Synthesis and application of L-serine derivative ligands in Diels-Alder reaction between cyclopentadiene and methyl acrylate

The synthesis of bicyclic compounds has detained large significance due to their use as synthetic intermediates in the preparation of a vast variety of compounds of chemical, biological, and pharmaceutical interest. 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, the most important used Lewis acids being Al(III), Fe(III) or boron complexes, as well as Ti(IV) and Sn(IV) halides. The use of such strong acids is needed because ester dienophiles (as acrylate ones) are not very reactive. In this work we studied the use of moderate strength and environmentally safe Lewis acids as catalysts for Diels-Alder reaction between CPD and methyl acrylate by complexation – resulting in solubilization – of otherwise insoluble metal ions with a suitable ligand. L-Serine is a cheap and readily available natural aminoacid and it has three functional groups that can be easily functionalized, thus allowing chemical and structural design. In this context, new L-serine derivative ligands were prepared and tested as co-catalyst in the Diels-Alder reactions. Bidentate serine ligands showed good ability to coordinate medium strength Lewis acids, thus boosting their catalytic activity and hence being an alternative for the usual employment of strong Lewis acids. The synthesis of the L-serine ligands proved to be highly efficient and straightforward. Additionally, the prepared bidentate L-serine derivative ligands were silylated in order to allow posterior grafting onto inorganic materials for posterior employment in heterogeneous catalysis.