1,3-dipolar cycloadditions of N-benzyl-2,3-O-isopropylidene-D-glyceraldehyde nitrone to methoxyallene - Control of site- and diastereoselectivity of isoxazolidine formation by Lewis acids

The siteselectivity of cycloadditions of the D-glyceraldehyde-derived nitrone I and methoxyallene (2) is strongly influenced by Lewis acids. The uncatalyzed reaction results in the formation of isomeric isoxazolidines 3a-3d and 4a-4c, whereas in the presence of different Lewis acids exclusive formation of 4-methylene-substituted isoxazolidines 4a-4d is observed. Furthermore, the diastereofacial selectivity of the methoxyallene addition to nitrone I can be controlled, thus giving rise to both diastereomeric isoxazolidines 3,4'-anti-4 or 3,4'-syn-4, just by employing different Lewis acids. The redox ring-opening of isoxazolidines 4 using Murahashi's protocol yields a-methylene-beta-amino acid esters 5a,b and 7a,b. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).