Stereoselective syntheses using carbohydrates as chiral auxiliaries

H. Kunz
1995 Pure and Applied Chemistry  
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
more » ... vity. The formation of a by-product during Diels-Alder reactions catalyzed with diethylaluminum chloride gave rise to the development of a new synthesis of p-branched carboxylic acids consisting of the 1,4 addition of dialkylaluminum chlorides to a,p-unsaturated N-acyl urethanes. The process combined with subsequent trapping reactions of the intermediates by electrophiles resulted in stereoselective syntheses of a-functionalized p-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. COMPLEXATION -A PRINCIPLE TO ACTIVATE AND ARRANGE CARBOHYDRATE RJNCTIONALITIES Carbohydrate side chains of glycoproteins obviously play major roles in biological recognition processes and in the directed distribution of glycoproteins and glycolipids in multicellular organisms (1). In the course of our syntheses of glycopeptides (2) we came across the problem of base-catalyzed p-elimination of the carbohydrate portion from 0-glycosylserine and -threonine derivatives (3). Since it is known from peptide chemistry that 0-tert-butyl and 0-benzyl ethers of serine and threonine are stable to bases, the easy basecatalyzed p-elimination of the carbohydrate from glycosylserine and -threonine is surprising. We ascribed the remarkable leaving group tendency of the carbohydrates in these compounds to their complexing abilities towards cations 1. This activation of carbohydrates by complexation could be demonstrated in the development of a new peptide synthesis (4). Carbohydrate esters of amino acids, e.g. 2, do not react with COOtBu 1 1627
doi:10.1351/pac199567101627 fatcat:zxhdn2n6svaxbgf6uzgn5uze3q