A DIRECT COMPARISON STUDY OF ASYMMETRIC BORANE REDUCTION OF C = N DOUBLE-BOND MEDIATED BY CHIRAL OXAZABOROLIDINES

A comparison study on asymmetric borane reduction of ketoxime ethers a nd N-substituted ketimines possesing C = N double bond mediated by the selected chiral oxazaborolidines (1-6) was investigated. Thus, an aro matic ketoxime O-alkyl ether acetophenone oxime 0-methyl ether was red uced to the corresponding amine (1-phenylethylamine 8a) with optical y ields, such as 58% ee with 1, 86% ee with 2, 3% ee with 3, 99% ee with 4, 60% ee with 5, and 73% ee with 6. However, the reduction of an ali phatic ketoxime derivative 2-heptanone oxime O-methyl ether provided l ow optical inductions (7-13% ee). For ketoxime O-trimethylsilyl ethers , the reduction of acetophenone O-trimethylsily ether afforded 8a with optical yields which were 90% ee with 1, 40% ee with 2, 2% ee with 3, 62% ee with 4, 5% ee with 5, and 60% ee with 6. The reduction of 2-he ptanone 0-trimethylsilyl ether also gave the product amine with low op tical yields (10-40% ee). In the case of N-substituted ketimines, the reduction of acetophenone N-phenylimine afforded the corresponding ami ne with 79% ee, 78% ee, 9% ee, 73% ee, 78% ee and 67% ee using 1, 2, 3 , 4, 5, and 6, respectively, whereas low optical inductions (5-18% ee) for 2-heptanone N-phenylimine were achieved.