Central-to-axial chirality transfer and induced circular dichroism in 6,7-dihydro-5H-dibenz[c,e]azepine derivatives of α- and β-amino esters

Circular Dichroism Sensing: Strategies and Applications

The analysis of the absolute configuration, enantiomeric composition, and concentration of chiral compounds are frequently encountered tasks across the chemical and health sciences. Chiroptical sensing methods can streamline this work and allow high-throughput screening with remarkable reduction of operational time and cost. During the last few years, significant methodological advances with innovative chirality sensing systems, the use of computer-generated calibration curves, machine learning assistance, and chemometric data processing, to name a few, have emerged and are now matched with commercially available multi-well plate CD readers. These developments have reframed the chirality sensing space and provide new opportunities that are of interest to a large group of chemists. This review will discuss chirality sensing strategies and applications with representative small-molecule CD sensors. Emphasis will be given to important milestones and recent advances that accelerate chiral compound analysis by outperforming traditional methods, conquer new directions, and pioneering efforts that lie at the forefront of chiroptical high-throughput screening developments. The goal is to provide the reader with a thorough understanding of the current state and a perspective of future directions of this rapidly emerging field.

Determination of sequence and absolute configuration of peptide amino acids by HPLC-MS/CD-based detection of liberated N-terminus phenylthiohydantoin amino acids

We report a method for the simultaneous determination of the sequence and absolute configuration of peptide amino acids using a combination of Edman degradation and HPLC–MS/CD. Phenylthiohydantoin (PTH) derivatives of 20 pairs of standard d- and l-amino acids were synthesized by the Edman reaction. The CD spectra of the derivatives revealed that each pair of the PTH derivatives exhibited the absorption with opposite signs at around 270 nm. These standard PTH derivatives showed well-resolved resolution without interference from byproducts in the ion chromatogram and clear positive/negative CD absorptions when subjected on a reversed phase HPLC–MS system coupled with a CD-2095 HPLC detector. This method was applied for the detection of a synthetic pentapeptide and a natural depsipeptide (halicylindramide C). The sequence and configuration of the pentapeptide and up to eight residues of halicylindramide C were successfully analyzed by this method. The amino acid configuration of the pentapeptide was also determined successfully by subjecting its acid hydrolysates to the Edman reaction followed by HPLC–MS/CD.

Central-to-axial chirality induction in biphenyl chiroptical probes for the stereochemical characterization of chiral primary amines

Flexible biphenyls have been applied as efficient and practical chiroptical probes for absolute configuration assignment to chiral primary amines. The mechanism of the central-to-axial chirality transfer from the amine moiety to the conformationally flexible biphenyl system has been determined by NMR and computational studies. This allowed proposing a general non-empirical rule in order to establish, simply by looking at the sign of the 250 nm A band in the ECD spectrum of the biphenyl derivative, the torsion of the biphenyl and thus the absolute configuration of the amine. The method proved to be very reliable and sensitive, allowing treatment of samples on the μmol scale and permitting the simultaneous determination of the amine sample's absolute configuration and enantiopurity.

Axial stereocontrol in tropos dibenz[c,e]azepines: the individual and cooperative effects of alkyl substituents

6,7-Dihydro-5H-dibenz[c,e]azepines, a class of secondary amine incorporating a centre-axis chirality relay, can be prepared from N-(2-bromobenzyl)-N-(1-arylalkyl)methanesulfonamides via Pd-catalysed intramolecular direct arylation, and methylated at C(7) via the 5,7-trans diastereoselective addition of methylmagnesium bromide to the derived N-benzylazepinium tetraphenylborate. Using these methods, the 4,5-dimethylated and 4,5,7-trimethylated homologues 13 and 14 were obtained and shown by ¹H NMR spectroscopy to be axially biased in opposite senses, as defined by the respective pseudoaxial or pseudoequatorial orientation of the 5-substituent in the preferred conformers, while retaining their tropos nature (the Arrhenius activation energy, E_A, for the conformational exchange process in 14 was estimated to be 57 kJ mol^-1 using 2D-EXSY NMR spectroscopy at 233-248 K). These results serve to illustrate how substituent effects might be exploited in new designs of bridged biaryl ligand in which tropos dynamics operate in combination with a pre-existing axial stereochemical bias.

A tendril perversion in a helical oligomer: trapping and characterizing a mobile screw-sense reversal

Helical oligomers of achiral monomers adopt domains of uniform screw sense, which are occasionally interrupted by screw-sense reversals. These rare, elusive, and fast-moving features have eluded detailed characterization. We now describe the structure and habits of a screw-sense reversal trapped within a fragment of a helical oligoamide foldamer of the achiral quaternary amino acid 2-aminoisobutyric acid (Aib). The reversal was enforced by compelling the amide oligomer to adopt a right-handed screw sense at one end and a left-handed screw sense at the other. The trapped reversal was characterized by X-ray crystallography, and its dynamic properties were monitored by NMR and circular dichroism, and modelled computationally. Raman spectroscopy indicated that a predominantly helical architecture was maintained despite the reversal. NMR and computational results indicated a stepwise shift from one screw sense to another on moving along the helical chain, indicating that in solution the reversal is not localised at a specific location, but is free to migrate across a number of residues. Analogous unconstrained screw-sense reversals that are free to move within a helical structure are likely to provide the mechanism by which comparable helical polymers and foldamers undergo screw-sense inversion.

Dibenzazepinyl ureas as dual NMR and CD probes of helical screw-sense preference in conformationally equilibrating dynamic foldamers

Dibenzazepinyl ureas act as probes to allow conformational analysis of screw-sense preference in dynamic foldamers by both NMR and CD spectroscopy.

Chirality sensing using stereodynamic probes with distinct electronic circular dichroism output

Circular dichroism (CD) spectroscopy is one of the most useful techniques for the stereochemical analysis of chiral biopolymers and fine chemicals. It has become invaluable for the assignment of the absolute configuration, the study of conformational isomers, and the determination of racemization kinetics of CD active chiral compounds. Molecular interactions between a nonracemic chiral substrate and a chromophoric, CD-silent probe that is achiral or exists as a racemic mixture of rapidly interconverting enantiomeric conformations or configurations can induce a strong, characteristic chiroptical readout. A covalent or noncovalent binding event that coincides with a well-defined asymmetric induction process can effectively imprint the chiral information of the substrate on the stereodynamic sensor and thus generate intense Cotton effects in the UV region of the latter. The probe can thus function as a stereochemical reporter unit and analysis of the CD spectrum often provides accurate information about the absolute configuration and enantiomeric composition of the substrate used. In this review, recent developments in circular dichroism analysis of chiral compounds with stereodynamic probes are described and particular emphasis is given to sensor design, chiral induction processes and applications scope.