Synthesis and Chiroptical Properties of Cryptophanes Having C1-Symmetry

Synthesis and VCD Spectroscopic Characterization of a Series of Azacryptands from a Chiral Valine-Based Derivative of Tris(2-aminoethyl)amine (TREN)

Utilizing experimental and computational vibrational circular dichroism (VCD) spectroscopy, we explored the conformational preferences of a series of chiral C3 -symmetric octaazacryptands with tris(2-aminoethyl)-amine head groups derived from valine. While the spectra of the smallest azacryptand with p-phenyl linkers and its elongated derivative with p-biphenyls linker were found to match well with the computed spectra, the computed conformational preferences of the m-biphenyl-based azacryptand did not seem to reflect the conformations dominating in chloroform solution. A detailed analysis revealed that structural changes resulting in a collapsed cage structure gave a notably better match with the experiment. It could subsequently be concluded from the VCD analysis, that the octaazacryptands prefer a collapsed structure, which is not predicted by density functional theory (DFT) calculations as the global minimum structures. These findings are expected to have consequences also for future studies on inclusion complexes of such azacryptands.

Enantiopure cryptophane derivatives: Synthesis and chiroptical properties

This review addresses the synthesis of enantiopure cryptophane and the study of their chiroptical properties. Cryptophane derivatives represent an important class of macrocyclic compounds that can bind a large range of species in solution under different conditions. The overwhelming majority of these host molecules is chiral, and their chiroptical properties have been thoroughly investigated. The first part of this review is dedicated to the optical resolution and the synthesis of enantiopure cryptophane derivatives. In a second part, the study of the chiroptical properties of these molecular hosts by different techniques such as electronic and vibrational circular dichroism and Raman optical activity is detailed. These techniques allow the determination of the absolute configuration of cryptophane derivatives and provide useful information about their conformation in different conditions.

Chiroptical study of cryptophanes subjected to self-encapsulation

In 1,1,2,2-tetrachloroethane-d₂ , the ¹²⁹ Xe NMR spectrum of the Xe@cryptophane-223 complex bearing seven acetate groups (Xe@1 complex) shows an unusually broad signal compared with that of its congeners (Chapellet, LL. et al. J. Org. Chem. 2015;80:6143-6151). To interpret this unexpected behaviour, a ¹ H NMR analysis and a thorough study of the chiroptical properties of 1 as a function of the nature of the solvent have been performed. The ¹ H NMR spectra of 1 reveal that a self-encapsulation phenomenon takes place in DMSO-d₆ and 1,1,2,2-tetrachloroethane-d₂ solvents. Thanks to the separation of the two enantiomers of 1 by HPLC on chiral stationary phase, the two enantiomers of 1 have been studied in detail by polarimetry, electronic (ECD), and vibrational (VCD) circular dichroism spectroscopies. Except for ECD spectroscopy, these chiroptical techniques reveal spectroscopic changes as a function of the nature of the solvent. For instance, in DMSO and 1,1,2,2-tetrachloroethane, in which the self-encapsulation phenomenon takes place, the sign of the specific optical rotation of [CD(-)₂₅₄ ]-1 and [CD(+)₂₅₄ ]-1 is changed. These results have then been compared with those obtained with cryptophane-223 bearing only one acetate group on the propylenedioxy linker (compound 2) and with cryptophane-223 bearing six acetate groups (compound 3). A self-encapsulation phenomenon is also observed with compound 2. Finally, compounds 2 and 3 show different chiroptical properties compared with those obtained with the two enantiomers of compound 1.

Theoretical study on the photophysical properties of boron-fused double helicenes

Seven boron-fused double helicenes containing different electron-donors or acceptors and their combinations were investigated to shed light on the effective way to enhance the second-order NLO response.

Nickel catalyzed reduction of arenols under mild conditions

The mild Ni-catalyzed reduction of arenols features a broad substrate scope, non-sensitivity to steric hindrance and no over-reduction of the aryl ring.

Immobilization of cryptophane derivatives onto SiO2/Au and Au substrates

The synthesis of a cryptophane molecule bearing five methoxy substituents and an alkanethiol chain, 4, as well as its subsequent grafting onto a gold surface, is reported. Immobilization of cryptophane derivatives onto silica (SiO2/Au) surfaces was also performed by reacting a cryptophane molecule bearing one or six acid functions, 5 or 6, respectively, with an amino-terminated self-assembled monolayer (SAM). Polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS) was used to characterize the two types of cryptophane monolayers. Surface coverage of cryptophane monolayers was estimated by comparing the PM-IRRAS intensity of cryptophane bands with that calculated from the optical constants of pentamethoxy-cryptophane for a compact monolayer. A very efficient grafting of 4 onto a gold surface was found, with a surface coverage close to 100%. On the other hand, the reaction of mono-acid, 5, or hexa-acid, 6, cryptophanes with amino-terminated SAM was less efficient, since the surface coverage did not exceed 15%. Finally, a good surface coverage (75%) was also obtained by using a cysteamine coupling agent to modify 5 before its grafting onto a gold surface.

Enantiopure Cryptophane-129Xe Nuclear Magnetic Resonance Biosensors Targeting Carbonic Anhydrase

The (+) and (-) enantiomers for a cryptophane-7-bond-linker-benzenesulfonamide biosensor (C7B) were synthesized and their chirality confirmed by electronic circular dichroism (ECD) spectroscopy. Biosensor binding to carbonic anhydrase II (CAII) was characterized for both enantiomers by hyperpolarized (hp) ¹²⁹Xe NMR spectroscopy. Our previous study of the racemic (+/-) C7B biosensor-CAII complex [Chambers, et al., J. Am. Chem. Soc. 2009, 131, 563-569], identified two "bound" ¹²⁹Xe@C7B peaks by hp ¹²⁹Xe NMR (at 71 and 67 ppm, relative to "free" biosensor at 64 ppm), which led to the initial hypothesis that (+) and (-) enantiomers produce diastereomeric peaks when coordinated to Zn²⁺ at the chiral CAII active site. Unexpectedly, the single enantiomers complexed with CAII also identified two "bound" ¹²⁹Xe@C7B peaks: (+) 72, 68 ppm and (-) 68, 67 ppm. These results are consistent with X-ray crystallographic evidence for benzenesulfonamide inhibitors occupying a second site near the CAII surface. As illustrated by our studies of this model protein-ligand interaction, hp ¹²⁹Xe NMR spectroscopy can be useful for identifying supramolecular assemblies in solution.

Biomembrane interactions of functionalized cryptophane-A: combined fluorescence and 129Xe NMR studies of a bimodal contrast agent

Fluorescent derivatives of the (129)Xe NMR contrast agent cryptophane-A were obtained by functionalization with near infrared fluorescent dyes DY680 and DY682. The resulting conjugates were spectrally characterized, and their interaction with giant and large unilamellar vesicles of varying phospholipid composition was analyzed by fluorescence and NMR spectroscopy. In the latter, a chemical exchange saturation transfer with hyperpolarized (129)Xe (Hyper-CEST) was used to obtain sufficient sensitivity. To determine the partitioning coefficients, we developed a method based on fluorescence resonance energy transfer from Nile Red to the membrane-bound conjugates. This indicated that not only the hydrophobicity of the conjugates, but also the phospholipid composition, largely determines the membrane incorporation. Thereby, partitioning into the liquid-crystalline phase of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine was most efficient. Fluorescence depth quenching and flip-flop assays suggest a perpendicular orientation of the conjugates to the membrane surface with negligible transversal diffusion, and that the fluorescent dyes reside in the interfacial area. The results serve as a basis to differentiate biomembranes by analyzing the Hyper-CEST signatures that are related to membrane fluidity, and pave the way for dissecting different contributions to the Hyper-CEST signal.

Host-guest complexes between cryptophane-C and chloromethanes revisited

Cryptophane-C is composed of two nonequivalent cyclotribenzylene caps, one of which contains methoxy group substituents on the phenyl rings. The two caps are connected by three OCH(2)CH(2)O linkers in an anti arrangement. Host-guest complexes of cryptophane-C with dichloromethane and chloroform in solution were investigated in detail by nuclear magnetic resonance techniques and density functional theory (DFT) calculations. Variable temperature proton and carbon-13 spectra show a variety of dynamic processes, such as guest exchange and host conformational transitions. The guest exchange was studied quantitatively by exchange spectroscopy measurements or by line-shape analysis. The conformational preferences of the guest-containing host were interpreted through cross-relaxation measurements, providing evidence of the gauche+2 and gauche-2 conformations of the linkers. In addition, the mobility of the chloroform guest inside the cavity was studied by carbon-13 relaxation experiments. Combining different types of evidence led to a detailed picture of molecular recognition, interpreted in terms of conformational selection.

Enantiopure supramolecular cages: synthesis and chiral recognition properties

Enantiopure compounds are ubiquitous in the chemical sciences and present a particular interest in the field of molecular recognition and host-guest systems. Indeed, chiral molecular receptors are at the basis of numerous biological recognition processes and have important implications in biochemistry or pharmacology. Chemists have been investigating this field for several decades, which has led to the development of the synthesis of chiral hosts, their enantiomeric differentiation, and the studies of their recognition properties towards important and bio-relevant chiral guest substrates. The design of molecular cages is a rather difficult task that is even more demanding when enantiopure molecules are required. In this review we chose to present the main families of synthetic organic supramolecular cages that have been developed, whose structures contain stereogenic centers or present an inherent chirality, giving rise to chiral supramolecular cages. Particular attention is given to obtaining enantiopure compounds. Their recognition properties are also underlined. A last important aspect of the review is to present how chiroptical spectroscopies can be used to characterize the recognition phenomena displayed by supramolecular cages.

Synthesis of enantiopure, trisubstituted cryptophane-A derivatives

The efficient synthesis of enantiopure, trisubstituted cryptophane-A derivatives, organic host molecules with unusually high xenon affinity, is reported. Synthesis and chromatographic separation of (±) tri-Mosher's acid substituted cryptophane diastereomers gave ready access to the enantiopure cryptophanes, which are critical components in the design of enantiomerically pure (129)Xe biosensors. Hyperpolarized (129)Xe NMR spectroscopy identified single resonances for both trisubstituted cryptophane diastereomers that were separated by 9.5 ppm. This highlights opportunities for using enantiopure xenon biosensors in the simultaneous detection of (129)Xe in different biochemical environments.

NMR investigation of guest-host complex between chloroform and cryptophane C

Guest-host complex between cryptophane C, possessing two non-equivalent caps, and chloroform is investigated by NMR spectroscopy. The kinetics of the chloroform exchange between the bound and free sites is determined by (1)H exchange spectroscopy. Moreover, the preferential orientation of chloroform molecule with respect to the cryptophane C frame is examined by the NOESY and ROESY experiments. The experimental findings are compared to the results of quantum chemical calculations.

Determination of the absolute configurations of synthetic daunorubicin analogues using vibrational circular dichroism spectroscopy and density functional theory

The absolute configurations of three synthesized anthracycline analogues have been determined using vibrational circular dichroism (VCD) spectroscopy and the density functional theory (DFT) calculations. The experimental VCD spectra of the three compounds have been measured for the first time in the film state, prepared from their CDCl(3) solutions. Conformational searches for the monomers and some dimers of the three compounds have been performed at the DFT level using the B3LYP functional and the 6-311G** and 6-311++G** basis sets. The corresponding vibrational absorption and VCD spectra have been calculated. The good agreement between the experimental and the calculated spectra allows one to assign the absolute configurations of the three compounds with high confidence. In addition, the dominant conformers of the three compounds have also been identified.

Conformational changes in cryptophane having C1-symmetry studied by vibrational circular dichroism

Vibrational circular dichroism (VCD) measurements and density functional theory (DFT) calculations were used to obtain the absolute configuration of a cryptophane molecule having C1-symmetry (labeled cryptophane-H). This molecule exhibits chiroptical properties different from those published for cryptophane-A having D3-symmetry [Brotin et al. J. Am. Chem. Soc. 2006, 128, 5533-5540]. In particular, we have shown that the conformation of the aliphatic linkers is very dependent on the solvent used and its ability to enter (CDCl3 solution) or not (C2D2Cl4 solution) in the cryptophane cavity. Calculations performed at the DFT (B3PW91/6-31G*) level establish, besides the absolute configuration, the preferential anti and gauche conformations of the aliphatic linkers of the chloroform@cryptophane-H complex and the empty cryptophane-H molecule, respectively. Polarimetric measurements performed in several solvents reflect also the change of conformation of the bridges upon guest encapsulation.