María T. Baumgartner

Effect of different C3-aryl substituents on the antioxidant activity of 4-hydroxycoumarin derivatives.

The antioxidant activity of 4-hydroxycoumarin synthetic derivatives and 4-methylumbelliferone were determined taking 4-hydroxycoumarin as the reference compound. Six 3-aryl-4-hydroxycoumarin derivatives were synthesized from 4-hydroxycoumarin as precursor in order to evaluate changes in their antioxidant properties due to C3-aryl substituent nature. Free radical scavenging capacities of these compounds against two different species DPPH(·) and ABTS(·+) and the protecting ability towards the β-carotene-linoleic acid co-oxidation enzymatically induced by lipoxygenase were measured. In addition, the relationship between the activities of these molecules against DPPH radical and the bond dissociation energy of O-H (BDE) calculated using methods of computational chemistry was evaluated.

Photostimulated reactions of haloarenes with 2-naphtylamide ions. A facile synthesis of 1-aryl-2-naphthylamines.

Abstract The photostimulated reactions of aryl iodides with 2-naphthylamide ions in liquid ammonia gave 1-aryl-2-naphthylamines as the major substitution product. This reaction is proposed to occur by the S RN 1 mechanism of nucleophilic substitution

Photostimulated reaction of aryl iodides with 2-naphthoxide ions by the SRN1 mechanism

The photostimulated reactions of p-iodoanisole (2) and 1-iodonaphthalene (7) with 2-naphthoxide ions in liquid ammonia gave the corresponding 1-aryl-2-naphthols as the only substitution product. These reactions are proposed to proceed via the SRN1 mechanism for nucleophilic aromatic substitution.

The reactivity of oxygen nucleophiles with aryl radicals in the SRN1 mechanism

Abstract The photostimulated reaction of 2-naphthoxide ions 1 with o-dihalobenzenes in liquid ammonia gives the halosubstituted product 4 and the cyclized substituted product 5. This is the first report about the coupling of an aromatic σ radical with an oxygen functionality in the chain propagation cycle of the SRN1 mechanism.

Theoretical study of the relative reactivity of chloroethenes with atmospheric oxidants (OH, NO3, O(3P), Cl(2P) and Br(2P))

The reactivity of a series of chloroethenes with different electrophiles of tropospheric and stratospheric interest is analyzed by frontier molecular orbital theory and a correlation with calculated orbital energies is investigated. The reactions of CH2CHCl; CH2CCl2; (Z)-CHClCHCl; (E)-CHClCHCl; CHClCCl2 and CCl2CCl2 with O(3P), Cl(2P), Br(2P) atoms and with OH and NO3 radicals were studied using semiempirical methods (AM1 and PM3) and ab initio calculations at the HF and B3LYP levels of theory with the 6-31G** basis set, using the Gaussian 98 suite of programs. In contrast to the majority of reaction series of small radicals and molecules with alkenes and alkanes, the rate constants for the reactions with halogenated ethenes do not correlate with the ionization potential of the halogenated ethene. The energy and the carbon–carbon π-bonding form of the HOMO change on addition of chlorine atoms as substituents to the carbon–carbon σ-bonding framework of the alkenes. For the reactions studied the complete interaction HOMO–SOMO was considered, taking into account the contribution of the different atomic orbitals to the HOMO of the chloroethene through the atomic orbital coefficients, and a good correlation with the experimental values was obtained.

Conformational studies of novel antiretroviral analogs of zidovudine.

Abstract Conformational properties of three novel zidovudine analogs, namely 3′-azido-3′-deoxy-5′-O-isonicotinoylthymidine (AZT-Iso, 2), (−)-trans-(5S,6S)-5-bromo-6, 5′-epoxy-5,6-dihydro-3′-azido-3′-deoxythymidine (3) and (+)-trans-(5R,6R)-5-bromo-6,5′-epoxy-5,6-dihydro-3′-azido-3′-deoxythymidine (4), have been investigated by AM1 calculations and NMR studies, and compared with those of the parent nucleoside (AZT, 1). Based on the results obtained the following correlation may be established, a) AZT and AZT-Iso exhibit a conformational behavior analog to other pyrimidinic nucleosides, displaying a dynamic equilibrium in solution where the two conformers (North and South) undergo a constant transformation. b) Compounds 3 and 4 show a different conformational profile. The estimate of the pseudorotation phase angle reveals the rigid structures of the latter compounds, which do not evidence conformational equilibrium in solution; theazide group being the only group free to rotate. c) Diastereoisomers 3 and 4 exhibit an extra conformational parameter compared with other pyrimidinic nucleosides: the chair or boat conformation in the third ring formed between the sugar and the base. In all cases, a reasonable correlation was obtained between theoretical and NMR spectroscopic data.

Steric effects of nucleophile-radical coupling reaction. Determination of rate constants for the reaction of aryl radicals with 2-naphthoxide anion

The absolute rate constants for the reaction of different aryl radicals with 2-naphthoxide anion were determined using an indirect method, a competition of the coupling reaction with the H-atom abstraction. We here show that the radical-ambident nucleophile reactions are sensitive to steric hindrance. A lower reactivity is found for 2-anisyl radical with respect to 4-anisyl and for 2-methoxy-1-naphthyl radical with respect to 1-naphthyl (k2b < k2a and k2d < k2c). The ortho substitution to the radical centre decreases the rate constant. The reactivity is in agreement with energy barriers determined by theoretical calculations.

Regioselectivity of the coupling between radicals and ambident nucleophiles. A theoretical study

We here report a theoretical study on the reaction of the anions of phenol, 2-naphthol, the pyrimidine bases (uracil, thymine, cytosine), pyrrole, imidazole and benzimidazole with phenyl and 4-nitrobenzyl radicals with the aim of interpreting the factors that control the regiochemical outcome of these reactions; their main features being C–C bond formation with phenyl radicals and C–heteroatom bond formation with 4-nitrobenzyl radicals. The preferred coupling positions were investigated by evaluation of the frontier molecular orbital theory (FMO) indexes and the potential energy surfaces both at the AM1 and DFT/B3LYP levels. Within the latter methodology, the solvent effect was considered under a continuum model. While FMO predicts the experimental regioselectivity of phenyl radicals with good accuracy, it fails in the nitrobenzyl case. On the other hand, studies of transition state stabilities provide a good prediction of the observed regioselectivity for both types of radicals, mainly when the solvent is taken into account.

A theoretical study of nitrogen radicals. Generation, reactivity and selectivity in electron transfer reactions

A theoretical study was made on the fragmentation of different radical anions from sulfonamides, sulfenamides and haloamides. Good correlation was obtained between theoretical and experimental results. The correlation indicated the validity of the procedure to determine the identity of the precursors of nitrogen radicals and their reactivity and regioselectivity in the reactions with nucleophiles.

Diels–Alder Reactions of d-Glucose-Derived Dienophiles with Cyclopentadiene: A Computational Study

Abstract AM1 and B3LYP calculations were performed for the Diels–Alder reactions of a series of d -glucose-derived dienophiles with cyclopentadiene. The preferred β vs. α diastereofacial addition and exo vs. endo selectivities are explained on the basis of products and transition states stabilities. Computed exo/endo ratios change in the correct direction although they do not agree quantitatively with available experimental data. The relative reactivity of the dienophiles bearing a carbonyl group can be interpreted in terms of FMO theory as well as of repulsive interactions in the corresponding transition states. The calculations fail to explain the observed lack of reactivity for the dienophile bearing a cyano group.