Structural Effects on the Intrinsic Basicities of alpha,beta-Unsaturated Lactones and Ketones

The proton affinities of 2(5H)-furanone, 1 (836 kJ/mol), 5,6-dihydro-2H-pyran-2-one, 2 (862 kJ/mol), cyclopentenone, 3 (857 kJ/mol), and cyclohexenone, 4 (863 kJ/mol), have been measured by Fourier transform ion cyclotron resonance techniques. A comparison is made with (reexamined) data concerning saturated cyclic and unsaturated aliphatic analogs. Three general observations are made. First, the basicity is found to increase with the size of the ring. Second, unsaturated lactones are more basic than their corresponding aliphatic unsaturated esters. Third, unsaturated and saturated lactones have almost identical gas-phase basicities, while unsaturated and saturated lactones have almost identical gas-phase basicities, while unsaturated cyclic ketones are more basic than their saturated analogs. All these experimental findings have been rationalized by means of ab initio calculations up to the G2(MP2,SVP) level. The basicity trends along the series are the result of two main factors: the different hybridization pattern of the carbonyl carbon as the size of the ring changes and, in the case of lactones, the nonbonding interaction between the proton attached to the carbonyl group and the ether-like oxygen which contributes to the enhanced stability of the protonated form. For unsaturated ketones the C=C double bond participates fully in the change in charge distribution induced by the protonation, while for unsaturated lactones the existence of an oxygen atom within the ring impedes this shift of the electron density.     

Cytosporacin,a highly unsaturated polyketide: application of the ACCORD-ADEQUATE experiment to the structural determination of natural products

Cytosporacin (1), a novel antibacterial polyketide containing naphthopyranone and isochromandione moieties, was isolated from the fermentation broth of the fungus Cytospora rhizophorae. A (1)H-detected ACCORD-ADEQUATE pulse sequence that distinguished (2)J(CH) from (3)J(CH) correlations provided critical information for structural determination. NOE studies established the relative configuration and revealed the presence of two rotamers. A biosynthetic (13)C-labeling experiment indicated that cytosporacin was derived from acetate origin.     

Isomerization of unsaturated radicals. VI. Isomerization of 3-cyclopenlenyl and pentamethylene radicals

The 184.9 nm photochemistry 0f gaseous 3-methylcyclopentene and 3-methyl-1,4-pentadiene have been studied. Both photoexcited species decompose mainly through the primayy rupture of the C-CH₃ bond. Vibrationally excited 3-cyclopenennyl and pentamethylene radicals are formed in the primayy decomposition in the former and latter systems respectively. These radicals are connected through isomerization reactions: in the presence of DI, the isomers cyclopenten,, and trans-1,3-pentadtene and/or vinylcyclopropane are formed in both systems. The quantum yields depend on the pressure and the starting monomer: cyclopentene and cyclopentadiene are the major products from the photolysis of 3-methylcyclopentene + DI mixtures and only minor quantities of the other C₅H₈ compounds are formed. Cyclopentadiene is the major product of the photolysis of 3-methyl-1,4-pentadiene + O₂ mixtures whereas vinylcyclopropane and trans-1,3-pentadiene are the major C₅ producss of the photolysis of 3-methyl-1,4-pentadiene + DI mixtures. The geometries of 3-cyclopentenyl and of the structures at the six critical points in the torsional potential energy curve (TPEC) for rotation about the 2- and 3-C-C bonds in the open chain pentamethylene species have been optimized completely by ab initio RHF-SCF gradient methods. For the open-chain structures the bond orders, bond lengths and the free valence (primarily associated with the central carbon atom) all correspond to 1,4-pentadien-3-yl conformations. In the ground state there is a high barrier to formation of 3-cyclopentenyl from 1,4-pentadien-3-yl. The features (relative energies and torsionll barriers) of the TPEC for 1,4-pentadien-3-yl explain the ESR observations for the open chain C₅H₇ radical rotamers.     

Études calorimétriques en milieu solvant organique IV. Dissociation de LiAlH4 dans le tétrahydrofuranne

From calorimetric measurements a model of solution is proposed for LiAlH₄ in THF. It is ionised as LiAlH₄ ? Li⁺ + AlH^?₄. For this reaction, ΔH_i = 3.05 kcal mol^?1 and the dissociation constant is K = 0.11.     

Solid-phase syntheses of 6-arylpyridazin-3(2H)-ones

The 3-chloropyridazine moiety was immobilized on a Wang resin, using two different methodologies. The first of these involved direct nucleophilic substitution of 3,6-dichloropyridazine with the alcoholate of Wang resin. The experimental conditions were optimized. The second method involved a Mitsunobu reaction between the Wang resin and 6-chloropyridazin-3-ol during which a problem of regioselectivity was observed. The so-obtained chloropyridazine-containing resins were subsequently reacted with various arylboronic acids under Suzuki conditions. Acid cleavage yielded 6-arylpyridazin-3(2H)-ones with high chemical purity.     

Crosslinking photosensitized by a ruthenium chelate as a tool for labeling and topographical studies of G-protein-coupled receptors

The purpose was to apply oxidative crosslinking reactions to the study of recognition and signaling mechanisms associated to G-protein-coupled receptors. Using a ruthenium chelate, Ru(bipy)(3)(2+), as photosensitizer and visible light irradiation, in the presence of ammonium persulfate, we performed fast and efficient covalent labeling of the B(2) bradykinin receptor by agonist or antagonist ligands possessing a radio-iodinated phenol moiety. The chemical and topographical specificities of these crosslinking experiments were investigated. The strategy could also be applied to the covalent labeling of the B(1) bradykinin receptor, the AT(1) angiotensin II receptor, the V(1a) vasopressin receptor and the oxytocin receptor. Interestingly, we demonstrated the possibility to covalently label the AT(1) and B(2) receptors with functionalized ligands. The potential applications of metal-chelate chemistry to receptor structural and signaling studies through intramolecular or intermolecular crosslinking are presented.     

Nitration in the imidazo[1,2-a]pyrazine series. Experimental and computational results

Nitration was carried out on a series of imidazo[1,2-a]pyrazine derivatives. The reactivities of diversely substituted derivatives and of all positions of substitution were analysed and experimental results compared with ¹³-nmr data and semi empirical calculations (AMI). Although the unsubstituted heterocycle is highly resistant to nitration, electron-donating groups such as alkoxy or alkylamino on position 8 enhance the reactivity of the imidazo[1,2-a]pyrazine derivatives towards electrophilic substitution and, more specifically, nitration. The ¹³-nmr experiments, electronic distributions and Molecular Electrostatic Potential isodensity surfaces calculated on the neutral forms are in good agreement with experimental results indicating position 3 is the most reactive position towards nitration.     

Catalyst-controlled diastereoselective hetero-Diels-Alder reactions

[reaction: see text] The diastereoselective hetero-Diels-Alder reaction between Danishefsky's diene and chiral aldehydes is catalyzed by chiral chromium-Schiff base complexes. High levels of catalyst control are obtained in several cases, allowing access to all four stereoisomeric products through appropriate choice of aldehyde and catalyst enantiomers.