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Molina de Aragón, Spain

Merino P.,Laboratorio Of Sintesis Asimetrica | Tejero T.,Laboratorio Of Sintesis Asimetrica

Mannich-type reactions with nitrones, oximes, and hydrazones provide efficient approaches to β-amino carbonyl derivatives with nitrogen functionality existing in an intermediate oxidation state, as in the cases of hydroxylamine and hydrazine derivatives. By employing chiral substrates and catalysts enantiomerically pure substances can be prepared by using these processes. © Georg Thieme Verlag Stuttgart - New York. Source

Marca E.,Laboratorio Of Sintesis Asimetrica | Valero-Gonzalez J.,University of Zaragoza | Delso I.,Laboratorio Of Sintesis Asimetrica | Tejero T.,Laboratorio Of Sintesis Asimetrica | And 2 more authors.
Carbohydrate Research

A series of β-(1,3)-d-glucans have been synthesized incorporating structural variations specifically on the reducing end of the oligomers. Both O- and C-glucosides derived from di- and trisaccharides have been obtained in good overall yields and with complete selectivity. Whereas the O-glycosides were obtained via a classical Koenigs-Knorr glycosylation, the corresponding C-glycosides were obtained through allylation of the anomeric carbon and further cross-metathesis reaction. Finally, the compounds were evaluated against two glycosidases and two endo-glucanases and no inhibitory activity was observed. © 2013 Elsevier Ltd. All rights reserved. Source

Delso I.,Laboratorio Of Sintesis Asimetrica | Melicchio A.,Laboratorio Of Sintesis Asimetrica | Melicchio A.,University of Calabria | Isasi A.,Laboratorio Of Sintesis Asimetrica | And 2 more authors.
European Journal of Organic Chemistry

A full experimental study of the activation energy required for the hitherto unknown neutral 2-aza-Cope rearrangement is presented. A kinetic study of the process showed activation energies in the range of 22.91-24.06 kcal/mol, in agreement with a process operating at moderate temperature (70°C). Calculations at B3LYP/6-311+G(d,p) and M06-2X/6-311+G(d,p) levels of theory considering solvent (dimethyl sulfoxide (DMSO) and toluene) effects (PCM model) predict reaction energy barriers that are in agreement with the values obtained from 1H NMR-based kinetic experiments. Results obtained by using enantiomerically pure substrates demonstrate that the rearrangement takes place with complete transfer of chirality, in contrast to previously described cationic processes. The effects of solvent and acid catalysis, which converts the process into the more common cationic rearrangement, have also been studied. DFT calculations also predict correctly the acceleration of the process under acid catalysis, estimating energy barriers in the range of 16.80-18.57 kcal/mol. The hitherto unknown neutral 2-aza-Cope rearrangement of nitrones takes place under thermal conditions with complete transfer of chirality. The process can be catalyzed by acid through a classical cationic 2-aza-Cope rearrangement. Kinetic 1H NMR experiments and DFT theoretical studies have been used to estimate the activation parameters and determine the energy of activation of the process. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

Delso I.,Laboratorio Of Sintesis Asimetrica | Delso I.,CSIC - Institute of Materials Science | Marca E.,Laboratorio Of Sintesis Asimetrica | Mannucci V.,Laboratorio Of Sintesis Asimetrica | And 4 more authors.
Chemistry - A European Journal

The diastereofacial selection in addition reactions to biased rigid systems can be modulated by the action of Lewis acids. As an example, the stereoselectivity of the nucleophilic addition of vinyl magnesium bromide (VMB) to cyclic nitrones in the presence of diethylaluminum chloride (DEAC) shows a strong dependence on the temperature and the carbon substituent adjacent at the reaction center; it is remarkable that whereas a high selectivity is obtained at higher temperatures, in the presence of DEAC, a trend to invert the stereochemical course of the reaction is observed at lower temperatures, provided the substituent at C3 of the pyrrolidine ring allows delivery of the vinyl moiety. This behavior and difference in selectivity is to be attributed to the high conformational barriers of the intermediate nitrone-DEAC-VMB complex. A clear inversion of the selectivity is observed at -78°C for the reaction of the nitrone protected as an Omethyl derivative. The present study provides a rationalization for the stereocontrolled addition of nucleophiles to rigid systems (cyclic nitrones). © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

Merino P.,Laboratorio Of Sintesis Asimetrica | Tejero T.,Laboratorio Of Sintesis Asimetrica | Diez-Martinez A.,Laboratorio Of Sintesis Asimetrica
Journal of Organic Chemistry

A full theoretical study of the reaction between a novel type of ylide, i.e. nitrone ylides, and alkenes has been carried out. Both concerted and polar stepwise mechanisms have been considered. Only the zwitterionic mechanism predicts correctly the experimentally observed adducts. Depending on the level of theory, the mechanism moves from concerted to polar stepwise, as demonstrated by the corresponding IRC analyses. The regio- and stereoselectivity of the reaction is well explained for both mono- and disubstituted alkenes. In the case of methyl acrylate a pathway leading to the two diastereoisomers obtained experimentally is predicted. For methyl fumarate a stereospecific mechanism is predicted as a consequence of a C-H⋯O=C interaction present in a Li-tricoordinated transition structure. The stereospecificity in the reaction with methyl maleate comes from a less hindered coordination around the lithium atom. Calculations with B3LYP and M06-2X functionals indicate that only the latter provides energy values in good agreement with experimental findings. © 2014 American Chemical Society. Source

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