Percorrer por autor "Lemos, A."
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- [4+2] Cycloadditions of 3-Tetrazolyl-1,2-diaza-1,3-butadienes: synthesis of 3-Tetrazolyl-1,4,5,6-tetrahydropyridazinesPublication . Lopes, Susana M. M.; Brigas, Amadeu; Palacios, Francisco; Lemos, A.; Melo, Teresa M. V. D. Pinho eThe synthesis of new 3-tetrazolyl-1,4,5,6-tetrahydropyridazines by Diels–Alder reactions of ethyl 3-(tetrazol-5-yl)-1,2- diaza-1,3-butadiene-1-carboxylates is reported. These 1,2- diaza-1,3-dienes reacted with electron-rich heterocycles, nucleophilic olefins and cumulenes, as well as with electrondeficient dienophiles, to give the target compounds regioselectively. Computational studies corroborated the rationalization of the observed reactivity and selectivity.
- Addition and cycloaddition reactions of Phosphinyl- and Phosphonyl-2H-Azirines, Nitrosoalkenes and AzoalkenesPublication . Lemos, A.An overview of the use of 2H-azirines, conjugated nitrosoalkenes and conjugated azoalkenes bearing phosphorus substituents in addition and cycloaddition reactions is presented, focused on strategies for the synthesis of aminophosphonate and aminophosphine oxide derivatives.
- Azabicyclo[3.2.0]heptan-7-ones (carbapenams) from pyrrolePublication . Gilchrist, Thomas L.; Lemos, A.; Ottaway, Carol J.The azabicyclo[3.2.0]heptan-7-ones 4, 10, 16 and 24 have been prepared from pyrrole. The same general approach has been used for all these derivatives; namely, substitution of pyrrole at the 2- and 5-carbon atoms, catalytic hydrogenation to produce pyrrolidine-2-acetic acid derivatives, and cyclisation using tris(1,3-dihydro-2-oxobenzoxazol-3-yl)phosphine oxide 6. The catalytic hydrogenation of 2,5-disubstituted pyrroles gives only the corresponding cis-2,5-disubstituted pyrrolidines. The hydrogenation proceeds more easily when the nitrogen atom bears a tert-butoxycarbonyl substituent. The N-tert-butoxycarbonylpyrroles 8 and 21 bearing an á-substituent in the acetate side chain were hydrogenated with a high degree of facial stereoselectivity. This allowed the 6-phthalimidoazabicyclo- [3.2.0]heptan-7-one 24 to be isolated as a single diastereoisomer. The X-ray crystal structure of a precursor, the triester, 22a, has been obtained.
- Cycloaddition reactions of conjugated azoalkenesPublication . Lemos, A.An overview of the use of 2H-azirines, conjugated nitrosoalkenes and conjugated azoalkenes bearing phosphorus substituents in addition and cycloaddition reactions is presented, focused on strategies for the synthesis of aminophosphonate and aminophosphine oxide derivatives.
- Cycloaddition reactions of nitrosoalkenes, azoalkenes and nitrile oxides mediated by hydrotalcitePublication . Lemos, A.; Lourenço, J. P.Mg:Al 3:1 hydrotalcite (Ht), used in catalytic quantities, promotes the generation of nitrosoalkenes, azoalkenes and nitrile oxides. These can be intercepted in situ by heterocycles and olefins in [4+2] and [3+2] cycloaddition reactions, producing dihydro-1,2-oxazines, tetrahydropyridazines and isoxazolines. The regeneration and reuse of Ht without loss of activity and the absence of organic solvent are the main advantages of this methodology.
- Diels–Alder reactions of 3-(1H-tetrazol-5-yl)-nitrosoalkenes: synthesis of functionalized 5-(substituted)-1H-tetrazolesPublication . Lopes, Susana M. M.; Palacios, Francisco; Lemos, A.; Melo, Teresa M. V. D. Pinho eA general route to 1,2-oxazines and open chain oximes bearing a 1H-tetrazolyl substituent via DielseAlder reaction of 3-(tetrazol-5-yl)-nitrosoalkenes is reported. It was also demonstrated that reduction of these adducts followed by deprotection of the tetrazolyl group give 5-(1-aminoalkyl)-1H-tetrazoles, a-amino acid analogues.
- Ethyl 2-(Diisopropoxyphosphoryl)-2H-azirine-3-carboxylate: reactions with Nucleophilic 1,3-DienesPublication . Lemos, A.; Alves, Maria José ChãoEthyl 2-(diisopropoxyphosphoryl)-2H-azirine-3-carboxylate, the first example of an azirine bearing simultaneously ester and phosphonate groups was generated in situ and reacted with a number of 1,3-dienes. Cycloadducts or their ensuing rearranged products were isolated in moderate yields.
- Ethyl 3-(2-pyridyl)-2H-azirine-2-carboxylate : synthesis and reactivity with dienesPublication . Alves, Maria José Chão; Fortes, Gil; Lemos, A.; Martins, Cristina I.Ethyl 3-(2-pyridyl)-2H-azirine-2-carboxylate, the first example of an 3-heteroamatic 2H-azirine has been prepared. The reaction of ethyl 3-(2-pyridyl)-2H-azirine-2-carboxylate with conjugated 1,3-dienes, under mild conditions and in the absence of Lewis acid catalyst, afforded cycloadducts in good yield and high stereoselectivity.
- Functionalization of dipyrromethanes via hetero-Diels–Alder reaction with azo- and nitrosoalkenesPublication . Pereira, Nelson A. M.; Lemos, A.; Serra, Arménio C.; Melo, Teresa M. V. D. Pinho e5,50-Diethyl- and 5-phenyldipyrromethanes participate in cycloadditions with azo- and nitrosoalkenes giving dipyrromethanes with side chains containing open chain oximes and hydrazones. By controlling reaction stoichiometry it is possible to get mono or 1,9-disubstituted derivatives. The reported methodology gave access to a range of dipyrromethanes with good structural features for various applications. It was demonstrated that reduction of dipyrromethanes containing a-oximino ester groups opens the way to new a-amino esters.
- A hetero-Diels–Alder approach to functionalized 1H-tetrazoles: synthesis of tetrazolyl-1,2-oxazines, -oximes and 5-(1-aminoalkyl)-1H-tetrazolesPublication . Lopes, Susana M. M.; Lemos, A.; Melo, Teresa M. V. D. Pinho eThis work describes the first and unprecedented examples of inverse electron demand Diels–Alder reactions of 5-(1-nitrosovinyl)-1-phenyl-1H-tetrazole, generated in situ from the corresponding bromooxime, with electron rich alkenes and heterocycles, providing in good overall yields tetrazolyl- 1,2-oxazines and -oximes. Upon subsequent reduction these allowed the access to 5-(1-aminoalkyl)- 1H-tetrazoles, paving the way for a new entry into this important class of compounds, bioisosteres of a-amino acids.