Pellegrino La Manna

Exploiting the hydrophobicity of calixarene macrocycles for catalysis under “on-water” conditions

An example of calixarene-mediated catalysis under “on-water” conditions is here reported. In the presence of thioureido-calixarenes a rate acceleration for the Vinylogous Mukaiyama Aldol Reaction (VMAR) was observed, under on-water conditions. The ability of the calixarene catalyst to accelerate the VMAR under on-water conditions is closely related to its hydrophobicity and to its recognition abilities toward the substrate.

Threading of an Inherently Directional Calixarene Wheel with Oriented Ammonium Axles

The threading of monostoppered alkylbenzylammonium axles 7+ and 8+ with the calix[6]-wheel 3 can occur by both routes of entering the macrocycle 3 in the cone conformation: passage through the upper rim and the through the lower rim. Thus, under thermodynamic conditions, with both the axles 7+ and 8+, the two possible orientations of calix[2]pseudorotaxane, namely, endo-benzyl and endo-alkyl, are formed by a stereoselectivity controlled by the endo-alkyl rule. Interestingly, by 1H NMR monitoring of the threading process between 8+ and 3, we revealed two calix[2]pseudorotaxane isomers in which the calix-wheel adopts 1,2,3-alternate and cone conformations, which represent the kinetic and thermodynamic species, respectively. Finally, the synthesis of ammonium-based oriented calix[2]rotaxane is here described.

The hexameric resorcinarene capsule as an artificial enzyme: ruling the regio and stereochemistry of a 1,3-dipolar cycloaddition between nitrones and unsaturated aldehydes

When a 1,3-dipolar cycloaddition between nitrones and α,β-unsaturated aldehydes catalyzed by L-proline derivatives takes place inside the hexameric resorcin[4]arene capsule the products are obtained in higher yields and selectivity, with respect to the reaction in bulk solution. The 4-formyl regioisomers were obtained preferentially, with an excellent preference for the endo-diastereoisomers and with moderate to good enantioselectivities. An extended in silico study was conducted to rationalize the observed outcome and to investigate the equilibria between the reagents outside and inside the capsule.

Supramolecular Organocatalysis in Water Mediated by Macrocyclic Compounds

In the last decades many efforts have been devoted to design supramolecular organocatalysts able to work in water as the reaction medium. The use of water as solvent provides promising benefits with respect to environmental impact. In this context, macrocyclic compounds played a role of primary importance thanks to their ease of synthesis and their molecular recognition abilities toward the reactants. The aim of this review is to give an overview of the recent advances in the field of supramolecular organocatalysis in water, focusing the attention on calixarene and cyclodextrins derivatives. Calixarenes and cyclodextrins, thanks to their hydrophobic cavities, are able to host selectively the substrates isolating they from the reaction environment. In addition, the synthetic versatilities of these macrocycles permits to introduce useful functional groups in close proximity of the hydrophobic binding sites. Regarding the cyclodextrins (CDs), we have here reviewed the their most recent uses as organocatalysts for the synthesis of heterocyclic compounds, in multi-component reactions and in carbon-carbon bond forming reactions. Examples have been reported in which CD catalysts are able to drive the regiochemistry of common organic reactions. In addition, cyclodextrins bearing catalytically active chiral groups, have shown excellent enantioselectivity in the catalysis of organic reactions. Recently reported results have shown that calixarene derivatives are able to accelerate organic reaction under “on-water” conditions with a significant selectivity toward the reactants. Under “on-water conditions” the hydrophobic effect, induced by insoluble calixarene derivatives, forces the reactants and the catalyst to aggregate and thus accelerating the reaction between them thanks to an amplification of weak secondary interactions. Regarding the use of water-soluble calixarene organocatalysts, we have here reviewed their role in the acceleration of common organic reactions.

Supramolecular synthons in the gamma-hydroxybutenolides

The supramolecular organization in the solid state of five novel gamma-hydroxybutenolides is described. The functionalities on their framework drive the construction of the crystalline packing through weak non-covalent forces such as H-bonds, CH–π, π–π and CH⋯O interactions. Gamma-butenolides bearing an unsubstituted aromatic or heteroaromatic ring as the side arm exhibit a helical assembly, in which the helical sense is determined by the chirality of the molecule, promoted by a three-center, bifurcated intra-intermolecular H-bond. In this assembly, the aromatic residues form an “aromatic zipper” between adjacent homochiral helices. Crystal structures of gamma-butenolide with p-substituted aromatic rings exhibit dimers, promoted by mutual O–H⋯OC intermolecular hydrogen-bond interactions. Finally, when the p-position of the aromatic ring is occupied by a potential H-bond acceptor substituent, such as CN group, the aromatic substituent is involved in an exclusive intermolecular H-bond in the observed crystal packing. The ability of gamma-butenolides to control the type of packing induced by the substitution pattern of their side arm makes them interesting for the engineering of novel crystalline assemblies.

Exploiting the p-Bromodienone Route for the Formation and Trapping of Calixarene Oxenium Cations with Enamine Nucleophiles

This study shows that calixarene p-bromodienone derivatives can act as precursors for the formation of oxenium cations, which can be trapped with enamine C-nucleophiles. When calixarene p-bromodienones were treated with enamines, in the presence of AgClO4, the lower rim-substituted C-O-C products were obtained by an electrophilic attack of the intermediate calixarene-oxenium cation with a contemporary cone-to-partial-cone inversion of the involved aromatic ring.