Quinoline Oligoamide Research Articles

Metadynamics simulations on the key factors of handedness induction of quinoline oligoamide foldamers with a terminal chiral group

AbstractMetadynamics simulation has been used to determine the conformational energy landscapes of several helical quinoline oligoamides bearing β‐pinene‐derived pyridine at either the C or N‐terminus. Based on the experimental results, the helix‐sense preference for four types of foldamers with the chiral terminal group has been verified. To compare to the key factors inducing handedness to helix‐sense preference, a terminal group with three hydrogen bond sites is designed and corresponding foldamers are built. The calculated results show that the delocalization effect and steric hindrance are mainly responsible for a particular helix‐sense preference for the investigated foldamers. The more hydrogen bonds between the terminal group and oligoamide units are formed, the more stable foldamers are.

Distance-Dependent Chiral Communication between Two Quinoline Oligoamide Foldamers Connected by Alkyl Chains.

A series of macrocycles that contain two quinoline oligoamide foldamers (QOFs) using various length of alkyl chains as linkers (2, 3, 6, 8 or 12 hydrocarbons) were synthesized. The two QOFs interact with each other through the linkers and the intramolecular helix chiral communications between the two QOFs were studied by 1 H NMR spectroscopy and crystal structures. Investigations show that the intensity of the intramolecular helix chiral communications between the two QOFs is dependent on the length of the linkers, and the interaction between the two QOFs increases with decreasing length of the linkers. When the length of the linkers decreased to C2 linkers, only one conformer is present in solution. Moreover, increasing the length of the foldamers would enhance the intramolecular helix chiral communication if the linkers are short, indicating that the length of the foldamers also has significant impact on intramolecular helix chiral communication.

Hyper-Rayleigh Scattering as a New Chiroptical Method: Uncovering the Nonlinear Optical Activity of Aromatic Oligoamide Foldamers.

Molecular helices based on self-organized aromatic oligoamide foldamers have been designed and prepared in their two enantiomeric forms in order to probe their second-order nonlinear chiroptical properties in solution. The quinoline oligoamides were rationally functionalized by electron-donating and electron-withdrawing groups to afford a gradual increase of the electronic polarization of the helical architectures. Their hyper-Rayleigh scattering (HRS) responses in solution were accordingly assessed, using either linearly polarized or circularly polarized incident light. Both methods allowed us to observe nonlinear optical activity that was quantified, for the first time for molecular systems, through circular differential scattering intensity ratios. The hyper-Rayleigh optical activity study reveals important charge-transfer differences within the aromatic oligomers, depending on the helix handedness and on the extent of electronic polarization induced by the appended substituents. The origin of the enantiomeric difference is discussed considering both achiral and chiral contributions. Overall, using aromatic oligoamide foldamers as a chiral model, we demonstrate the capabilities of HRS as a complementary chiroptical method, ideally suited for the analysis of various chiral molecular and supramolecular systems in solution. The reliability and chiral discrimination sensitivity of the method can be further improved through dynamic measurements using standard polarization modulation and heterodyning techniques.

Chiral Induction and Remote Chiral Communication in Quinoline Oligoamide Foldamers for Determination of Enantiomeric Excess and Absolute Configuration of Chiral Amines and Their Derivatives.

Two pentameric foldamers, Q5 and Q5C-S, containing a C-F bond were synthesized based on quinoline oligamide foldamers for the measurement of enantiomeric excess and for the determination of absolute configuration of chiral amines, diamines, amino alcohols, and α-amino acid esters. Chiral induction of Q5 was triggered in situ when the chiral analytes reacted with the C-F bond in Q5 by a N-nucleophilic substitution reaction, leading to a linear correlation between the CD amplitude at the region of quinoline chromophores and the ee values of the chiral analytes, which can be used for the ee determination of chiral analytes. Furthermore, the CD intensity of Q5C-S containing a chiral motif at its C-terminus enhances via remote, favorable chiral communication when the chiral induction was triggered in situ by chiral analytes at the N-terminus matches the original helicity of Q5C-S, but decreases via remote, conflicted chiral communication when the chiral induction is triggered in situ by chiral molecules at the N-terminus mismatches the original one. The system can thus be used for determination of the absolute configuration of chiral analytes, given that the chirality of the chiral motif at the C-terminus of Q5C-S is known.

Absolute control of helicity at the C-termini in quinoline oligoamide foldamers by chiral oxazolylaniline moieties

Absolute one-handed chiral quinoline tetramers andoctamers containing different oxazolylanilines at the C-terminus have been synthesized. The absolute one-handed sense and diastereomeric excess values were valued by 1H NMR. X-ray crystal diffractionand CD studies reveal that the S-oxazolylaniline always induces a P-handed helicity and the absolute helicity is driven by the stable three-center hydrogen bonding between protons in the amide and N atoms in oxazolylaniline and adjacent quinoline ring. CPL investigations demonstrated that S-CQn-a⿼d are CPL active and its glum values are dependent on its length. Interestingly, the sizes of the substituents in the chiral centers are different, however, they exert no effect on the dissymmetric factors gabs and glum of quinoline oligoamide foldamers.

Novel Helical Foldamers Based on the Conformational Properties of Aromatic Amides

In this review, we focus on our recent work on the development of novel helical foldamers based on the cis conformational properties of aromatic amide bonds. First, we describe the conformational properties of N-alkylated pyrroleamides and their oligomers. The conformation of the amide bond on the pyrrole ring is altered by N-alkylation, and the ratio of the cis conformer is lower in N-methylated amides with an N-pyrrole ring compared with that in N-methylbenzanilide. Pyrrole-containing oligomers, in which benzene and pyrrole rings are linked alternately by amide bonds with chiral N-substituents, show significant CD signals that depend on the chain length, temperature, and solvent properties, indicating that the oligoamides take folded conformations in solvents. Second, we describe the design and synthesis of alternately N-alkylated aromatic oligoamides as novel helical oligoamides with larger cavities compared with those of poly(N-alkylated p-benzamides). Spectroscopic studies and calculated optimized structures indicate that these oligoamides adopt helical structures with a cavity of about 9 Å in diameter. Finally, we describe the synthesis of a series of strained aromatic oligoamide macrocycles based on the cis conformational preference of the aromatic N-alkylated amide bond, building on the work of Huc et al. on the construction of large macrocycles by cyclization of helical quinoline oligoamides. The crystal structures are also presented. Our findings demonstrate that N-alkylated amides with the cis conformation are useful building blocks for unique aromatic foldamers.

Significant Enhancement of Circularly Polarized Luminescence Dissymmetry Factors in Quinoline Oligoamide Foldamers with Absolute Helicity.

When S- or R- oxazolylaniline enantiomers were attached to achiral quinoline oligoamide foldamers (QOFs), a single diastereomerically pure P- or M-handed foldamer was observed and exhibits negative or positive circularly polarized luminescence with the emission dissymmetry factors | glum| up to 0.038, which is significantly larger than that of QOF with incomplete chiral induction. More importantly, the CPL dissymmetry factors, together with the absorption dissymmetry factors, are enhanced with increases in the lengths of QOFs.

Chiral bisphosphine ligands based on quinoline oligoamide foldamers: application in asymmetric hydrogenation.

A series of chiral bisphosphine ligands were designed and synthesized based on single-handed quinoline oligoamide foldamers. The bisphosphine ligands can coordinate with Rh(cod)2BF4 in a 1 : 1 stoichiometry and the resulted chiral Rh(i) catalysts were applied in the asymmetric hydrogenation of α-dehydroamino acid esters, in which excellent conversions and promising levels of enantioselectivity were achieved.

Aromatic oligoamide foldamers as versatile scaffolds for induced circularly polarized luminescence at adjustable wavelengths.

Quinoline oligoamide foldamers appended with non-chiral fluorophores and derivatized with a camphanyl chiral inducer display strong chiroptical properties at tunable wavelengths as proved by CD and CPL spectroscopies. Induced CPL activity with high luminescence dissymmetry factors was observed in the visible range at wavelengths specific to the fluorophores.

Controlling Helix Sense at N- and C-Termini in Quinoline Oligoamide Foldamers by β-Pinene-Derived Pyridyl Moieties.

A series of quinoline oligoamide foldamers bearing a β-pinene-derived pyridyl group at the N-terminus or the C-terminus were synthesized, and the efficiencies of chiral inductions have been evaluated by 1H NMR and CD spectra. The chiral inductions were quantitative when chiral pyridyl acid was appended at the N-terminus, but were inferior when chiral pyridyl amine was appended at the C-terminus. Unexpectedly, N-oxidation on the pyridine ring at the C-terminus does not notably enhance the chiral induction efficiency in spite of the presence of three-center hydrogen bonds.

Solution Observation of Dimerization and Helix Handedness Induction in a Human Carbonic Anhydrase-Helical Aromatic Amide Foldamer Complex.

The design of synthetic foldamers to selectively bind proteins is currently hindered by the limited availability of molecular data to establish key features of recognition. Previous work has described dimerization of human carbonic anhydrase II (HCA) through self-association of a quinoline oligoamide helical foldamer attached to a tightly binding HCA ligand. A crystal structure of the complex provided atomic details to explain the observed induction of single foldamer helix handedness and revealed an unexpected foldamer-mediated dimerization. Here, we investigated the detailed behavior of the HCA-foldamer complex in solution by using NMR spectroscopy. We found that the ability to dimerize is buffer-dependent and uses partially distinct intermolecular contacts. The use of a foldamer variant incapable of self-association confirmed the ability to induce helix handedness separately from dimer formation and provided insight into the dynamics of enantiomeric selection.

Chiral separation by a terminal chirality triggered P-helical quinoline oligoamide foldamer

A P-helical quinoline oligoamide foldamer was grafted on silica and applied as an HPLC stationary phase for chiral separation. The P-handedness of the quinoline oligoamide foldamer was induced by a (1S)-camphanyl group, which was introduced at the N-terminus of a tetrameric quinoline oligoamide foldamer (Cmp-Q4). To immobilize the foldamer on porous silica particles, a trimethoxysilyl group was introduced at the opposing end of the foldamer. Elemental analysis indicated that the amount of foldamer on the silica surface was 0.57μmol/m2. Circular dichroism and vibrational CD spectra of Cmp-Q4 and Cmp-Q4-immobilized silica (Sil-Q4-Cmp) suggested that the helical structure of Cmp-Q4 was altered on the silica surface whilst retaining a chiral structure. The chiral recognition ability of Sil-Q4-Cmp was evaluated with various aromatic enantiomers. Sil-Q4-Cmp showed enantio-selectivity for axially chiral molecules (e.g., αTrigger’s base=1.26 and αBinaphthol=1.07). Sil-Q4-Cmp showed remarkable recognition of helical octameric quinoline oligoamides with isobutoxy and triethylene glycol side chains (α=10.35 and 14.98, respectively). In contrast, an (1S)-camphanyl group-immobilized porous silica showed no chiral recognition for any enantiomers tested in this study. To elucidate the chiral separation mechanism of Sil-Q4-Cmp, thermodynamic parameters were calculated using van’t Hoff plots. HPLC results and thermodynamic parameters suggested that the chiral recognition of Sil-Q4-Cmp is based on the helical structure of Cmp-Q4 and other thermally dependent interactions such as hydrophobic effects associated with aromatic stacking. This work represents the first known application of aromatic foldamers in chiral separation.

Solid-Phase Synthesis of Water-Soluble Helically Folded Hybrid α-Amino Acid/Quinoline Oligoamides

We report here a solid phase synthesis methodology that allows the incorporation of α-amino acids (X) into quinoline (Q) oligoamide foldamer sequences. Water-soluble hybrid oligoamides based on the XQ2 trimer repeat motif were shown to adopt helical conformations presenting α-amino acid side chains in a predictable linear array on one face of the helix. In contrast, sequences based on the XQ dimer motif expressed less well-defined behavior, most likely due to local conformational variability precluding long-range order. Also presented is a full structural investigation by NMR of a dodecameric XQ2-type foldamer containing four different amino acid residues (Lys, Ala, Asp, and Ser).

Solid phase synthesis of oligoethylene glycol-functionalized quinolinecarboxamide foldamers with enhanced solubility properties

Abstract A series of octameric quinoline oligoamide foldamers has been synthesized consisting exclusively of monomers which display mono-, di-, tri- or tetra-ethylene glycol side-chains. These oligomers adopt stable helical conformations. New Fmoc-acid monomer precursors were first developed. The microwave assisted solid-phase synthesis (SPS) methodology for oligomer preparation is described, and it is demonstrated that small adjustments in side-chain length translate into large differences in the solubility profile of the oligomers. The impact of such modifications on foldamer preparation, handedness inversion kinetics and potential applications is also discussed.

Unusual Regioselective Electrophilic Substitutions in Quinoline Foldamers: Conceptual DFT and Frontier Molecular Orbital Analysis Reveal the Crucial Role of Folding and Substituents

We carried out a detailed computational investigation of an earlier experimentally observed, unusual, regioselective, electrophilic halogenation in helically folded quinoline oligoamides. In the experimental studies, halogenation occurred selectively at a given monomer of a foldamer substituted with electron-withdrawing groups at the N terminus, although apparently identical reactive sites were available to react with the incoming electrophile. On the other hand, the selectivity was lost with weakly electron-donating groups. To gain an insight into the regioselective preference of bromination in quinoline foldamers, conceptual DFT was used to calculate the local nucleophilicity index of various foldamers of different sizes and with different substituents, and it was found that the predicted reaction centers were in line with the experimental results. Frontier molecular orbital analysis was used to understand this behavior. A detailed study of the hypothetical linear conformation of the tetramer for comparison with the folded conformation was carried out. In the case of a linear conformer, the HOMO is localized on specific monomers irrespective of substitution, but upon folding delocalization is observed, which is larger for the weakly electron-donating groups when compared with the electron-withdrawing groups. In the case of strongly donating groups there is no delocalization, even upon folding. The behavior remains the same when the size of the helix is increased (octamer). Thus, it is clearly seen in this work that the combined effects of conformations and substituents dictate the regioselectivity in the folded oligoamides; this knowledge will have a profound effect on the field of foldamer chemistry.

Heteromeric double helix formation by cross-hybridization of chloro-and fluoro-substituted quinoline oligoamides

Oligoamides of 8-chloroquinoline have been synthesized and shown to assemble into double helical dimers both in solution and in the solid state, and to undergo cross-hybridization with 8-fluoroquinoline oligoamide analogues; the handedness of these double helices can be controlled via chiral residues.

Proteomorphous Objects from Abiotic Backbones.

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