Estudos envolvendo reações de azalactonas e alcalóides com a luz visível

Abstract

Photochemical catalysis has been widely used as a powerful tool for the activation of certain molecules and in the construction of new carbon-carbon bonds that were previously unattainable under thermal control, its use has prominence in cycloaddition reactions and also in the synthesis of alkaloids. In this work the cycloaddition [2+2] of Erlemeyer-Ploch azlactones using the combination of nickel catalysis and photorredox catalysis is described, and the study of the total synthesis of the alkaloid Melodinin K using the alkaloid pachysiphine as substrate has also been initiated. In the cycloaddition [2+2] of azlactone, iridium photocatalyst and nickel catalyst were used to perform dual catalysis and a product was obtained which was believed to be the asymmetric cycloadduct with the hydrogens of cyclobutane in cis configuration however, from the data of X-ray diffraction analysis of the crystal of the product, it is suggested that in fact the product is the symmetric cycloadduct with the azlactone portions open and the hydrogens of cyclobutane in trans configuration. From this result, some tests were made to evaluate whether the use of nickel could bring some advantage in the azlactone cycloaddition reactions with different catalysts, but no satisfactory results were obtained. In the study of the total synthesis of Melodinin K, we investigated the formation of the iminium ion, indispensable for the development of the total synthesis, in tabersonin and pachysiphin via photorredox cyanation. The photorredox cyanation was performed on tabersonine, using 2.5 mol% iridium photocatalyst, 2 equivalents of TMSCN in methanol, irradiated by blue LED of 60 W power for 6 hours, leading to the desired product with 89% conversion and optimal selectivity. In the photoredox cyanation of pachysiphine, 2.5 mol % of ruthenium photocatalyst, 2 equivalents of TMSCN in methanol, irradiated by blue LED of 30 W power for 3.5 hours, which led to the desired product with 50% yield, but with low selectivity. Due to the impossibility of getting the necessary substrates for further testing, we set out for attempts to functionalize cyanated tabersonin, but did not achieve any desired results.