In search of exo-selective catalysts for enantioselective 1,3-dipolar cycloaddition between acryloyloxazolidinone and diphenylnitrone

1,3-Dipolar cycloaddition (1,3-DC) between acryloyloxazolidone I 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 Mg-II- and Ni-II-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-angstrom molecular sieves (MS). Different results were observed with the catalysts derived from this ligand and Co-II or Zn-II, which gave good levels of exo enantioselectivity, with 84% ee of (3'S,4'S)-4. When 8c was used as ligand, Mg-II, Co-II and Ni-II 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. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).