Michèle Cesario

Binding of acetylcholine and other quaternary ammonium cations by sulfonated calixarenes. Crystal structure of a [choline-tetrasulfonated calix[4]arene] complex

Abstract The water-soluble tetra- and hexasulfonated calix[4] and calix[6]arenes bind very strongly the neurotransmitter acetylcholine and other quaternary ammonium cations with association constants Ks up to 4×105 M−1 for NEt4 +. The high affinities for choline and acetylcholine (Ks=5×104 to 8×104 M−1) are comparable to those of the biological recognition sites. The crystal structure of the [choline-tetrasulfonated calix[4]arene] complex was determined. Two complexes A and B were observed in the asymmetric unit. In each one, the choline inserts its N-terminal inside the cavity of the receptor. The calix[4]arene adopts a single cone conformation but the choline substrate is in an extended conformation in complex A and adopts two other folded conformations in complex B (75 % –25 %). The very high association constants and the crystal structure provide important information about the nature of the binding in this biologically most relevant complex.

Identification of des-A-triterpenoid hydrocarbons occurring in surface sediments

Abstract Molecular structures of eight tetracyclic hydrocarbons occurring in sediments were established either by synthesis of the corresponding reference compounds or spectroscopic studies (MS, NMR, X-ray diffraction) of the natural products isolated from a pond mud. These products are derived from higher plant 3-oxygenated pentacyclic triterpenes (lupane, oleanane and ursane series) by microbially mediated loss of ring A and subsequent aromatisations.

Benzohopanes, a novel family of hexacyclic geomarkers in sediments and petroleums.

Abstract A novel family of hexacyclic hopanoids, the benzohopanes 1 (C32–C35), has been identified in geological sources by X-ray crystallography. These compounds must be formed from bacterial C35 hopanoid precursors during early stages of sedimentation.

Structure and electronic properties of (N,N′-bis(4-methyl-6-tert-butyl-2-methyl-phenolato)-N,N′-bismethyl-1,2-diaminoethaneFeIII (DBSQ). Spectroelectrochemical study of the red-ox properties. Relevance to intradiol catechol dioxygenases

Abstract The species LFe III Cl ( 1 ) was synthesized where L 2− is the dianion N , N ′-bis(4-methyl-6-tert-butyl-2-methyl-phenolato)- N , N ′-bismethyl1,2-diaminoethane. It crystallizes in the triclinic space group P -1 with a = 14.704(6), b = 17.421(7), c = 17.328(8) A , α = 89.45(8) , β = 129.76(9), γ = 102.71(9)°, V = 3277(2) A 3 and Z = 2 . The molecule has approximately a square pyramidal structure. In the presence of DBCH 2 (3,5-di-tert-butylcatechol), this complex gives LFe III (DBSQ) ( 2 ), a stable Fe(III)-semiquinonato complex (DBSQ − stands for the 3,5-di-tert-butyl- o -benzosemiquinone monoanion). It crystallizes in the monoclinic space group C 2/ c with a = 36.24(2), b = 10.438(5), c = 23.928(12) A , β = 115.31(5)°, V = 8183(7) A 3 and Z = 8 . This X-ray study allows the structure of the DBSQ − monoanion complexed to Fe(III) to be compared with that of the DBC 2− dianion complexed to Fe(III) as found in several complexes already described (see for instance H.G. Jang, D.D. Cox and L. Que, Jr., J. Am. Chem. Soc., 113 (1991) 9200–9204). A clear alternation is found in CC bond lengths in the DBSQ − monoanion. Magnetic coupling between the S Fe = 5 2 electronic spin on Fe(III) and the S R = 1 2 electronic spin on the DBSQ − anion radical has been deduced from the behavior of the magnetic susceptibility as a function of temperature. It has been found antiferromagnetic with J = −206 cm −1 (with the notation H = − JS Fc S R ). This is a weaker coupling than that found in other analogous complexes. Mossbauer spectroscopy confirms the S = 2 nature of the ground state. Analysis of the 57 Fe hyperfine coupling parameters prove that the earlier description of the electronic structure of 2 is correct. It is possible to reduce 2 to get [LFe III (DBC)] − ( E 0 = −0.3 V/SCE in AcN). This species has UV-Vis and EPR properties typical of this type of complex. In the absence of protons, [LFe III (DBC)] − is stable under pure O 2 but the addition of protons results in its oxidation to form 2 . The insensitivity of [LFe III (DBC)] − to O 2 is compatible with the idea that the system has to go through an Fe(II) (DBSQ) form by internal electron transfer to allow the attack of the aromatic ring by O 2 (see L. Que and R.Y.N. Ho, Chem. Rev., 96 (1996) 2607–2624, for a review). Observation of the lowest energy LMCT band in [LFe III (DBC)] − at 620 nm (2.00 eV, AcN) suggests that the Fe(II) (DBSQ) form is indeed at high energy ( ΔG 0 ) above the catechol form. This contributes from Marcus theory to a high activation energy. In intradiol dioxygenases the decoordination of one of the tyrosine ligands (D.H. Ohlendorf, A.M. Orville and J.D. Lipscomb, J. Mol. Biol., 244 (1994) 586–608) is certainly important for the deprotonation of the catechol substrate but could also accelerate the formation of the postulated Fe(II)-semiquinonato intermediate. The stability of 2 under O 2 also demonstrates the importance of the Fe(II) oxidation state in order to lead to a peroxo form of the oxygen adduct. The insensitivity of [LFe III (DBC)] − to O 2 argues against the view of the dioxygenase activity as the result of an electrophilic attack of the catecholato by O 2 in model complexes.

Chirality directed self-assembly. Resolution of 2,5-diazabicyclo[2.2.2]octane-3,6-dione and crystal structures of its racemic and (−) enantiomeric forms

The chiral bicyclic bis-lactam 1 has been resolved and the crystal structures of the racemate (±)-1 and of enantiomerically pure (−)-1 have been determined. They show that the chiral nature of the substance determines the supramolecular architecture generated by hydrogen bonding self-assembly, giving an infinite zig-zag chain for the racemate and a cyclic tetramer for the enantiomer.

Polyaza-macrocycles of cyclophane type: synthesis, structure of a chloroform inclusion complex and anion binding

Abstract Polyazamacrocycles (1)–(7) have been obtained via efficient (amine + aldehyde) polycondensation processes. The tetraimine (1) forms a chloroform inclusion complex whose crystal structure has been determined; the protonated tetraamine (5) binds dicarboxylate substrates.

Synthesis and absolute configuration of chiral (C3) cyclotriveratrylene derivatives. Crystal structure of (M)-(–)-2,7,12-triethoxy-3,8,13-tris-[(R)-1-methoxycarbonylethoxy]-10,15-dihydro-5H-tribenzo[a,d,g]-cyclononene

The absolute configuration of chiral C3-cyclotriveratrylene derivatives has been determined unambigously as follows. First, the absolute configuration of 2-(2-ethoxy-4-hydroxymethylphenoxy)propionic acid (5b) was established as R-(+) by chemical correlations and circular dichroism measurements. Upon treatment with perchloric acid, compound (+)-(5b) leads to the title compound (–)-(3), isolated in 14% yield, m.p. 127 °C, [α]D25–16.1°(CHCl3). The crystal and molecular structure of (–)-(3) have been determined by single crystal X-ray analysis: orthorhombic, a= 23.428, b= 22.165, c= 15.145 A space group P212121, Z= 8. The M absolute configuration has been ascribed to (–)-(3), on the basis of internal comparison with groups –CH(Me)CO2Me whose absolute stereochemistry is derived from that of (+)-(5b). Seven chiral C3-cyclotriveratrylene derivatives have been chemically related to (M)-(–)-(3). The absolute configurations so established are identical with those deduced independently from an exciton analysis of circular dichroism spectra.

Synthesis and photochromism of two new 1,2-bis(thiazolyl)perfluorocyclopentenes with chelating sites

Two new dithiazolylethenes, 1a {1,2-bis[5′-methyl-2′-(2′′-pyridyl)thiazolyl]perfluorocyclopentene} and 2a {1,2-bis[4′-methyl-2′-(2′′-pyridyl)thiazolyl]perfluorocyclopentene}, have been synthesized. Their photochromic behavior has been fully investigated in solution as well as in the crystalline phase. They display both fatigue-resistant and thermally irreversible photochromic reactions, with more than 80% of the closed-ring forms (1b and 2b, respectively) in the photo-stationary state. Upon UV irradiation, a colorless solution of 1a turns purple while the yellow color of a 2a solution becomes more intense. Such marked differences in their electronic absorption spectra are reproduced by DFT calculations. Finally, although structurally closely related, only 1a shows photochromism in the crystalline phase whereas 2a displays reversible photo-modulation of fluorescence in solution.

Synthesis of chiral isoquinuclidines and determination of their absolute configuration

Abstract A route to 1,2-dihydropyridines N1-substituted with chiral auxiliaries has been developed starting from commercially available chiral amines. Cycloaddition between these dihydropyridines and methyl acrylate gave, in moderate d.e., isoquinuclidines of good enantiomeric purity whose absolute configuration has been established.

New MnII Complexes with an N/O Coordination Sphere from TripodalN-Centered Ligands − Characterization from Solid State to Solution and Reaction with Superoxide in Non-Aqueous and Aqueous Media

The crystal structure of a dimeric bis(µ‐carboxylato)MnIIMnII complex (2) with a tetradentate N‐centered tripodal ligand, N,N‐bis[(1‐methylimidazol‐2‐yl)methyl]glycinate, is presented. The carboxylates are bridging monodentate. This complex shows some striking differences to the previously published parent compound 1 obtained with the closely related ligand N‐[(1‐methylimidazol‐2‐yl)methyl]‐N‐(2‐pyridylmethyl)glycinate. The carboxylato bridges in both structures are shown to be disrupted in solution. The reactivities in solution of compounds 1 and 2 toward superoxide were studied. In anhydrous DMSO solutions, a pathway involving a bis(µ‐oxo)MnIIIMnIV complex was identified. In water solutions, IC50 values were measured by the Fridovich test (1.7 ± 0.2 µM for 2 and 2.0 ± 0.2 µM for 1), which showed these complexes to be good superoxide scavengers. Association constants for the complexes were estimated by means of a method based on IC50 measurements of the complexes under several conditions (alone and in the presence of excess ligand). They were found to be consistent with literature data.