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Background: The Hetero-Diels-Alder reaction (HDAR) is a method extensively used in organic chemistry as a tool in the synthesis of innumerous polycyclic compounds in particular nitrogen compounds, presents in many natural products, medicinally relevant substances and organic materials. The literature describes innumerable studies of HDAR using classic methods and modern developments such as reactions on the solid phase, the use of catalysts, transformations in aqueous solution and under microwaves. Objective: This review describes a variety of HDAR focused on obtaining nitrogen-containing compounds of considerable chemical and biological interest, and highlighting reported biological activity. Conclusion: This review has shown the importance of the HDA reaction as a tool of organic chemistry in the synthesis of nitrogen compounds. This type of reaction presents important properties including bond-forming economy, high regio- and stereoselectivities and thus provides highly efficient routes to access a wide range of polycyclic compounds. In addition to the variety of nitrogen compounds synthetized successfully by this method, they present relevant biological properties.

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The conventional biodiesel process, although it reaches high conversion yields and productivity, faces problems related to the use of homogeneous catalysts. This work aims to study mixtures of calcium oxide (CaO) and niobium oxide (Nb 2 O 5) as the heterogeneous catalyst. It was used a full 2 3 factorial design with four central points to analyze how the mass percentage of the oxides, the molar ratio of reactants, and the reaction temperature affect the conversion yield to methyl esters. The best conversion yield was found near to 89% using 1.8% of catalyst, a 1:36 oil to methanol ratio and at 77 °C as reaction temperature. Finally, it was performed a simplified simulation to compare the heterogeneous catalyst process with the conventional process, and an algorithm to compare the effects of the exit streams of each process would have on the environment. The simulations results display a better performance for the heterogeneous catalyst process studied.

New Schiff base compound N,N'-bis-(4-hexadecanate)-salicylethylenediamine (named IM) was synthesized, and characterized via infrared and nuclear magnetic ressonance spectroscopies. IM was tested as corrosion inhibitor and antioxidant in soybean biodiesel. Corrosion studies were developed with AISI 1020 carbon steel in a three-step accelerated corrosion assay and IM inhibitory activity was evaluated using microscopic techiniques: optical microscopy (OM), scanning electron microscopy (SEM) and atomic force microscopy (AFM). In 750 ppm concentration, IM showed better inhibitory activity compared to commercial compounds tert-butylhydroquinone and pyrogallol, reducing root-mean-square roughness (Rq) from 37.8 to 11.7 nm and peak-peak height (PP) from 454.1 to 120.2 nm after 8 days of immersion. Antioxidant studies were conducted using the PetroOXY equipment. In 250 ppm concentration, IM showed better antioxidant activity as metal chelator, reducing biodiesel oxidation induced by copper on biodiesel. In this concentration, IM increased the induction period from 3.0 to 5.8 h. Furthermore, the new Schiff base acts as an oxygen scavanger. This is a great property because it reduces oxygen concentration in biodiesel, reducing metal corrosion reactions.

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Using niobium compounds as heterogeneous catalysts in biodiesel production is a promising methodology from economic and environmental viewpoints. However, the application of niobium catalysts still is a challenge due to the high temperatures and pressures for moderate biofuel yields. Therefore, easily handled and applied materials have been developed to optimize biofuel production, which is the goal of this study. Nb 2 O 5 and ammonium niobium oxalate (AmNO) were activated in reflux and ultrasound-assisted system. Nb 2 O 5 showed better activity under reflux, using methanol. The characterizations conclude that the Lewis-acid sites are determinant for higher conversion rather than surface area. AmNO has better activity also in the reflux system at 70 °C, against 170 °C for Nb 2 O 5 , reaching above 70% conversion. In addition, reactions in ultrasound-assisted systems are also appealing due to the lower time and temperature, with conversion rates above 40%. Both catalysts showed interesting results under milder conditions than those in the literature.