In Search of exo‐Selective Catalysts for Enantioselective 1,3‐Dipolar Cycloaddition between Acryloyloxazolidinone and Diphenylnitrone

Abstract 1,3‐Dipolar cycloaddition (1,3‐DC) between acryloyloxazolidone 1 and diphenylnitrone 2 was catalysed by complexes of three (4R)‐phenylbis(oxazolines) [box = bis(oxazolines)] − either 5‐unsubstituted (8a), or 4,5‐cis‐ and ‐trans‐diphenyl‐substituted (8b, 8c) − with several perchlorates of divalent cations. Normal endo selectivity was obtained with MgII‐ and NiII‐8a catalysts, and the formation of the endo enantiomers (3′R,4′S)‐ or (3′S,4′R)‐3 depended upon the presence of 4‐Å molecular sieves (MS). Different results were observed with the catalysts derived from this ligand and CoII or ZnII, which gave good levels of exo enantioselectivity, with 84% ee of (3′S,4′S)‐4. When 8c was used as ligand, MgII, CoII and NiII give exo‐selective catalysts, and the enantiomer (3′R,4′R)‐4 was obtained with good dr and excellent ee. The unknown absolute configuration of the exo enantiomers 4 was established by structure correlation with one exo diastereoisomer obtained from the 1,3‐DC between 2 and (S)‐3‐acryloyl‐4‐benzyl‐2‐oxazolidinone (10). The flexibility of the catalysts derived from box 8a and 8c, all with 4R configuration, is remarkable since a change in the cation allows the endo‐3 or the exo‐4 enantiomers to be obtained enantioselectively with ees in the 84−99% range. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)