Aziridino alcohols as catalysts for the enantioselective addition of diethylzinc to aldehydes

The chiral aziridino alcohols 1 -3 have been prepared either from amino acids (1a from serine; 1b - 1i and 3 from threonine; 2a - 2e from allo-threonine) or via asymmetric synthesis (1j, 1k, 1l and 2f from methyl cinnamate). These easily available ligands act as catalysts for the enantioselective addition of diethylzinc to benzaldehyde, with up to 90% stereoselectivity. The absolute configuration of the alcohol product is dependent on the substitution pattern of the aziridine ring, and different transition state models are proposed to explain the observed switch in enantioselectivity. The C-2-symmetric aziridino diols 4a - 4h have been prepared from tartaric acid, and also catalyze the organozinc addition reaction with high chemicals yields and up to 94% e.e. C-2-symmetric ligands 5a - 5c, with ether sidechains, were less efficient (46% e.e. at best). The most efficient ligand (4c) was also tested in the addition to aldehydes 9 - 11, with 97% ee at best. (C) 1998 Elsevier Science Ltd. All rights reserved.