摘要:
Acyclic-alpha-amino nitriles 1, derived from alpha-alkylbenzylamines 3 and aldehydes 4 via a Strecker-type reaction, readily epimerize in MeOH at the newly emerged asymmetric center alpha-to the cyano group. The equilibrium diastereomeric ratio [(R*,R*)-1]:[(R*,S*)-1] varies from 73:27 for le to 90:10 for 1h. The latter ratio is unprecedentedly high for thermodynamically controlled 1,3-asymmetric induction. The aromatic group and a bulky alkyl group (R1) of the parent amine 3 are required to obtain high concentrations of (R*,R*)-1, whereas the bulkiness of the alkyl group (R2) of the parent aldehyde 4 has little effect. The reasons for the thermodynamic predominance of (R*,R*)-1 over (R*,S*)-1 were addressed by MM2 calculations, which predicted the most stable conformers of (R*,R*)-1 and (R*,S*)-1. The structures of these conformers were verified by H-1 NMR spectroscopy and X-ray crystallography.