Stereoselective syntheses using carbohydrates as chiral auxiliaries

Abstract

Carbohydrates contain numerous functional groups and chiral centers in one molecular unit. Furthermore, they can exert marked complexing capabilities towards Lewis acids. Utilizing these properties, carbohydrates were applied for effective diastereodifferentiation in reactions on prochiral faces or groups of substrates. In this sense, Lewis acid-catalyzed Diels-Alder reactions of carbohydrate-linked dienophiles furnished the corresponding cycloadducts in high diastereoselectivity. The formation of a by-product during Diels-Alder reactions catalyzed with diethylaluminum chloride gave rise to the development of a new synthesis of β-branched carboxylic acids consisting of the 1,4 addition of dialkylaluminum chlorides to α,β-unsaturated N-acyl urethanes. The process combined with subsequent trapping reactions of the intermediates by electrophiles resulted in stereoselective syntheses of a-functionalized β-branched carboxylic acid derivatives. Glycosylamines proved to be particularly useful in stereoselective syntheses of a-amino acid derivatives, in asymmetric hetero Diels-Alder reactions and in Mannich reactions. In a new stereoselective reaction, Schiff bases of these glycosylamines with allylsilane gave chiral homoallylamines of high optical purity. © 1995 IUPAC