Gisele F. Gauze

Effect of Sulfur Oxidation on the Transmission Mechanism of 4JHH NMR Coupling Constants in 1,3-Dithiane

Long-range 4J(HH) couplings in 1,3-dithiane derivatives are rationalized in terms of the effects of hyperconjugative interactions involving the S=O group. Theoretical and experimental studies of 4J(HH) couplings were carried out in 1,3-dithiane-1-oxide (2), cis-1,3-dithiane-1,3-dioxide (3), 1,3-dithiane-1,1,3-trioxide (4), and 1,3-dithiane-1,1,3,3-tetraoxide (5) compounds. Hyperconjugative interactions were studied with the natural bond orbital, NBO, method. Hyperconjugative interactions involving the LP(O), oxygen lone pair and sigma*(C2-S1) and sigma*(S1-C6) antibonding orbitals yield an increase of 4J(H(eq)-H(eq)) couplings. Long-range 4J(H(ax)-H(ax)) couplings were also observed between hydrogen atoms in axial orientation, which are rationalized as originating in hyperconjugative interactions involving the bonding sigma(C6-H(ax)) and antibonding sigma*(S=O) orbitals. The symmetry for orbital interactions is possible only when the S=O group is in the axial orientation.

Theoretical and experimental investigation of the polyeletrophilic β-enamino diketone: straightforward and highly regioselective synthesis of 1,4,5-trisubstituted pyrazoles and pyrazolo[3,4-d]pyridazinones

Obtaining a new precursor enamino diketone with five electrophilic centers is reported, along with theoretical and experimental studies of its reactivity against mono- or dinucleophiles. The Fukui function showed that the β-carbon is the most electrophilic center, followed by the carbonyl ketone and the carbonyl ester, respectively. The reaction of enamino diketone with aniline and hydrazines allowed for the synthesis of a new enamino diketone and 1,4-disubstituted pyrazoles-5-carboxylates, respectively. The regiochemistry and mechanism of syntheses of 1,4-disubstituted pyrazoles-5-carboxylates were determined from reaction monitoring by ESI-MS, NMR analysis and crystallographic data, and fully agreed with the theoretical results. The versatility and efficiency of the enamino diketone was demonstrated by the reaction with hydrazines furnishing multi-functionalized pyrazoles and pyrazolo[3,4-d]pyridazinone derivatives with high regioselectivity.

The conformational analysis of 2-halocyclooctanones.

The establishment of the most stable structures of eight membered rings is a challenging task to the field of conformational analysis. In this work, a series of 2-halocyclooctanones were synthesized (including fluorine, chlorine, bromine and iodine derivatives) and submitted to conformational studies using a combination of theoretical calculation and infrared spectroscopy. For each compound, four conformations were identified as the most important ones. These conformations are derived from the chair-boat conformation of cyclooctanone. The pseudo-equatorial (with respect to the halogen) conformer is preferred in vacuum and in low polarity solvents for chlorine, bromine and iodine derivatives. For 2-fluorocyclooctanone, the preferred conformation in vacuum is pseudo-axial. In acetonitrile, the pseudo-axial conformer becomes the most stable for the chlorine derivative. According to NBO calculations, the conformational preference is not dictated by electron delocalization, but by classical electrostatic repulsions.

Infrared and theoretical calculations in 2-halocycloheptanones conformational analysis

2-Halocycloheptanones (Halo=F, Cl, Br and I) were synthesized and their conformational analysis was performed through infrared spectroscopy data. The corresponding conformers geometries and energies were obtained by theoretical calculations at B3LYP/aug-cc-pVDZ level of theory in the isolated state and in solution. It was observed, by both approaches, that the conformational preferences were very sensitive to the solvent polarity, since its increase led to an increase in the population of the more polar conformer. An analysis of these conformational equilibria showed they suffer also the influence of stereoelectronic effects, like hyperconjugation and steric effects. These results were interpreted using natural bond orbital (NBO) analysis, which indicated that the electronic delocalization to the orbital π*(C=O) is directly involved in the stability increase of conformers I and II. The relative effect of the period of the halogen can also be noted, with changes in the conformational preferences and in the energies involved in the interactions of NBO.

Theoretical aspects of the unexpected regiospecific synthesis of pyrazole-5-carboxylates from unsymmetrical enaminodiketones

Pyrazoles are heterocycles with economic importance because of the wide applications of such compounds in the pharmaceutical and agrochemical industries. Pyrazoles can be prepared from the cyclocondensation of unsymmetrical enaminodiketones with hydrazines. However, this method often suffers from the formation of a regioisomeric mixture of pyrazoles with generally poor selectivity. Only in few cases, the regiospecific synthesis was possible. Nevertheless, up to now, the factors behind the regiospecificity of this reaction were unknown. Thus, in this work, we use density functional theory (M06-2X/6-31++G(d,p)) to analyze the reaction mechanism. Our results show that the activation-free energy leading to the possible products are similar, but the products present very different stabilities which explain the formation of a single regioisomer experimentally observed. In this way, the regiospecificity is governed by thermodynamical factors.

Cholinesterases Inhibition by Novel cis- and trans-3-Arylaminocyclohexyl N,N-Dimethylcarbamates: Biological Evaluation and Molecular Modeling

The present study describes the synthesis, assessment of the anticholinesterase activity and the inhibition type of novel cis- and trans-3-arylaminocyclohexyl N,N-dimethylcarbamates. In vitro inhibition assay by Ellmans method with human blood samples showed that carbamates were selective for butyrylcholinesterase (BuChE) with compound concentration that inhibits 50% of enzyme activity (IC50) between 0.11 and 0.18 mmol L-1. cis- and trans-3-(4-Methoxyphenylamino)cyclohexyl N,N-dimethylcarbamate hydrochloride were the most active for BuChE, showing that the presence of methoxyl group enhanced the anticholinesterase activity. The enzyme kinetics studies indicate a noncompetitive inhibition against acetylcholinesterase (AChE) and mixed type inhibition for BuChE. Molecular modeling studies confirm the ability of carbamates to bind both the active and peripheral sites of the BuChE.

Molecular modeling and anticholinesterasic activity of novel 2-arylaminocyclohexyl N,N-dimethylcarbamates

This work reports a detailed theoretical and experimental study of the novel isomer series cis- and trans-2-arylaminocyclohexyl N,N-dimethylcarbamates as potential inhibitors of cholinesterases. In vitro inhibition assay by Ellmans method with human blood samples showed that the new carbamates are selective to the inhibition of enzyme butyrylcholinesterase (BuChE) with maximum inhibition of 90% and IC50 of 6 and 8 mmol L-1 for the more actives compounds of the series. Molecular modeling studies point to significant differences for the conformations of the compounds in the active sites of enzymes BuChE and acetylcholinesterase (AChE). The results show that the compounds interact more effectively with the active site of enzyme BuChE since the carbamate group is close to the key residues of the catalytic triad.

Targeting the Homoserine Dehydrogenase of Paracoccidioides Species for Treatment of Systemic Fungal Infections

ABSTRACT This work evaluated new potential inhibitors of the enzyme homoserine dehydrogenase (HSD) of Paracoccidioides brasiliensis, one of the etiological agents of paracoccidioidomycosis. The tertiary structure of the protein bonded to the analogue NAD, and l-homoserine was modeled by homology. The model with the best output was subjected to gradient minimization, redocking, and molecular dynamics simulation. Virtual screening simulations with 187,841 molecules purchasable from the Zinc database were performed. After the screenings, 14 molecules were selected and analyzed by the use of absorption, distribution, metabolism, excretion, and toxicity criteria, resulting in four compounds for in vitro assays. The molecules HS1 and HS2 were promising, exhibiting MICs of 64 and 32 μg · ml−1, respectively, for the Pb18 isolate of P. brasilensis, 64 μg · ml−1 for two isolates of P. lutzii, and also synergy with itraconazole. The application of these molecules to human-pathogenic fungi confirmed that the HSD enzyme may be used as a target for the development of drugs with specific action against paracoccidioidomycosis; moreover, these compounds may serve as leads in the design of new antifungals.