Ipso-Nitration of calix[6]azacryptands: intriguing effect of the small rim capping pattern on the large rim substitution selectivity

Synthesis, Crystal Structure, and Biological Activity of a Multidentate Calix[4]arene Ligand Doubly Functionalized by 2-Hydroxybenzeledene-Thiosemicarbazone

The design and synthesis of a novel tert-butyl-calix[4]arene functionalized at 1, 3 positions of the lower rim with two terminal 2-hydroxybenzeledene-thiosemicarbazone moieties is reported. The new ligand with multi-dentate chelating properties was fully characterized by several techniques: ESI-Mass spectroscopy, FT-IR, 1H-NMR, and single crystal X-ray diffraction. The solid state structure confirms that the calix[4]arene macrocycle has the expected open cone conformation, with two opposite phenyl rings inclined outwards with large angles. The conformation of the two alkoxythiosemicarbazone arms produces a molecule with a C2 point group symmetry. An interesting chiral helicity is observed, with the two thiosemicarbazone groups oriented in opposite directions like a two-blade propeller. A water molecule is encapsulated in the center of the two-blade propeller through multiple H-bond coordinations. The antibacterial, antifungal, anticancer, and cytotoxic activities of the calix[4]arene-thiosemicarbazone ligand and its metal derivatives (Co2+, Ni2+, Cu2+, and Zn2+) were investigated. A considerable antibacterial activity (in particular against E. coli, MIC, and MBC = 31.25 μg/mL) was observed for the ligand and its metal derivatives. Significant antifungal activities against yeast (C. albicans) were also observed for the ligand (MIC = 31.25 μg/mL and MBC = 125 μg/mL) and for its Co²⁺ derivative (MIC = 62.5 μg/mL). All compounds show cytotoxicity against the tested cancerous cells. For the Saos-2 cell line, the promising anticancer activity of ligand L (IC₅₀ < 25 μg/mL) is higher than its metal derivatives. The microscopic analysis of DAPI-stained cells shows that the treated cells change in morphology, with deformation and fragmentation of the nuclei. The hemo-compatibility study demonstrated that this class of compounds are suitable candidates for further in vivo investigations.

Direct Synthesis of Chiral Porphyrin Macrocyclic Receptors via Regioselective Nitration

Nitration of tetraphenylporphyrin cage compound 1, at -40 °C, leads to the regioselective formation of the chiral mononitro compound 2 (75% isolated yield) and, at -30 °C, to the achiral syn-dinitro-derivative 3 and the chiral anti-dinitro derivative 4 in a diastereomeric ratio of 5:2, which were separated by chromatography (46 and 20% yields, respectively). The structures of the compounds were confirmed by X-ray crystallography.

Supramolecular modeling of mono-copper enzyme active sites with calix[6]arene-based funnel complexes

Supramolecular bioinorganic chemistry is a natural evolution in biomimetic metallic systems since it constitutes a further degree of complexity in modeling. The traditional approach consisting of mimicking the first coordination sphere of metal sites proved to be very efficient, because valuable data are extracted from these examples to gain insight in natural systems mechanisms. But it does not reproduce several specific aspects of enzymes that can be mimicked by the implementation of a cavity embedding the labile active site and thus controlling the properties of the metal ion by noncovalent interactions. This Account reports on a strategy aimed at reproducing some supramolecular aspects encountered in the natural systems. The cavity complexes described herein display a coordination site constructed on a macrocycle. Thanks to a careful design of the cavity-based ligands, complexes orienting their labile site specifically toward the inside of the macrocycle were obtained. The supramolecular systems are based on the flexible calix[6]arene core that surrounds the metal ion labile site, thereby constraining exogenous molecules to pass through the conic funnel to reach the metal center. Such an architecture confers to the metal ion very unusual properties and behaviors, which in many aspects are biologically relevant. Three generations of calix[6]-based ligands are presented and discussed in the context of modeling the monocopper sites encountered in some enzymes. A wide range of phenomena are highlighted such as the impact that the size and shape of the access channel to the metal center have on the selectivity and rate of the binding process, the possible remote control of the electronics through small modifications operated on the cavity edges, induced-fit behavior associated with host-guest association (shoe-tree effect) that affects the redox properties of the metal ion and the electron exchange pathway, consequences of forbidden associative ligand exchange allowing a redox switch to drive an "antithermodynamic" ligand exchange, drastic effects of the full control of the second coordination sphere, and dioxygen activation in a confined chamber conducted to a selective and unusual four-electron redox process. All these findings bring new clues for better understanding the control exerted by the proteic environment on a metal center, allow the identification of new reaction pathways, and lead to new proposals for enzymatic catalytic cycle (such as the formation of an alkylhydroperoxide intermediate for mononuclear Cu-hydroxylases). The supramolecular systems may also be exploited for designing highly selective and sensitive probes for molecules of particular function and shape or to design new selective catalysts.

An induced-fit process through mechanical pivoting of aromatic walls in host-guest chemistry of calix[6]arene aza-cryptands

The per-ipso-nitration of a TMPA-capped calix[6]arene has been achieved. The substitution of the six bulky tBu substituents for nitro groups has a strong impact on the behavior of the ligand during guest recognition. The complexation of the aza cap (by H(+) or Cu(+)) associated with the encapsulation of a guest triggers an induced-fit process leading to the loss of the cone conformation of the host in favor of alternate conformations. Such a "pivoting" response of one or two walls of the calixarene core induces a large mechanical motion of the corresponding aromatic units. This stands in strong contrast with the "breathing" phenomena previously identified with other calix[6]arene-based complexes that expand or shrink the size of their cone as a function of the guest. Because of the covalently attached rigid TMPA cap, three arene units of this new calixarene host have a restricted mobility, which forces it to respond in a different manner to a supramolecular stress.