THE USE OF HETEROGENEOUS CATALYSIS IN DIELS-ALDER REACTIONS OF N-ACETYL-ALPHA,BETA-DEHYDROALANINATES

Several solids have been tested as heterogeneous catalysts in the Diels-Alder reactions of the relatively unreactive dienophiles methyl (1a) and (-)-menthyl (1b) alpha,beta-dehydroalaninates with cyclopentadiene, which is the key step in the asymmetric synthesis of cycloaliphatic alpha-amino acids. Reactions were carried out in toluene and in the absence of a solvent, the latter method leading to higher percentages of conversion. Silica gels modified with Lewis acids are the best catalysts for the reaction of the non-chiral dienophile 1a, but they are completely inefficient in the reaction of the chiral dienophile 1b, probably due to diffusional limitations. In the case of the chiral dienophile 1b, silica gel is the best catalyst. Both exolendo and diastereofacial selectivities are very similar to those obtained using homogeneous catalysts. The complete diastereofacial selectivity obtained in exo cycloadducts constitutes the best asymmetric induction described to date in asymmetric Diels-Alder reactions with heterogeneous catalysts.