Solid supported chiral auxiliaries in asymmetric synthesis. Part 2: Catalysis of 1,3-dipolar cycloadditions by Mg(II) cation

1,3-Dipolar cycloadditions of supported Evans' chiral auxiliary with nitril e oxides and nitrones in the presence of Mg(II) cation as catalyst were eva luated. The presence of acetonitrile as co-solvent was found to be fundamen tal for the Lewis acid catalysis on solid-phase. The regio- and stereochemi cal outcome of nitrile oxide cycloadditions is influenced by nearly stoichi ometric quantities of the cation, whilst catalytic amounts of Mg(II) influe nce both the reactivity and the stereoselectivity of the nitrone cycloaddit ions. The results obtained support a reaction mechanism involving the coord ination of the Mg(II) to the dicarbonyl fragment of the chiral auxiliary. ( C) 2001 Elsevier Science Ltd. All rights reserved.