyl derivative (S,S)-11, which contains a urea unit by treating the former with 3-(triethoxysilyl)propyl isocyanate. Next, (S,S)-11 was heated with spherical HPLC quality silica gel in toluene to obtain (S,S)-CSP-12. In order to acetylate the 3-aminopropylsilyl groups bonded to the silica gel during immobilization of the triethoxysilyl derivative (S,S)-11, we pumped acetic anhydride and triethylamine in DMF through the column to give the modified chiral stationary phase (S,S)-CSP-20. Triflate (S,S)-13 was first transformed to a pyridino-18-crown-6 ether derivative (S,S)-15, which contains a 4-(methoxycarbonyl)phenyl substituent at the 4-position of the pyridine ring by a Suzuki carbon–carbon coupling reaction. The hydrolysis of ester (S,S)-15 gave carboxylic acid (S,S)-21. Carboxylic acid (S,S)-21 was reacted with an excess of thionyl chloride to form the appropriate acyl chloride, which was treated with 3-(triethoxysilyl)propylamine in the presence of triethylamine in THF to furnish triethoxysilyl derivative (S,S)-16 containing an amide unit. Triethoxysilyl derivative (S,S)-16 was heated with spherical HPLC quality silica gel in toluene to give the chiral stationary phase (S,S)-CSP-17. The enantiomer separating ability of chiral stationary phases (S,S)-CSP-12, (S,S)-CSP-17 and (S,S)-CSP-20 were tested by using mixtures of enantiomers of 1-(1-naphthyl)ethylamine hydrogen perchlorate (1-NEA), 1-(2-naphthyl)ethylamine (2-NEA), 1-(4-bromophenyl)ethylamine (Br-PEA) and 1-(4-nitrophenyl)ethylamine hydrogen chloride (NO2-PEA). Chiral stationary phase (S,S)-CSP-17 showed the best enantiomer separating ability for the mixtures of enantiomers of amine compounds amongst the pyridino-crown ether-based CSPs ever synthesized. The high enantioselectivity is probably due to the strong π–π interaction of the extended π system of the aryl–substituted pyridine unit. [Copyright &y& Elsevier]

Abstract: In this paper, we have shown that the [RuCl2(p-cymene)]2 complex associated with simple hemisalen ligands is able to racemize (S)-1-phenylethanol. The influence on the racemization process of the ligand’s structure as well as the nature of a co-catalyst have been evaluated and optimized. This [RuCl2(p-cymene)]2/Ligand/TEMPO racemization system was then associated with the Candida Antarctica B lipase in order to carry out dynamic kinetic resolution experiments on rac-phenylethanol. This led us to identify the best conditions for effective DKR, which was then applied to various secondary benzylic and aliphatic alcohols. It was thus possible to obtain (R)-1-cyclohexylethyl acetate from rac-1-cyclohexylethanol in quantitative conversion and with high enantioselectivity (98%). [Copyright &y& Elsevier]

Abstract: Triple monoamine reuptake inhibitors have been implicated in the development of a new generation of antidepressants with higher efficacy than the currently existing therapies. In this paper, we have developed an alternative efficient synthetic route for triple monoamine reuptake inhibitor D-142 in 18.5% overall yield in 11 steps starting from diphenylmethane. D-142 was developed by us recently. The key step of the present synthetic strategy is the preferential formation of a bromohydrin from olefin via a cis-bromonium intermediate, which introduced significant efficiency in the overall synthesis. Furthermore, we have developed an efficient way to recycle the optically active intermediate diol back to the desired chiral epoxide. [Copyright &y& Elsevier]

Abstract: This paper reports efficient procedures for the preparation of optically active dibromide, diazide, diamine, and unsaturated diesters derived from a chiral bicyclic diol with C 2 symmetry namely (11R,12R)-9,10-dihydro-9,10-ethaneanthracene-11,12-dimethanol 3. Esterification of 3 with saturated and unsaturated carboxylic acids and acids chlorides leads to the corresponding normal-, olefinic- and acetylenic diesters in average yields of 81%. Also more efficient techniques for the preparation of starting diol 3 in higher yields as well as for a very simple separation of DCU (N,N′-dicyclohexylurea) from reactions carried out following the DCC/DMAP method are described. [Copyright &y& Elsevier]

Abstract: Controlled living polymerization of a broad range of monomers is a radical process known as ATRP (atom transfer radical polymerization) and is mediated by a variety of metals. A complex of copper has been found to be the most efficient catalyst, with a copper(I)/copper(II) catalytic cycle. The radical, enantioselective catalytic Reformatsky reaction mediated by Me2Zn can be efficiently promoted by copper(I) complexes avoiding the use of other promoters such as air and oxidant, giving more reproducible and affordable conditions. The CuCN-mediated enantioselective addition of ethyliodoacetate to functionalized ketones is described in this paper. [ABSTRACT FROM AUTHOR]

Abstract: This paper describes an anomalous behaviour of the Sharpless dihydroxylation of a terminal olefin. The synthesis of a new chiral pyrrolidine and its application as an organocatalyst in the Michael addition of cyclohexanone into nitrostyrenes are described. [Copyright &y& Elsevier]

Abstract: A novel family of eleven new tetrahydroisoquinoline (TIQ)–oxazoline intermediates and five corresponding copper(II) catalysts has been developed and applied to the catalytic asymmetric Henry reaction of various aldehydes with nitromethane to provide β-hydroxy nitroalkanes in high conversion (>99%). This paper describes the synthesis of the TIQ compounds from l-dihydroxyphenylalanine (l-DOPA) as the starting material. The chiral ligands were complexed in situ with various transition metals such as Cu, Sc, Co, Zn, Ni and Mn and tested as a chiral catalyst for the Henry reaction. The reaction was optimized in terms of the metal, counter ion, solvent, temperature and over a range of substrates. The corresponding catalyst with copper(II) acetate and 2-propanol as the solvent provides the best enantioselectivities (up to 77% ee) of the corresponding nitroalcohol for 4-chlorobenzaldehyde. [Copyright &y& Elsevier]

Abstract: In this paper we present our results concerning the rhodium/olefin-catalyzed reaction of arylboronic acids with an α-acetamido acrylic ester. With a chiral norbornadiene ligand rather low enantioselectivities (up to 21% ee) were obtained. Besides the expected conjugate adduct, we also observed the formation of a significant amount of Mizoroki–Heck-type product. The ratio of the conjugate adduct/Mizoroki–Heck product could be adjusted by a proper choice of the olefin ligand. [Copyright &y& Elsevier]

Abstract: This paper describes a method for the intramolecular anodic oxidation of ω-hydroxyl amides. Products were obtained in acceptable yields from different starting materials using various applied currents, charge, and reactant concentrations. Compounds 1a and 1d afforded a single diastereomer in 67% yield when the applied current and reactant concentration were 100mA and 0.04M. [Copyright &y& Elsevier]

Abstract: Recently, a pinene-derived amino alcohol [(1R,2R,3S,5R)-3-amino-2,6,6-trimethylbicyclo[3.1.1]heptan-2-ol] has been experimentally employed as an effective chiral catalytic precursor in the borane-mediated asymmetric reduction of prochiral ketones to produce the corresponding secondary alcohols, which provides the products in 96% ee. In this paper, we suggest a mechanism for this reduction process and then theoretically investigate it in detail by density functional theory. Fully geometry-optimized reactants, products, transition structures, and intermediates were obtained at the B3LYP/6-31G (d,p) level, and the results reveal that this reaction has five steps. Further calculations show that the solvent effect of THF has no great influence on the enantioselectivity of this reduction. [Copyright &y& Elsevier]

Abstract: This paper describes studies on the feasibility of an asymmetric Favorskii rearrangement of a meso-dihaloketone substrate. In the racemic series, metal amide bases in the presence of amines give poor to reasonable yields of ring-contracted unsaturated cyclopentyl amides, whilst amines in aqueous solvent mixtures afford cyclopentyl amides in good to excellent yields. A range of chiral non-racemic amines are screened, a tiny diastereo-bias is observed and a tentative mechanistic rationale for the diastereoselective process is proposed. [Copyright &y& Elsevier]

Abstract: The most recent papers describing the stereoselective synthesis of cyclic quaternary α-amino acids are collected in this review. The diverse synthetic approaches are classified according to the size of the ring and taking into account the bond that is formed to complete the quaternary skeleton. [Copyright &y& Elsevier]

Abstract: This paper reports on the synthesis and chiroptical spectroscopic characterization of Rh2(OAc)4− n (O-Phe-Z) n (n =1–4, abbreviated as Rh2L1, Rh2L2, Rh2L3 and Rh2L4) complexes. Mixtures of these complexes were prepared in MeCN and CHCl3 solvents from a Rh2(OAc)4 core complex and the chiral N-protected amino acid N-benzyloxycarbonyl-l-phenylalanine (Z-Phe-OH). The components of the in situ mixture of complexes proved to be stable enough to be separated by flash chromatography followed by preparative HPLC. The in situ mixtures prepared at different Z-Phe-OH to Rh2(OAc)4 concentration ratios contained different amounts of the Rh2L1–Rh2L4 complexes. ECD spectra of the individual complexes were measured in MeCN, CHCl3 and EtOH. This is the first time that dinuclear rhodium complexes comprising 1–4 chiral ligands have been characterized by ECD spectroscopy. It was found that the structure of the complexes depends not only on the number of the chiral Z-Phe-O − ligands but also on the donor character of the solvent. [Copyright &y& Elsevier]

Abstract: This paper describes the preparation of a new chiral building block containing a benzylic quaternary stereogenic center via the highly enantioselective PLE-mediated hydrolysis of dimethyl 2-(2-chloro-5-methoxyphenyl)-2-methylmalonate, as well as the absolute configuration of the new chiral building block, which has been elucidated through the formal total synthesis of (−)-physostigmine. [Copyright &y& Elsevier]

Abstract: The stereoselective synthesis of oxaziridines with three stereogenic centres is presented in this paper. The chirality is provided by asymmetric induction, and a possible mechanism for the induced stereoselectivity is also discussed. These small heterocycles undergo the [3+2] cycloaddition reaction with aryl ethenes to afford enantiomerically pure isoxazolidines of controlled configurations. This class of heterocycles is present in the structure of various compounds of biological and pharmacological interest, which can undergo further synthetic modifications. [Copyright &y& Elsevier]

Abstract: A simple method for the resolution of racemic trans-1,2-cyclohexanediol enantiomers is presented in this paper. The chiral discrimination was performed by tartaric acid via diastereomeric complex formation. The diastereomeric complexes were formed by adding the resolving agent to the racemic diol in a 0.5:1 molar ratio. The (2R,3R)-(+)-tartaric acid forms stable diastereomeric complex with (1R,2R)-(−)-cyclohexanediol. Supercritical carbon dioxide extraction was applied to the separation of the mixture of diastereomeric complexes and uncomplexed diol enantiomers. We found unexpected optimal conditions according to the 32 factorial design on the resolution efficiency within the studied range of the extraction pressure and temperature. In the best cases, the (1S,2S)- and (1R,2R)-diol enantiomers were obtained with ee(1S,2S) =62% and ee(1R,2R) =93% enantiomeric excess in one equilibrium stage, respectively. [Copyright &y& Elsevier]

Abstract: Our interest in the glycoaminocyanation reaction led us to apply this methodology to introduce amino acids on a monosaccharide using the N-terminal position. The GAAs described in this paper are characterized by having the amino and carboxylic acid functionalities on a disubstituted position of the saccharide backbone leading to α,α-disubstituted glycines. These new sugar amino acids showed a restricted conformation involving a spontaneous intramolecular cyclization between the C- and N-terminal positions during hydrolysis or hydrogenolysis to give the corresponding oxopiperazine. Tripeptide mimics were obtained by the introduction of an additional amino acid using one-pot conditions starting from these cyclic by-products under basic media. We demonstrated that these pseudo tripeptides are good candidates for extension of the peptidic scaffold and cyclization. [Copyright &y& Elsevier]

Abstract: In this paper, a novel general route to the stereospecific synthesis of chiral dihydropyridazines relying on the stereospecific formation of 1,4-diones is presented. The chirality is provided through the application of bicyclic lactams to the synthesis of the required diones. Treatment of the diones with hydrazine produces the pyridazine targets. In this report, the synthesis of four chiral 6-phenyl-4,4-disubstituted-1,4-dihydropyridazines is also presented. [Copyright &y& Elsevier]

Abstract: This paper outlines our efforts to study the influence of an oxygen atom adjacent to the stereogenic center of β-aminoalcohol derivatives used as ligands for catalysts in the asymmetric alkylation of aldehydes. Thirty-four enantiomerically pure (S)-dihydrofuran-2-yl, (S)-tetrahydrofuran-2-yl-, and furan-2-yl-β-dialkylamino alcohols have been prepared from 1,4:3,6-dianhydromannitol, 1,4:3,6-dianhydrosorbitol, and aminoacids, and then have been evaluated as ligands for the enantioselective addition of diethylzinc to benzaldehyde. Attention has been focused on the structural features governing the extent of chiral induction, the reaction rate, and the chemical yield of 1-phenyl-1-propanol which has been promoted by this wide collection of β-dialkylamino alcohols. [Copyright &y& Elsevier]

Abstract: This paper describes the enantioselective hydrogenation of vinylthioethers. We show that thioether derivatives of maleic esters can be hydrogenated with full conversion and up to 60% ee, and that α-thioether cinnamic acids can be hydrogenated in 51% ee with modest conversion. [Copyright &y& Elsevier]

Abstract: This paper reports a new chiral separation technology—biphasic recognition chiral extraction for the separation of aromatic acid enantiomers such as α-cyclohexyl-mandelic acid (CHMA) and naproxen (NAP). The biphasic recognition chiral extraction system was established by adding hydrophobic d(l)-isobutyl tartrate in 1,2-dichloroethane organic phase and hydrophilic β-cyclodextrin (β-CD) derivative in aqueous phase, which preferentially recognize the (R)-enantiomer and (S)-enantiomer, respectively. These studies involve an enantioselective extraction in a biphasic system, where aromatic acid enantiomers form complexes with the β-cyclodextrin derivative in the aqueous phase and d(l)-isobutyl tartrate in the organic phase, respectively. Factors affecting the extraction mechanism are analyzed, namely the influence of the concentrations of the extractants and aromatic acid enantiomers, the types of the extractants, pH, and temperature. The experimental results show that the biphasic recognition chiral extraction is of much stronger chiral separation ability than the monophasic recognition chiral extraction, which utilizes the cooperations of the forces of the tartrate and the β-CD derivative. Hydroxypropyl-β-cyclodextrin (HP-β-CD), hydroxyethyl-β-cyclodextrin (HE-β-CD), and methyl-β-cyclodextrin (ME-β-CD) have stronger recognition abilities for the (S)-aromatic acid enantiomers than those for (R)-aromatic acid enantiomers, among which HP-β-CD has the strongest ability. d-Isobutyl tartrate preferentially recognizes (R)-CHMA and (S)-NAP, while l-isobutyl tartrate preferentially recognizes (S)-CHMA and (R)-NAP. The maximum enantioselectivities of CHMA and NAP are 2.49 and 1.65, under conditions at which the pH values of the aqueous phases are 2.7 and 2.5, at the ratio of 2:1 of [isobutyl tartrate] to [HP-β-CD]. [Copyright &y& Elsevier]

Abstract: This paper reports the synthesis and application of quasi-enantiomeric trianglamine macrocycles for chiral analysis using ion trap ESI mass spectrometry. The quasi-enantiomeric macrocycles were shown to display chiroselection towards a series of enantiomerically pure carboxylic acids and therefore act as chiral probes for anions in the gas phase and in solution. [Copyright &y& Elsevier]

Abstract: In this paper, successive batch steps of an osmium-catalyzed asymmetric dihydroxylation reaction using Sharpless conditions and nanofiltration of post-reaction mixture were coupled, allowing us to enhance the cumulative catalyst turn over number to about 3.7 times over six cycles. The nanofiltration step provides for isolation of the chiral product, whereas the catalytic system (osmium and chiral ligand) is re-used in the following batch cycle. In this work the osmium average rejection through the selected nanofiltration membrane, Starmem™120, was 83%; this result may indicate the existence of free osmium in solution and implies residual product osmium contamination at an average value of 1.5mgOs/g-product. Effective application of this methodology to the model reaction requires improvement of catalyst rejection, which calls for an effective complexation of osmium by the ligand. Nevertheless, the enantioselectivity of the reaction was maintained constant over the six cycles at a value of 69%. [Copyright &y& Elsevier]

Abstract: This paper concentrates on studies of the acylation of alcohols with 3,3-difluoro-4-phenylazetidin-2-one rac-1, trans-3-fluoro-4-phenylazetidin-2-one rac-2 and 4-phenylazetidin-2-one rac-3 in the presence of immobilized lipase PS from Burkholderia cepacia in dry tert-butyl methyl ether (TBME). Fluorine activation in the compounds studied was essential in order to split the β-lactam ring with lipase PS. The highly enantioselective ring opening of rac-1 and rac-2 with methanol (1-butanol was also studied) allowed the preparation of the (R/(3R,4R))-β-lactams as the unreacted enantiomers and (S/(2S,3S))-β-amino esters as the product enantiomers with an ee >99%. Under the same conditions, rac-3 was totally unreactive. The possibility for a competing hydrolysis caused by water in the enzyme preparations is also discussed. [Copyright &y& Elsevier]

Abstract: This paper describes the establishment of a new synthetic route to the hydrobromide salt of (+)-coniceine 1 with high enantiopurity in five steps and 8% overall yield. The key step is the reductive photocyclization of an acyclic chiral dienamide, for which a convenient one-pot synthesis is described. [Copyright &y& Elsevier]

Abstract: The most recent papers describing new procedures for the synthesis of acyclic α,α-dialkylamino acids are collected in this review along with coverage of extensions of well established procedures. [Copyright &y& Elsevier]

Abstract: This paper reports the regio-, diastereo- and highly enantioselective vinylogous aldol reaction of 2-trimethylsilyloxyfuran (TMSOF) promoted by the SiCl4/Lewis base catalytic system. Several electron-pair donors proved to be effective as SiCl4 activators versus the TMSOF γ-selective addition to aldehydes giving rise to different diastereoisomeric ratios, while Denmark’s chiral bis-phosphoramide (R,R)-7 gave the highest enantioselectivity for both the anti- and syn-diastereoisomers. Furthermore, by using ambident electrophiles such as α,β-unsaturated aldehydes, the SiCl4/Lewis base-promoted process leads exclusively to the 1,2-addition products. [Copyright &y& Elsevier]

Abstract: This paper reports the preparation and testing of a new pyridino-18-crown-6 ether based chiral stationary phase (CSP). The chiral crown ether was covalently bound to silica gel. Circular dichroism (CD) spectroscopy was used for probing the complex formation of the chiral crown ether with the enantiomers of protonated primary arylalkylamines. The (S,S)-dimethylpyridino-18-crown-6 ether selector having a terminal double bond was first transformed to a triethoxysilyl derivative by regioselective hydrosilylation, and then heated with spherical HPLC quality silica gel to obtain the CSP. The discriminating power of the HPLC column filled with the above CSP was tested by using the hydrogenperchlorate salts of racemic α-(1-naphthyl)ethylamine (1-NEA), α-(2-naphthyl)ethylamine (2-NEA) and the hydrochloride salts of aromatic α-amino acids and α-amino acids containing different aromatic side-chain protecting groups. [Copyright &y& Elsevier]

Abstract: This paper presents an experimental and modelling approach to understanding the key conditions for diastereomeric resolutions of chiral amines by diacid resolving agents. The model is used to calculate resolution yields and enantiomeric excesses on the basis of acid–base chemical equilibria, formation of diastereomeric salts and their solubilities. Model parameters were determined experimentally and used to model predicted process variables, which were then compared to experimental resolution data as a function of the resolving agents/amine molar ratio. Based on this model, suggestions for the rational design of diastereomeric resolutions are made, highlighting the benefits of employing molar ratios higher than 1.5. [Copyright &y& Elsevier]

Abstract: The paper reports the first enantioselective synthesis of all the possible collagen reduced cross links as described: (2S,2′S,5R)- and (2S,2′S,5S)-5-hydroxylysinonorleucine 3a and 3b, (2S,2′S,5R,5′R)-5,5′-dihydroxylysinonorleucine 4a, (2S,2′S,5R,5′S)-5,5′-dihydroxylysinonorleucine 4b and (2S,2′S,5S,5′S)-5,5′-dihydroxylysinonorleucine 4c. The Williams’ glycine template methodology was used both for the introduction of a stereogenic at the α-position and for an easy protection of the amino acidic functionalities during the synthesis of the dimeric amino acids. [Copyright &y& Elsevier]

Abstract: Glycosylating complex molecules remains a major synthetic challenge, making it hard to explore carbohydrate diversity in biologically active glycosylated natural products and their derivatives. In this paper we compare the efficiency of chemical and enzymatic methods for glycosylation of glycopeptides related to vancomycin and describe the parameters that should be considered in designing synthetic approaches to glycosylated natural product derivatives. [Copyright &y& Elsevier]

This paper describes the preparation of nitrogen-containing bicycles by the aza-Diels–Alder reaction of nonactivated iminodienophiles and cyclopentadiene. Readily available starting materials such as (S)-(−)-lactate and l-amino acids were used for the preparation of chiral aldehydes with high enantiomeric excess. The improved oxidation procedure by Dess–Martin periodinane was employed for the synthesis of l-alanine derived phthalimide protected aldehyde 14, which was difficult to obtain in high enantiomeric excess by other methods. The influence of different Lewis acids on the stereoselectivity of the aza-Diels–Alder reaction was also investigated: It was found that the use of a combination of BF3·Et2O and TFA in the cycloaddition leads to complete racemization of the imine prepared from 14 whereas the use of TiCl4 gives the cycloaddition products 17a and 17b with high enantioselectivity (90%). [Copyright &y& Elsevier]

This paper deals with a study of the ability of a β-glucuronidase from bovine liver to catalyse transglucuronidation reactions. Our results show that when sodium (p-nitrophenyl β-d-glucopyranosid)uronate was used as a donor and as an acceptor, the sodium (sodium β-d-glucopyranosyluronate)-(1→3)-(p-nitrophenyl β-d-glucopyranosid)uronate and the (1→2) regioisomer were obtained in yields of 18 and 15%, respectively. A different regioselectivity was observed using p-nitrophenyl α-d-galactopyranoside as an acceptor leading to the synthesis of p-nitrophenyl (sodium β-d-glucopyranosyluronate)-(1→4) α-d-galactopyranoside and of the (1→2) regioisomer in yields of 16 and 21%, respectively. [Copyright &y& Elsevier]

The enantiomeric ratio (E) is commonly used to evaluate enzyme-catalysed kinetic resolutions. Chen et al. (1982) proposed a model for the enantiomeric ratio, which relates the extent of substrate conversion and the enantiomeric excess. The model, however, does not apply to cases where the substrate initially contains unequal amounts of enantiomers. This paper describes the adaptation of Chen''s model to cases where the reagent consists of a mixture of diastereomers present in unequal initial amounts and gives product as a single diastereomer. Diastereomeric ‘resolutions’ of (+)-limonene-1,2-epoxide and of (−)-carveol were used to validate this model. [Copyright &y& Elsevier]

In our previous work, racemic β-carbomethoxy-γ-lactams were subjected to enzymatic hydrolysis in the presence of α-chymotrypsin. The hydrolysis of three N-substituted lactams proved to be highly enantioselective, whereas an unsubstituted lactam was recovered in racemic form. Thus, in this paper we applied several molecular modeling protocols to explain the substrate specificity and the enantioselectivity of this enzyme. The adopted procedures involved accurate docking experiments of both enantiomers of each lactam to the protein active site, whose 3-D structure was obtained from X-ray crystallographic data, followed by extensive conformational and energetic analysis of the computer-generated complexes. The results obtained fully account for the experimental evidences on the enantioselective hydrolysis of these interesting, potential drugs by α-chymotrypsin. [Copyright &y& Elsevier]

Abstract: This paper presents a new recognition method using terahertz (THz) spectroscopy. By forming a guest–host diastereomer with an optically active compound (host) as a chiral recogniser, the chirality of the enantiomer (guest) can be clearly discriminated according to its absorption spectra. In this proof-of-concept study, we examined a pair of host–guest (h–g) complexes made of (R)- or (S)-α-methylbenzylamine (guest) and cholic acid (host). [Copyright &y& Elsevier]