DIELS-ALDER REACTIONS OF ALPHA-AMINO-ACID PRECURSORS BY HETEROGENEOUS CATALYSIS - THERMAL VS MICROWAVE ACTIVATION

The Diels-Alder reactions of methyl and (-)-menthyl 2 acetamidoacrylates with cyclopentadiene, a key step in the synthesis of cyclic alpha-amino acids, are carried out in the presence of heterogeneous catalysts. Given the low reactivity of these dienophiles the best results are obtained when the reactions are carried out in the absence of a solvent. A close relationship between the relative catalytic activity of the different solids and the nature of the dienophile exists. The selectivities obtained are similar to those described for the reactions catalyzed by homogeneous Lewis acids, in particular a complete diastereofacial selectivity is obtained. Microwave activation increases the reaction rate without decreasing the selectivity. However, it is necessary to choose a suitable input power in order to avoid the decomposition of the 2-acetamidoacrylate.