Encontrados 18 documentos, a visualizar página 1 de 2
Synthesis and kinetic investigation of the selective acydolysis of para-substit...
Lima, Sílvia M. M. A. Pereira; Pinto, Filipa C. S. C.; Ventura, Cristina; Albuquerque, Lídia; Maia, Raquel M. Gonçalves; Maia, Hernâni L. S.Several derivatives of N-phenylacetyl-N-benzyl-alpha,alpha-dimethylglycine cyclohexylamide and their alpha,alpha-dibenzylglycine analogues were synthesised by a Ugi-Passerini reaction. In addition, a few analogues of the former but having an N-phenyl instead of a benzyl group at the nitrogen atom were synthesised. The compounds in each of these three sets differed from each other at the position 4 of the N-benz...
Kinetic investigation of the effect of the amino acid side chains in the select...
Wei-Qun, Jiang; Lima, Sílvia M. M. A. Pereira; Ventura, Cristina; Costa, Susana P. G.; Albuquerque, Lídia; Maia, Raquel M. GonçalvesAccurate kinetic measurements of the rate constants for the acidolysis of five N-acetyl-N-(4-methoxybenzyl)-alfa,alfa-trialkyl glycine cyclohexyl amides in TFA were performed at 25.00 ºC and the reactions monitored by HPLC. The results were in all cases consistent with a first order behaviour with respect to the substrate. No direct correlation was obtained with this data between rate constant values and struct...
Criacionismo vs. Evolucionismo
Maia, Hernâni L. S.History shows that Criationism has been distorting scientific arguments to justify their views which not resist to the progress of Science. If the reason and consciousness represents the more differentiated expressions of humankind and praises the human genius within the Creation domain, it would be inadequate to try the use of reason to deny the Reason itself. For what? To blindly defend human interpretations ...
Synthesis and conformational investigation of tetrapeptide analogues of the fra...
Rodrigues, Lígia M.; Fonseca, José I.; Maia, Hernâni L. S.Tetrapeptides containing one of a set of four different alpha,alpha-dialkyl glycines at the C-terminus were synthesized by conventional methods in solution and their conformational behavior investigated by 1H NMR spectroscopy in connection with molecular mechanics calculations. The results were consistent with conformations stabilized by a gama-turn in the case of compounds with alkyl groups larger than methyl,...
Synthesis of novel derivatives of 4-amino-3,5-dicyanopyrazole
Campos, Ana M. F. Oliveira; Gonçalves, M. Sameiro T.; Rodrigues, Lígia M.; Proença, M. Fernanda R. P.; Griffiths, John; Maia, Hernâni L. S.Diazotisation of substituted arylamines followed by reaction with malononitrile gave substituted arylazomalononitriles. Cyclisation of these intermediates with chloroacetonitrile and triethylamine, as base, gave the corresponding new aminodicyanopyrazoles, in 22–80% yields.
Dialkyl imidazolinones from alpha,alpha-dialkyl glycines
Costa, Susana P. G.; Maia, Hernâni L. S.; Lima, Sílvia M. M. A. PereiraIn recent years, our group has been involved in the use of the Ugi-Passerini reaction for the synthesis of several alpha,alpha-dialkylglycines. The C-terminal amide bond of the resulting Ugi adducts 1 proved to be very sensitive to TFA, cleaving readily by a mechanism involving an oxazolinium-type intermediate 2. This intermediate allows in situ functionalization of the C-terminus by reaction with several nucle...
An improved approach for the synthesis of alfa,alfa-dialkyl glycine derivatives...
Lima, Sílvia M. M. A. Pereira; Costa, Susana P. G.; Maia, Hernâni L. S.A general and simple strategy for routine peptide synthesis with alfa,alfa-dialkyl glycines taking advantage of the four-component Ugi-Passerini reaction is presented. The isonitrile required for the reaction can be relatively simple and its selection based on cost, as the group it generates is easily removed under acidic conditions; in addition, this removal is not visibly affected by the bulkiness of the alfa...
Development of a temporary marker for peptides
Gonçalves, M. Sameiro T.; Maia, Hernâni L. S.3-[(N,N-dimethylaminophenyl)-4'-diazenyl]-benzoic acid was coupled with several amino acid esters and the product further acylated with Boc. The material thus obtained was then submitted to cleavage by electrolysis and nucleophilic attack in order to evaluate the possibility to use this chromophore as a temporary marker.
An improved approach to the synthesis of N-acyl-alpha,alpha-dialkyl glycines an...
Lima, Sílvia M. M. A. Pereira; Costa, Susana P. G.; Maia, Hernâni L. S.Owing to steric hindrance, syntheses of and reactions with alfa,alfa-dialkylglycines are very slow; thus, most methods of peptide synthesis are of little use to handle these compounds when the alfa-alkyl groups are larger than methyl. Ugi’s four-component reaction is appropriate to synthesise these amino acids [1], but for peptide synthesis it presents two drawbacks, i.e. (i) peptide isonitriles racemise above ...
Reductive cleavage of N-substituted aromatic amides as tert-butyl acylcarbamates
Ragnarsson, Ulf; Grehn, Leif; Maia, Hernâni L. S.; Monteiro, Luís S.Synthetic and spectroscopic details related to a set of heteroaromatic N-benzyl carboxamides and especially the corresponding tert-butyl acylcarbamates are reported. These compounds were required to study the postulated effect of various heterocycles (pyridine and pyrazine with and without condensed benzene rings) on the cleavage of acyl-N bonds by reduction. All compounds were initially characterized by cyclic...
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