Ring-Opening Reaction of 1-Phospha-2-Azanorbornenes via P-N Bond Cleavage and Reversibility Studies

The reactive P-N bond in 1-phospha-2-azanorbornenes is readily cleaved by simple alcohols to afford P-chiral 2,3-dihydrophosphole derivatives as a racemic mixture. The isolation of the products was not possible due to the reversibility of the reaction, which could, however, be stopped by sulfurization of the phosphorus atom, thus efficiently blocking the lone pair of electrons, as exemplified for 6b yielding structurally characterized 8b. Additionally, the influence of the substituent in the α position to the phosphorus atom (H, Ph, 2-py, CN) on the reversibility of the reaction was studied. Extensive theoretical calculations for understanding the ring-closing mechanism suggested that a multi-step reaction with one or more intermediates was most probable.

Trimeric and Dimeric Carbazole Alkaloids from Murraya microphylla

Seventeen new carbazole alkaloid derivatives, including a trimeric carbazole racemate, (±)-microphyltrine A (1), 15 dimeric carbazole racemates, (±)-microphyldines A–O (2–16), and a C-6–C-3″-methyl-linked dimeric carbazole, microphyldine P (17), were isolated from the leaves and stems of Murraya microphylla (Merr. et Chun) Swingle. The structures of the new compounds were elucidated on the basis of HRESIMS and NMR data analysis. The optically pure isomers of these isolated carbazole alkaloids were obtained by chiral HPLC separation and their absolute configurations were determined by electronic circular dichroism (ECD) data analysis.

Drug Stereochemistry: A Prodigy For Pharmacology and Drug Development

Stereochemistry has evinced the importance of many chiral drugs with respect to drug designing and development. A literature review was conducted for several chiral drugs involving pharmacokinetic and pharmacodynamic parameters of their enantiomers along with their uses in certain diseased conditions. This article mainly includes the pharmacological profile review of some chiral drugs and the aspects due to which the single enantiomer is of importance as compared to the racemic mixture of the drug. This was achieved by moderating the side effects or toxic effects; or by the potentiated activity of the single enantiomer. Resolution deals with the separation of racemic compounds which shows up the credibility to obtain the desired enantiomeric properties. As isomers vary in their pharmacokinetic and pharmacodynamic profiles, chiral drugs have showcased considerable importance in the drug development process. Both the enantiomers have a different pharmacological profile in the treatment of a disease, which differentiates them from drug racemates.

Toward High-Efficient Chiral Separation Using Hierarchically Porous HROP@Silica-Gel-Sheet Composite

Separating racemates is still a great challenge for their similarity in chemical structures and physicochemical properties. Despite exhibiting a significant potential in the adsorption separation due to their intrinsic characteristics, hierarchically porous materials utilized in enantioseparation have rarely been reported to date. Furthermore, the molding of such materials together with their hybrid organic-inorganic monoliths is generally required to meet various prerequisites in diverse large-scale industrial applications, but without sacrificing their inherently hierarchical architectures. In this work, a three-dimensional hierarchically porous organic-inorganic composite was simply and feasibly prepared via integrating the micro/meso-porous hyper-cross-linked resin organic polymer (HROP) with macroporous silica gel sheet (SGS), followed by a chiral selector postmodification, named as HROP@SGS. Racemic 1-phenylethanol, ibuprofen, and naproxen could be separated only using such a piece of HROP@SGS as the filler with a solid phase extraction technique. Herein, HROP@SGS exhibited extraordinary chiral resolution performances and succeeded in achieving a complete chiral resolution. Our findings suggest that this simple strategy proposed by us, that is, combining the chiral micro/mesoporous organic materials with macroporous inorganic substrates, can be employed to prepare an unprecedented enantioseparation material, which has a promising potential in large-scale industrial applications, such as fixed-bed and membrane separation.

Simulation of diffuse scattering in DL-norleucine

The diffraction patterns of DL-norleucine (SR-2-aminohexanoic acid, DL-Nle) crystals may show obvious diffuse scattering, usually described as `streaking', between the Bragg peaks. This phenomenon is obviously related to the non-ideal behaviour of the crystal. The normal interpretation is disorder in the stacking of weakly interacting 2D layers, known also for a number of other racemates of amino acids with linear hydrophobic side chains, as well as 1:1 complexes between different L- and D-enantiomers (quasi-racemates). Presented here is the first attempt to extract the information hidden in the diffuse scattering for this group of compounds by applying Monte Carlo simulations to the site distributions of two polymorphs in a block of 48 × 48 × 48 unit cells (four sites in each unit cell, 442 368 in total). The results demonstrate that it is indeed possible to model the diffuse scattering and relate it to processes expected to take place during phase transitions, characterized by slipping of molecular bilayers (or parts of them) relative to their neighbours. The understanding of the (intermediate) mixed phases in terms of domain size and defect density is consequently brought to a new level.

Chiral Purity of Crystals Using Low-Frequency Raman Spectroscopy

The pharmaceutical industry is in need of new techniques to identify the chirality of solids due to regulatory and safety concerns regarding the biological activity of enantiomers. In this study, we present for the first time the application of low-frequency Raman spectroscopy as a new and sensitive method for analyzing the chiral purity of crystals. Using this method, we were able to identify small amounts, as low as 1 % w/w, of an enantiomer in racemic crystals. To demonstrate the capabilities of the method, we used a model system based on chiral crystals of enantiopure, racemic crystals and their mixtures in various ratios. We found that the low-frequency Raman spectra of racemic and enantiopure crystals are significantly different, reflecting the different hydrogen bond networks. Moreover, a comparison of the sensitivity of enantiomeric excess in chiral crystals to that of circular dichroism and X-ray diffraction measurements showed that low-frequency Raman attains high sensitivity comparable to chiral optical methods used for solutions. Overall, our proposed approach of using Raman spectroscopy for determining enantiomeric excess in crystals is simple, fast, and offers a high degree of chiral sensitivity.

19 F NMR isotropic chemical shift for efficient screening of fluorinated fragments which are racemates and/or display multiple conformers

Fluorine ligand-based NMR spectroscopy is now an established method for performing binding screening against a macromolecular target. Typically, the transverse relaxation rate of the fluorine signals is monitored in the absence and presence of the target. However, useful structural information can sometimes be obtained from the analysis of the fluorine isotropic chemical shift. This is particularly relevant for molecules that are racemates and/or display multiple conformers. The large difference in fluorine isotropic chemical shift between free and bound state deriving mainly from the breaking and/or making of intramolecular and/or intermolecular hydrogen bonds allows the detection of very weak affinity ligands. According to our experimental results, racemates should always be included in the generation of the fluorinated fragment libraries. The selection or the availability of only one of the enantiomers for the fluorinated screening library could result in missing relevant chemical scaffold motifs.

Suppression of Oligomer Formation and Formation of Non-Toxic Fibrils upon Addition of Mirror-Image Aβ42 to the Natural l-Enantiomer

Racemates often have lower solubility than enantiopure compounds, and the mixing of enantiomers can enhance the aggregation propensity of peptides. Amyloid beta (Aβ) 42 is an aggregation-prone peptide that is believed to play a key role in Alzheimer's disease. Soluble Aβ42 aggregation intermediates (oligomers) have emerged as being particularly neurotoxic. We hypothesized that the addition of mirror-image d-Aβ42 should reduce the concentration of toxic oligomers formed from natural l-Aβ42. We synthesized l- and D-Aβ42 and found their equimolar mixing to lead to accelerated fibril formation. Confocal microscopy with fluorescently labeled analogues of the enantiomers showed their colocalization in racemic fibrils. Owing to the enhanced fibril formation propensity, racemic Aβ42 was less prone to form soluble oligomers. This resulted in the protection of cells from the toxicity of l-Aβ42 at concentrations up to 50 μm. The mixing of Aβ42 enantiomers thus accelerates the formation of non-toxic fibrils.

A 3D Homochiral MOF [Cd2(d-cam)3]•2Hdma•4dma for HPLC Chromatographic Enantioseparation

Up to now, some chiral metal-organic frameworks (MOFs) have been reported for enantioseparation in liquid chromatography. Here we report a homochiral MOF, [Cd2(d-cam)3]·2Hdma·4dma, used as a new chiral stationary phase for high-performance liquid chromatographic enantioseparation. Nine racemates of alcohol, naphthol, ketone, and base compounds were used as analytes for evaluating the separation properties of the chiral MOF packed column. Moreover, some effects such as mobile phase composition, column temperature, and analytes mass for separations on this chiral column also were investigated. The relative standard deviations for the resolution values of run-to-run and column-to-column were less than 2.1% and 3.2%, respectively. The experimental results indicate that the homochiral MOF offered good recognition ability, which promotes the application of chiral MOFs use as stationary phase for enantioseparation.

Probes for narcotic receptor mediated phenomena. Part 42: synthesis and in vitro pharmacological characterization of the N-methyl and N-phenethyl analogues of the racemic ortho-c and para-c oxide-bridged phenylmorphans

A new synthesis of N-methyl and N-phenethyl substituted ortho-c and para-c oxide-bridged phenylmorphans, using N-benzyl- rather than N-methyl-substituted intermediates, was used and the pharmacological properties of these compounds were determined. The N-phenethyl substituted ortho-c oxide-bridged phenylmorphan(rac-(3R,6aS,11aS)-2-phenethyl-2,3,4,5,6,11a-hexahydro-1H-3,6a-methanobenzofuro[2,3-c]azocin-10-ol (12)) was found to have the highest μ-opioid receptor affinity (K(i)=1.1 nM) of all of the a- through f-oxide-bridged phenylmorphans. Functional data ([³⁵S]GTP-γ-S) showed that the racemate 12 was more than three times more potent than naloxone as an μ-opioid antagonist.