Synthesis of Cryptophane-223-Type Derivatives with Dual Functionalization

Hemicryptophane Cages with a C1-Symmetric Cyclotriveratrylene Unit

Two new hemicryptophanes combining a cyclotriveratrylene unit with either an aminotrisamide or a tris(2-aminoethyl)amine (tren) moiety have been synthesized. Although a conventional synthesis approach was used, the molecular cages obtained are devoid of the expected C₃ symmetry. NMR analyses and X-ray crystal structure determination showed that these hemicryptophanes exhibited C₁ symmetry due to the unusual arrangement of the substituents of the cyclotriveratrylene unit. This unprecedented arrangement is related to a change in the regioselectivity of the Friedel-Crafts reactions that led to the CTV cap. This constitutes an original approach to access enantiopure chiral molecular cages with low symmetry.

Are the Physical Properties of Xe@Cryptophane Complexes Easily Predictable? The Case of syn- and anti-Tris-aza-Cryptophanes

We report the synthesis and optical resolution of C₃-symmetrical tris-aza-cryptophanes anti-3 and syn-4, as well as the study of their interaction with xenon via hyperpolarized ¹²⁹Xe NMR. These molecular cages are close structural analogues of the two well-known cryptophane-A (1; chiral) and cryptophane-B (2; achiral) diastereomers since these new compounds differ only by the presence of three nitrogen atoms grafted onto the same cyclotribenzylene unit. The assignment of their relative (syn vs anti) and absolute configurations was made possible, thanks to the combined use of quantum calculations at the density functional theory level and vibrational circular dichroism spectroscopy. More importantly, our results show that despite the large structural similarities with cryptophane-A (1) and -B (2), these two new compounds show a very different behavior in the presence of xenon in organic solutions. These results demonstrate that prediction of the physical properties of the xenon@cryptophane complexes, only based on structural parameters, remains extremely difficult.

Molecular Sensing with Host Systems for Hyperpolarized 129Xe

Hyperpolarized noble gases have been used early on in applications for sensitivity enhanced NMR. 129Xe has been explored for various applications because it can be used beyond the gas-driven examination of void spaces. Its solubility in aqueous solutions and its affinity for hydrophobic binding pockets allows "functionalization" through combination with host structures that bind one or multiple gas atoms. Moreover, the transient nature of gas binding in such hosts allows the combination with another signal enhancement technique, namely chemical exchange saturation transfer (CEST). Different systems have been investigated for implementing various types of so-called Xe biosensors where the gas binds to a targeted host to address molecular markers or to sense biophysical parameters. This review summarizes developments in biosensor design and synthesis for achieving molecular sensing with NMR at unprecedented sensitivity. Aspects regarding Xe exchange kinetics and chemical engineering of various classes of hosts for an efficient build-up of the CEST effect will also be discussed as well as the cavity design of host molecules to identify a pool of bound Xe. The concept is presented in the broader context of reporter design with insights from other modalities that are helpful for advancing the field of Xe biosensors.

Selective Capture of Thallium and Cesium by a Cryptophane Soluble at Neutral pH

We report in this article the synthesis of an asymmetrical cryptophane derivative (possessing only C₃-symmetry) bearing three phenol groups and three other carboxylic acid functions, each of these groups on the aromatic rings. Thanks to isothermal titration calorimetry experiments, we show that this compound binds large monovalent cations, such as Cs⁺ and Tl⁺, with a binding constant significantly lower than its congeners bearing a larger number of phenol groups grafted on the benzene rings. However, higher selectivity for Cs⁺ and Tl⁺ was observed with this compound since it does not show any affinity for other alkali cations. More importantly, due to the greater solubility of this derivative in pure water, we show for the first time that effective thallium(I) complexation takes place at neutral pH. This result demonstrates that cryptophane derivatives decorated with a higher number of phenol groups are promising host molecules for removing traces of thallium(I) from aqueous phases at neutral pH or above.