Paolo Lo Meo

Spectrophotometric study on the thermodynamics of binding of α- and β-cyclodextrin towards some p-nitrobenzene derivatives

Binding properties of native α- and β-cyclodextrin towards some nitrobenzene derivatives have been studied by means of UV-vis spectrophotometry. The former host is able to form complexes having 1 : 1 and 1 : 2 stoichiometric ratios with these guests, while only 1 : 1 complexes are detected with the latter host. A careful analysis of the thermodynamic parameters for complexation equilibria, under the perspective of the enthalpy–entropy compensation effect, reveals that binding abilities of the two different hosts are subject to different features.

Studies on the stereoselective selenolactonization, hydroxy and methoxy selenenylation of α- and β-hydroxy acids and esters. Synthesis of δ- and γ-lactones

Abstract The diastereoselective synthesis of hydroxy substituted γ- and δ-lactones was accomplished following two approaches. A 5- or 6-endo cyclization promoted by electrophilic selenium reagents of α- or β-hydroxy acids and a 5- or 6-exo cyclization of hydroxy esters obtained through a diastereoselective hydroxy selenenylation reaction of α- or β-hydroxy esters. Moreover, the diastereoselective methoxy selenenylation of the above compounds was investigated showing a case in which the compound that was unreactive in the hydroxy selenenylation conditions gave, in the methoxy selenenylation conditions, the deprotected diol. The usefulness of the methoxy selenenylation procedure was proven by the preparation of a symmetric compound unsymmetrically functionalized. Yields and selectivities were found to depend on substituents (Ph or alkyl groups at the carbon atom that undergoes the nucleophilic attack), mode of cyclization, kinetic or thermodynamic control conditions. In the latter case, silica gel played an important role.

A Study of the Influence of Ionic Liquids Properties on the Kemp Elimination Reaction

The morpholino-induced elimination of 5-nitrobenzisoxazole into the relevant 2-cyano-4-nitrophenolate has been used as a sample reaction in order to investigate molecular properties of some room temperature ionic liquids. The kinetic study was carried out at 298 K by means of spectrophotometric measurements. Ionic liquids, which differ in both their cation and anion properties, were used as solvent systems. In particular, aliphatic (pyrrolidinium, piperidinium, and ammonium) and aromatic (imidazolium and pyridinium) ionic liquids were used. For aromatic cations, imidazolium ions having different hydrogen-bond donor ability or a different alkyl-chain length were taken into account. The anions chosen ([BF(4)(-)], [PF(6)(-)], [SbF(6)(-)], and [NTf(2)(-)]; where NTf(2) = bis(trifluoromethansulfonyl)imide) showed different shape, size, and coordination ability. Solvent parameters of all ionic liquids used were determined by using spectroscopic probes, such as 4-nitroaniline, N,N-diethyl-4-nitroaniline, Nile Red, and Reichardts dye. Finally, in order to obtain information on the structural organization of the solvent systems, resonance light-scattering measurements were carried out. The collected data provide evidence that ionic liquids are solvent media which exhibit peculiar features, whose effects can be rationalized only considering all parameters affecting their three-dimensional structure.

Regiochemical control in the synthesis of tetrahydrofurans by acid-catalyzed cyclization of hydroxy selenides and hydroxy sulfides

Abstract An efficient regiochemical control in the synthesis of tetrahydrofurans was achieved by acid-catalyzed cyclization of hydroxy-selenides ( 10a ) and ( 11a ) which cyclize in the endolexo mode and by hydroxy-sulfides ( 10c ) and ( 11c ) which cyclize in the exo mode. By cyclization of the relevant hydroxy-selenides, the C15–C20 fragment of (+)-Rolliniastatin 1 was obtained. Semiempirical calculations (PM3) were used in rationalizing experimental results.

The question of exo vs endo cyclisation. A joint experimental and ab initio study on the stereoselective synthesis of tetrahydrofurans and tetrahydropyrans via seleniranium ions

Abstract The reactivity of hydroxy selenides with catalytic amounts of perchloric acid in dichloromethane was investigated. Four different cyclisation modes (4- exo , 5- endo , 5- exo and 6- endo ) of the intermediate seleniranium ion could occur. The reaction of the hydroxy selenides with primary and secondary hydroxyl groups in the same side chain led first to a mixture of tetrahydrofurans from the 5- exo cyclisation (kinetic products), then to a 68/32 mixture of tetrahydropyrans from the 6- endo cyclisation (thermodynamic products) probably via two diasteromeric seleniranium ions. Both experimental and ab initio (HF/3-21G ∗ ) studies showed that the order of cyclisation of the intermediate seleniranium ion was 5- exo - tet endo - tet endo - tet exo - tet. The factors on which regiochemistry of the 4- exo vs the 5- endo cyclisation and the 5- exo vs the 6- endo cyclisation of the seleniranium ions depend were analysed.

Spectrophotometric determinations of binding constants between cyclodextrins and aromatic nitrogen substrates at various pH values

Abstract The inclusion capacity of native β-cyclodextrin (1) and mono-(6-amino-6-deoxy)-β-cyclodextrin (2) versus aromatic compounds having a nitro or an amino group or both has been investigated at three different pH values. Molecular interactions in inclusion complexes have also been investigated by means of molecular mechanics (MM2/QD) models. Electrostatic and van der Waals interactions and the formation of a hydrogen bond between the donor amino group and the oxygen atom of the secondary hydroxyl group seem to be the more important contributions in determining complex stability.

Spectrophotometric determination of binding constants between some aminocyclodextrins and nitrobenzene derivatives at various pH values

Abstract The inclusion capacity of three modified cyclodextrins—namely mono-(6- N , N -dimethylamino-6-deoxy)- ( 3 ), mono-6-(2-aminoethyl)-amino-6-deoxy- ( 4 ) and mono-6-(2- N , N -dimethylaminoethyl)-amino-6-deoxy- ( 5 ) β-cyclodextrin, with six para -substituted nitrobenzenes ( A – F ) has been investigated at three different pH values. Molecular interactions in inclusion complexes have also been investigated by means of molecular mechanics (MM2/QD) models. The desolvation of the cyclodextrin is the most important factor in determining the binding ability of the various hosts. However, for a given host, electrostatic and van der Waals interactions and the formation of a hydrogen bond between the donor amino group and the oxygen atom of a secondary hydroxyl group are the most important contributions in determining the binding constant of different guests.

Cyclodextrin–calixarene co-polymers as a new class of nanosponges

Hyper-reticulated co-polymers jointly formed by cyclodextrin and calixarene units, which can be considered as a new class of nanosponges, were easily obtained by means of a click chemistry approach. In particular, we succeeded in preparing our materials by exploiting the copper-catalyzed 1,3-dipolar cycloaddition (CuAAC) reaction between heptakis-(6-deoxy)-(6-azido)-β-cyclodextrin and (5,11,17,23-tetra-tert-butyl)-(25,26,27,28-tetra-propargyloxy)-calix-[4]-arene, mixed in different proportions. These materials were fully characterized by means of combined FT-IR, thermogravimetric, 13C {1H} CP-MAS NMR and nitrogen adsorption/desorption techniques. In particular, 13C {1H} CP-MAS spectra allowed us to perform a quantitative analysis of the co-polymers obtained. Tests on their possible sequestering abilities towards some organic molecules, in particular nitroarenes and commercial dyes chosen as suitable pollutants or drug models, were successfully carried out. Results obtained point out that absorption abilities seem affected by polymer composition and porosity as well.

Stereoselective synthesis of tetrahydrofurans and tetrahydropyrans by acid-catalyzed cyclization of hydroxy selenides and hydroxy sulfides

Abstract The behaviour in acid media of hydroxy selenides and hydroxy sulfides ( 1a-c and 1′a-c ) was investigated. The protection of the primary hydroxyl group in compounds 1a and 1′a allowed the stereoselective synthesis of a substituted tetrahydrofuran ring, whereas compounds 1b-c and 1′b-c gave an efficient regiochemical control affording substituted tetrahydropyran rings. Tetrahydropyrans containing the phenylselanyl moiety were found to be in equilibrium in the cyclization reaction conditions, whereas tetrahydropyrans containing the phenylsulfanyl moiety were not. A mechanism for the above equilibration is proposed. Semiempirical (AM1, PM3) and ab initio (HF/3-21G ∗ ) calculations were used in an attempt to rationalize the experimental results.

The binary pyrene/heptakis-(6-amino-6-deoxy)-β-cyclodextrin complex: a suitable chiral discriminator. Spectrofluorimetric study of the effect of some α-amino acids and esters on the stability of the binary complex

Abstract The effect of some α-amino acids and their esters on the stability of the binary pyrene/heptakis-(6-amino-6-deoxy)-β-cyclodextrin (py/am-β-CD) complex has been studied by means of fluorescence spectroscopy at two pH values (8.0 and 9.0). The binary complex was generally stabilized by adding the ternary agent at pH 8.0. A more varied substrate effect is observed at pH 9.0 where am-β-CD is present in the uncharged form. The conditional constant ( β 2 ) values determined by l / d α-amino acids show that the binary complex is a suitable receptor for chiral recognition. The enantiomer selectivity values obtained, ranging from 1.2 up to 7.4, are generally higher than those reported for α-amino acids and their derivatives by modified cyclodextrins.