Influence of Helical Structure on Chiral Recognition of Poly(phenylacetylene)s Bearing Phenylcarbamate Residues ofL-Phenylglycinol and Amide Linage as Pendants

Positive Synergy between the Helical Poly(phenylacetylene) Backbones and the Helical L-Proline Oligopeptide Pendants for Enhanced Enantioseparation Properties

A series of optically active helical poly(phenylacetylene)s (PPA-Pro1, PPA-Pro3, PPA-Pro6, PPA-Pro9, and PPA-Pro12) bearing different chain lengths of L-proline oligopeptide in the side chains were obtained by polymerizing the corresponding novel phenylacetylene monomers. The monomer adopted a trans-rich helix structure when the L-proline oligopeptide chain length was longer, according to the optical activities and 2D-NMR analysis. The helical structure could be maintained and significantly influenced the polymers' helical conformation by introducing the L-proline oligopeptide to the pendants. By the way, the morphology of PPA-Pro3 was observed by atomic force microscope (AFM) on highly oriented pyrolytic graphite (HOPG), and the information on the helix direction, pitch, and chain arrangement was obtained. Also, the chiral separation properties of these polymer-based chiral stationary phases (CSPs) were investigated using high-performance liquid chromatography (HPLC). The poly(phenylacetylene)s showed enhanced enantioseparation properties toward various racemates depending on the longer chain length of the L-proline oligopeptide in the pendants and the positive synergy between the helical backbone and helical side chains. Particularly, PPA-Pro9 showed comparable or even superior enantioseparation properties for racemates 2 and 9 to four commercial columns (Daicel Chiralpak or Chiralcel AD, AS, OD, and OT), indicating that these poly(phenylacetylene)-based CSPs have potential practical values. This work presented here provides inspiration for the further development of CSPs based on a new paradigm.

Enantioseparation by high-performance liquid chromatography on proline-derived helical polyacetylenes

The nature, size, and position of the substituent on the phenyl ring remarkably influence the enantioseparation performance of polyacetylene-based CSPs.

Synthesis of poly(phenylacetylene)s containing chiral phenylethyl carbamate residues as coated-type CSPs with high solvent tolerability

Two novel helical poly(phenylacetylene) derivatives containing chiral phenylethyl carbamate residues in the end of each side chain (PPA-S and PPA-R) were synthesized by polymerization of the corresponding phenylacetylene monomers using Rh(nbd)BPh₄ as a catalyst in DMF. The enantioseparation properties of the polymers were evaluated as coated-type chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC). Under the same chromatographic conditions, PPA-S and PPA-R showed different enantioseparation properties, indicating that the different interactions between the analytes and the polymers, which result from the different chiral phenylethyl carbamate groups in the end of each side chains. Racemates 1, 7, and 8 could be better resolved on PPA-S, while racemate 6 was separated on PPA-R more efficiently. In addition, the coated-type CSPs showed good solvent tolerability and could work without any damage by introducing the polar solvents, such as CHCl₃ and THF, in eluent. Moreover, some racemates could be better resolved on these coated-type CSPs with the addition of THF to the eluent.

Synthesis and enantioseparation of proline-derived helical polyacetylenes as chiral stationary phases for HPLC

Proline-derived aliphatically substituted polyacetylenes with stable helical conformations exhibit an excellent enantioseparation ability as chiral stationary phases of HPLC.

Synthesis of helical poly(phenylacetylene) derivatives bearing diastereomeric pendants for enantioseparation by HPLC

Four novel helical poly(phenylacetylene) derivatives bearing diastereomeric pendants were prepared and used as CSPs in HPLC for enantioseparation.

Influence of different sequences of l-proline dipeptide derivatives in the pendants on the helix of poly(phenylacetylene)s and their enantioseparation properties

Novel helical poly(phenylacetylene)s bearing different sequences of l-proline dipeptide derivative pendants were prepared and used as CSPs in HPLC for enantioseparation.

Synthesis and evaluation of pseudopeptide chiral stationary phases for enantioselective resolution

Poly(2-oxazoline)s are regarded as bioinspired polymers due to their structural relation to polypeptides. In this work, a new kind of poly(2-oxazoline)s containing dipeptide segments in the side chains was synthesized through a bottom-up protocol, which involves ring-opening copolymerization of 2-(N-Boc-l-2-pyrrolidinyl)-2-oxazoline (PyOX_Boc) with 2-(3-butenyl)-2-oxazoline (BuOX) followed by deprotection and amide coupling with N-protected L-proline. The resulting vinyl-functionalized polymers were subsequently immobilized onto mercaptopropylated silica bead matrices by means of thio-click chemistry and their potential as the chiral stationary phase (CSP) for high-performance liquid chromatography was preliminarily evaluated with a series of structurally different racemates. The results showed that this class of pseudopeptide CSPs is particularly adapted to the enantiomeric separation of 1,1'-bi-2-naphthol and acyloin compounds (such as benzoin) under normal-phase conditions. Moreover, an increase in the length of polymer main chains is beneficial to the enhancement of both enantioselectivity and resolution ability. The chiral discrimination of analytes by the polymeric selectors stems primarily from hydrogen bonding and π-π interactions as well as steric hindrance.

Helical Polyacetylenes Induced via Noncovalent Chiral Interactions and Their Applications as Chiral Materials

Construction of predominantly one-handed helical polyacetylenes with a desired helix sense utilizing noncovalent chiral interactions with nonracemic chiral guest compounds based on a supramolecular approach is described. As with the conventional dynamic helical polymers possessing optically active pendant groups covalently bonded to the polymer chains, this noncovalent helicity induction system can show significant chiral amplification phenomena, in which the chiral information of the nonracemic guests can transfer with high cooperativity through noncovalent bonding interactions to induce an almost single-handed helical conformation in the polymer backbone. An intriguing "memory effect" of the induced macromolecular helicity is observed for some polyacetylenes, which means that the helical conformations induced in dynamic helical polyacetylene can be transformed into metastable static ones by tuning their helix-inversion barriers. Potential applications of helical polyacetylenes with controlled helix sense constructed by the "noncovalent helicity induction and/or memory effect" as chiral materials are also described.

Chiral stationary phases consisting of π-conjugated polymers bearing glucose-linked biphenyl units: reversible switching of resolution abilities based on a coil-to-helix transition

We have succeeded in developing a novel chiral stationary phase that can reversibly switch resolution abilities based on a coil-to-helix transition in a column.

Synthesis of Optically Active Poly(diphenylacetylene)s Using Polymer Reactions and an Evaluation of Their Chiral Recognition Abilities as Chiral Stationary Phases for HPLC

A series of optically active poly(diphenylacetylene) derivatives bearing a chiral substituent (poly-2S) or chiral and achiral substituents (poly-(2Sx-co-31-x)) on all of their pendant phenyl rings were synthesized by the reaction of poly(bis(4-carboxyphenyl)acetylene) with (S)-1-phenylethylamine ((S)-2) or benzylamine (3) in the presence of a condensing reagent. Their chiroptical properties and chiral recognition abilities as chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC) were investigated. Poly-2S and poly-(2Sx-co-31-x) (0.06 < x < 0.71) formed a preferred-handed helical conformation with opposite helical senses after thermal annealing despite possessing the same chiral pendant (h-poly-2S and h-poly-(2Sx-co-31-x)). Furthermore, h-poly-2S and h-poly-(2S0.36-co-30.64) emitted circularly polarized luminescence with opposite signs. h-Poly-2S showed higher chiral recognition abilities toward a larger number of racemates than poly-2S without a preferred-handed helicity and the previously reported preferred-handed poly(diphenylacetylene) derivative bearing the same chiral substituent on half of its pendant phenyl rings. h-Poly-(2S0.36-co-30.64) also exhibited good chiral recognition abilities toward several racemates, though the elution order of some enantiomers was reversed compared with h-poly-2S.

Supramolecular Helical Systems: Helical Assemblies of Small Molecules, Foldamers, and Polymers with Chiral Amplification and Their Functions

In this review, we describe the recent advances in supramolecular helical assemblies formed from chiral and achiral small molecules, oligomers (foldamers), and helical and nonhelical polymers from the viewpoints of their formations with unique chiral phenomena, such as amplification of chirality during the dynamic helically assembled processes, properties, and specific functionalities, some of which have not been observed in or achieved by biological systems. In addition, a brief historical overview of the helical assemblies of small molecules and remarkable progress in the synthesis of single-stranded and multistranded helical foldamers and polymers, their properties, structures, and functions, mainly since 2009, will also be described.

Chiral recognition in separation science - an update

Stereospecific recognition of chiral molecules is an important issue in various aspects of life sciences and chemistry including analytical separation sciences. The basis of analytical enantioseparations is the formation of transient diastereomeric complexes driven by hydrogen bonds or ionic, ion-dipole, dipole-dipole, van der Waals as well as π-π interactions. Recently, halogen bonding was also described to contribute to selector-selectand complexation. Besides structure-separation relationships, spectroscopic techniques, especially NMR spectroscopy, as well as X-ray crystallography have contributed to the understanding of the structure of the diastereomeric complexes. Molecular modeling has provided the tool for the visualization of the structures. The present review highlights recent contributions to the understanding of the binding mechanism between chiral selectors and selectands in analytical enantioseparations dating between 2012 and early 2016 including polysaccharide derivatives, cyclodextrins, cyclofructans, macrocyclic glycopeptides, proteins, brush-type selectors, ion-exchangers, polymers, crown ethers, ligand-exchangers, molecular micelles, ionic liquids, metal-organic frameworks and nucleotide-derived selectors. A systematic compilation of all published literature on the various chiral selectors has not been attempted.