Theoretical study of peptide model dimers. Homo versus heterochiral complexes

The study of possible chiral recognition of a series of peptide models (For-Gly-NH₂, For-Ala-NH₂ and four of their fluoro substituted derivatives) has been carried out by means of DFT calculations. Homo (L,L) and heterochiral (L,D) dimers formed by hydrogen bond (HB) complexation have been considered. Initially, the conformational preferences of the monomers have been calculated and used to generate all the possible homo and heterochiral dimers. The energetic results show that in most cases, the β monomers are the most stable while in the dimers, the γ–γ complexes show the strongest interaction energies. In three of the four chiral cases studied, a heterochiral dimer is the most stable one. In addition, the electron density and nuclear shielding of the complexes have been studied.     

Design and synthesis of novel imidazolium-based ionic liquids with a pseudo crown-ether moiety: diastereomeric interaction of a racemic ionic liquid with enantiopure europium complexes

A planar-chiral imidazolium salt with a tris(oxoethylene) bridge was synthesized, and its potential application as a room temperature ionic liquid with a molecular-recognition ability was demonstrated.     

Remarks on beyond leading order

In recent years a two-scale expansion was established to study reactions of the type NN→NNπ within chiral perturbation theory. Then the diagrams of some subclasses that are invariant under the choice of the pion field no longer appear at the same chiral order. In this Letter we show that the proposed expansion still leads to well defined results. We also discuss the appropriate choice of the heavy baryon propagator.     

Chiral separations by host-guest complexation with cyclodextrin and crown ether in capillary zone electrophoresis

Summary Capillary zone electrophoresis using cyclodextrins and a chiral crown ether as buffer constituents was studied for the enantiomeric separation of drugs and amino acids. Based on results obtained from separation of racemic -amino acids both chiral selectors are compared with respect to resolution, efficiency and retention time. For (±)-Quinagolide effects of buffer composition and temperature are examined using -cyclodextrin as chiral agent. Optimum conditions were pH 2.5 at 30 mmol L^–1 -cyclodextrin. A linear dependence of retention on -cyclodextrin concentration allowed calculation of formation constants of the host-guest complexes. Buffer concentration and temperature also influence resolution. The application of a chiral crown ether to the separation of optical isomers in capillary zone electrophoresis is described for the first time. Chiral recognition of solutes depends on the formation of protonated alkyl amines and separation is attributed to the formation of diastereomeric host-guest complexes with different interactions for each enantiomer. The effects of crown ether concentration on resolution are presented.     

Conformation control of bilatrienes by peptides: A survey

Summary A variety of bilipeptides has been synthesized and investigated by UV-VIS and CD spectroscopy to explore the conformational influence of peptide ligands on the covalently bound tetrapyrrol moiety. The results obtained are complemented by those reported previously. From the comprehensive data thus available several features of the peptidic chain regulating interactions with the bilatriene moiety have been elucidated. Accordingly, the presence of three amide linkages at the ligand is prerequiste for induction of pronounced conformational changes consisting either in a complete transition ofM intoP helical bilatriene species by inversion of all torsional angles, or in a stretching of the chromophore. The kind of conformational influence can be controlled by varying the position of the prolyl entity within a given peptide.Dedicated to Prof. Dr. Kurt Schaffner on occasion of his 60th birthday     

Enhanced chromatographic resolution of amine enantiomers as carbobenzyloxy derivatives in high-performance liquid chromatography and supercritical fluid chromatography

The carbobenzyloxy (cbz) protecting group is evaluated for it's potential to enhance the resolution of chiral amine enantiomers using high-performance liquid chromatography (HPLC) and supercritical fluid chromatography (SFC). A series of cbz derivatives of commercially available racemates was prepared and analyzed by enantioselective chromatography using a variety of mobile phases and polysaccharide and Pirkle-type chiral stationary phases (CSPs). The cbz-derivatized product consistently demonstrated enhanced chiral resolution under HPLC and SFC conditions. Improved selectivity and resolution combined with an automated preparative HPLC or SFC system can lead to the rapid generation of highly purified enantiomers of desirable starting materials, intermediates or final products.     

Synthesis of Some Allyl-β-Cyclodextrin Derivatives and their Properties as Capillary GC Stationary Phases

Summary Three -Cyclodextrin derivatives (-CDs) were synthesized by substituting the 2,6-OH groups of -CD with allyl groups and the 3-OH groups with three different acyl groups (valeryl, heptanonyl, octanoyl). The chromatographic properties of these -CDs as stationary phases for capillary gas chromatography (CGC) were studied. The test results showed that the three -CDs possessed good coating properties and that the capillary columns coated with them exhibited high column efficiency. These -CDs can separate not only the disubstituted benzene isomers but also some racemic compounds.     

Diastereoselective Three Component Reaction of N‐Glyoxyloyl Camphorpyrazolidinone,Amine and Allyltributylstannane: Facile Synthesis of Homoallylic Amines

Diastereoselective allylation of imine derived from N‐glyoxyloyl camphorpyrazolidinone and amines was treated with allyltributylstannane in the presence of triflic acid. The corresponding optically enriched homoallylic amines were obtained in reasonable to high material yields and with practical levels of stereoselectivity in 10 min at ambient temperature.     

Chiral Recognition in π Complexes of Alkenes,Aldehydes, and Ketones with Transition Metal Lewis Acids; Development of a General Model for Enantioface Binding Selectivities

Prochiral alkenes, aldehydes, and ketones constitute the most frequently used starting materials for enantioselective organic syntheses. Protocols often involve chiral binding agents or Lewis acids that can give two diastereomeric adducts, the ratios of which are measures of chiral recognition. With π adducts, the diastereomers differ in the enantioface of the C?C or O?C group bound to the Lewis acid. This review provides the first comprehensive analysis of such equilibria and related binding phenomena with chiral transition metal Lewis acids. An extensive body of data from the authors' laboratory for complexes of the pyramidal rhenium fragment [(η⁵?C₅H₅)Re(No)(PPh₃)]⁺ ( I ) affords particular insight. Literature data for other complexes are also summarized. A general model for chiral recognition based upon the relative steric properties of four quadrants is presented. This enables binding selectivities to be individually and rationally optimized for different classes of ligands. Electronic effects are also identified and correlated with specific structural properties. Relationships between binding equilibria, reactivity, and product configurations are discussed.     

Synthesis of stereocontrolled α,α-difluoro-β-hydroxycarbonyl materials

Synthesis and synthetic utilities of stereocontrolled α,α-difluoro-β-hydroxy-γ,δ-unsaturated carbonyl compounds via enzymatic resolution with lipase PS (Pseudomonas cepacia, Amano Pharmaceutical Co. Ltd.) or lipase MY (Candida rugosa, Meito Sangyo Co. Ltd.) were described, and then the absolute configuration of obtained chiral materials was determined by the modified Mosher's method.