New L-serine derivative ligands were prepared and tested as cocatalyst in the Diels-Alder reactions between cyclopentadiene (CPD) and methyl acrylate, in the presence of several Lewis acids. The catalytic potential of the in situ formed complexes was evaluated based on the reaction yield. Bidentate serine ligands showed good ability to coordinate medium strength Lewis acids, thus boosting their catalytic activity. The synthesis of the L-serine ligands proved to be highly efficient and straightforward. 1. Introduction The synthesis of bicyclic compounds has large significance due to their use as synthetic intermediates in the preparation of a vast variety of compounds of chemical, biological, and pharmaceutical interest [1, 2]. The most efficient and widely used method for the preparation of bicyclic compounds is the Diels-Alder reaction. Generally, activation by an electron-withdrawing group and a Lewis acid is required in order to achieve good conversion rates. The acid catalyzed Diels-Alder reactions, namely, between cyclopentadiene (CPD) and acrylates, is well documented [1, 3–15], the most important used Lewis acids being Al(III), Fe(III) or boron complexes. The use of such strong acids is needed because ester dienophiles (as acrylate ones) are not very reactive [15, 16]. Depending on the reactions, the solvent used is also a factor to consider regarding both the Lewis acid solubility and the reaction media’s polarity, dichloromethane being the most used solvent as combines both properties. In this work, we studied the use of moderate strength Lewis acids as catalysts for Diels-Alder reaction between CPD (1) and methyl acrylate (2) in dichloromethane (Scheme 1), by complexing insoluble metal ions with novel L-serine derivative ligands, as alternative to the usual strong Lewis acids. Scheme 1: Lewis acid catalyzed Diels-Alder reaction between CPD ( 1) and methyl acrylate ( 2). 2. Results and Discussion The work started with the study of several Lewis acids tested as catalysts in the Diels-Alder reaction between CPD (1) and methyl acrylate (2) using dichloromethane as solvent; the results are summarized in Table 1, as well as the reaction conditions. Table 1: Results of the Diels-alder reaction between 1 and 2, yield, and endo/ exo ratio of adduct 3. As expected, in the absence of catalyst, the reaction did not take place (entry 1). For the catalyzed reactions, it is noteworthy the correlation observed between the Lewis acid strength and the yield of the reaction: the best results were achieved when the stronger Lewis acids AlCl3, FeCl3, and TiCl4
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