1,3‐Dipolar Cycloadditions of <i>N</i>‐Benzyl‐2,3‐<i>O</i>‐isopropylidene‐<scp>D</scp>‐glyceraldehyde Nitrone to Methoxyallene – Control of Site‐ and Diastereoselectivity of Isoxazolidine Formation by Lewis Acids

Abstract

Abstract The siteselectivity of cycloadditions of the D ‐glyceraldehyde‐derived nitrone 1 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 1 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 α‐methylene‐β‐amino acid esters 5a , b and 7a , b .(© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2008)