Educación básica en Chile : reformas y procesos pedagógicos

Reunión: Congreso Brasileño de Educación, 5o, 1988, Brasilia, B

Relación entre la educación preprimaria y de primer año de primaria en escuelas fiscales en Chile

En IDL-783

Stereocontrolled [11C]Alkylation of N-Terminal Glycine Schiff Bases To Obtain Dipeptides

The use of various quaternary ammonium salts as chiral phase-transfer catalysts allowed effective and stereoselective radiochemical [11C]alkylation to obtain functionalized dipeptides. We herein report a broadly applicable procedure for the asymmetric [11C]alkylation of dipeptides to give labeled N-terminal peptides by using different [11C]alkyl halides. Contended stereoselectivities of the reactions were observed by using 11C-labeled alkyl halides, [11C]methyl iodide and [11C]benzyl iodide, and diastereomeric ratios with different specialized catalysts of 95:5 and 90:10 were achieved, respectively. Accordingly, the straightforward synthesis of enantioenriched compounds should play a vital role in peptide-based radiopharmaceutical development and positron emission tomography imaging

Enantioselective synthesis of carbon-11 labeled L-alanine using phase transfer catalysis of Schiff bases

Radiolabeled amino acids are an important class of compounds that can be used for Positron Emission Tomography (PET) imaging of the amino acid transporter status of various diseases e.g., cancer. Current radiochemistry techniques do not offer synthesis approaches that are generally applicable and result in high yields and enantiomeric purity. Here, the radiosynthesis of l-[11C]alanine is described employing an enantioselective alkylation of a Schiff base glycine precursor with [11C]methyl iodide. By conducting a comprehensive reaction conditions optimization and a strategic analysis of several phase-transfer catalysts that facilitate enantioselective alkylation, the radiosynthesis of l-[11C]alanine was achieved in good radiochemical conversion, short reaction times and above 90% enantiomeric excess. This new methodology is broadly applicable and could also be used for the radiolabeling of other amino acids with carbon-11

From Carbon-11-Labeled Amino Acids to Peptides in Positron Emission Tomography: the Synthesis and Clinical Application

Radiolabeled amino acids, their derivatives and peptides have a broad scope of application and can be used as receptor ligands, as well as enzyme substrates for many different diseases as radiopharmaceutical tracers. Over the past few decades, the application of molecular imaging techniques such as positron emission tomography (PET) has gained considerable importance and significance in diagnosis in today’s advanced health care. Next to that, the availability of cyclotrons and state-of-the-art radiochemistry facilities has progressed the production of imaging agents enabling the preparation of many versatile PET radiotracers. Due to many favorable characteristics of radiolabeled amino acids and peptides, they can be used for tumor staging and monitoring the progress of therapy success, while aromatic amino acids can be employed as PET tracer to study neurological disorders. This review provides a comprehensive overview of radiosynthetic and enzymatic approaches towards carbon-11 amino acids, their analogues and peptides, with focus on stereoselective reactions, and reflects upon their clinical application

A rapid and highly enantioselective C-11C bond formation of l-[11C]phenylalanine via chiral phase-transfer catalysis

A rapid method for the synthesis of carbon-11 radiolabeled phenylalanine was developed using a chiral phase-transfer catalyst and a sub-nanomolar quantity of [11C]benzyl iodide as a radio-precursor. Based on a reported synthesis of [11C]benzyl iodide, a Schiff base precursor was evaluated for stereoselective [11C]benzylation. Extensive and interactive screening of the precursor, catalyst, base, stirring and temperature was required to achieve high stereoinduction. The result is an efficient 5-step radiolabeling method to reliably synthesize l- or d-[11C]phenylalanine with an excellent enantiomeric excess of >90% and almost quantitative radiochemical conversion of >95% (n > 5). Additionally, a phase-transfer catalyzed alkylation was utilized on the preparative scale using automated platform. The application resulted in high specific activity ranging from 85-135 GBq μmol−1 of the enantiomerically pure [11C]phenylalanine, showing that the process is robust and amenable to broad use in PET

Escuela básica en sectores urbanos pobres : situación actual y proyecciones

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