Chiral C2-symmetric 2,4-disubstituted azetidines as chiral ligands in the addition of diethylzinc to aldehydes

Chiral C-2-symmetric 2,4-disubstituted azetidine derivatives having a beta-amino alcohol moiety have been successfully synthesized and their stereoselective catalytic abilities have been examined in the asymmetric addition of diethylzinc to aldehydes in hexane. When N-(2,2-diphenyl-2-hydroxyethyl)-(S,S)-2,4-bis(methoxymethyl)azetidine 12 was used as a catalytic chiral ligand, the production of sec-alcohols having an S-absolute configuration could be achieved in high chemical yields (92-99%) and high enantiomeric excesses (83-93% for arylaldehydes and 63-65% for aliphatic aldehydes). For some arylaldehydes, the enantioselectivities are higher than the corresponding chiral C-2-symmetric 2,5-disubstituted pyrrolidine ligands. However, when the chiral C-2-symmetric azetidine derivatives 13 and 14, which have bulky substituents on the 2,4-positions were used as the chiral ligands under the same reaction conditions, the enantiomeric excesses of the corresponding sec-alcohols decreased to 20-30%. (C) 1999 Elsevier Science Ltd. All rights reserved.