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 Diel s-Alder reactions of the relatively unreactive dienophiles methyl (1a) and (-)-menthyl (1b) alpha,beta-dehydroalaninates with cyclopentadien e, which is the key step in the asymmetric synthesis of cycloaliphatic alpha-amino acids. Reactions were carried out in toluene and in the a bsence of a solvent, the latter method leading to higher percentages o f conversion. Silica gels modified with Lewis acids are the best catal ysts for the reaction of the non-chiral dienophile 1a, but they are co mpletely inefficient in the reaction of the chiral dienophile 1b, prob ably due to diffusional limitations. In the case of the chiral dienoph ile 1b, silica gel is the best catalyst. Both exolendo and diastereofa cial selectivities are very similar to those obtained using homogeneou s catalysts. The complete diastereofacial selectivity obtained in exo cycloadducts constitutes the best asymmetric induction described to da te in asymmetric Diels-Alder reactions with heterogeneous catalysts.