Controlling factors in chiral bisoxazoline-catalyzed asymmetric lithium ester enolate-imine condensation producing a beta-lactam.
A catalytic amount of external chiral bisoxazoline ligand 3a bearing an isopropyl group as a stereocontrolling group catalyzed a reaction of a lithium ester enolate 4b, generated from 3-pentyl 2-methylpropionate, with benzaldehyde anisidine-imine 5 to afford corresponding beta-lactam 6 in higher 70% ee than that obtained by the reaction using a stoichiometric amount of the ligand. A bulkier ligand 3d bearing a phenyl group gave 81% and 6% ees in stoichiometric and catalytic reactions, respectively. Examination of the varying factors suggested the involvement of mixed aggregates as a reactive species. A working model is presented for prediction of the sense of asymmetric induction.
Kambara
T
T
Graduate School of Pharmaceutical Sciences, Kyoto University, Japan.
Tomioka
K
K
D002384:Catalysis; D004952:Esters; D007097:Imines / Q000737:chemistry*; D008094:Lithium / Q000737:chemistry*; D015394:Molecular Structure; D010080:Oxazoles / Q000737:chemistry*; D013057:Spectrum Analysis; D013237:Stereoisomerism; D047090:beta-Lactams / Q000138:chemical synthesis*
D004952:Esters; D007097:Imines; D010080:Oxazoles; D047090:beta-Lactams; D008094:Lithium