Lewis acid chiral heterogeneous

During recent years, the homogeneous Lewis acid-catalyzed asymmetric Diels-Alder reactions and hetero-Diels-Alder (HDA) reactions have each undergone extensive study. Various chiral Lewis acids including aluminum, titanium or boron, and chiral ligands such as chiral amino alcohols, diols, salen, bisoxazoline or N-sulfonylamino acids have been used as the catalysts [84]. Much efforts have also been made in the investigation of heterogeneous diastereoselective Diels-Alder reactions. 

A variety of solid Lewis and Br0nsted acids has been shown to catalyze Diels-Alder reactions. In several instances the results obtained with heterogeneous catalysts were better than those with homogeneous Lewis acid catalysts. Most of the reported reactions of interest in the synthesis of fine chemicals were catalyzed by (modified) zeolites, clays, alumina, or silica. Catalysts with interesting properties were obtained when support materials such as zeolites, alumina, or silica were treated with Lewis acids. These catalysts were moderately selective in diastereo-selective Diels-Alder reactions with chiral dienophiles and induced enantioselec-tivity (up to 31 % e. e.) in the reaction of cyclopentadiene with methacrolein after treatment with chiral derivatives. Excellent enantioselectivity in this reaction (up to 95 % e. e.) was observed with a polymer-supported chiral oxazaborolidinone. Because of their facile recovery and recycling, we expect that solid-acid catalysts will find increasing use in Diels-Alder reactions in the future. 

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