Ethers prochiral ketoxime

An early approach to the formation of chiral amines by nonenzymatic asymmetric synthesis was the reduction of prochiral ketoximes and their O-tetrahydropyranyl and O-methyl derivatives with lithium aluminum hydride-3-0-benzyl-1,2-0,0-cyclohexylidene-a-D-glucofuranose complex (16)33 in ether and prochiral ketoximes... 

Enantioselective Reduction of Oxime 0-Ethers. In addition to the reduction of prochiral ketones, oxazaborolidine (3) has been used (both stoichiometrically and catalytically with borane-THF) to catalyze the enantioselective reduction of prochiral ketoxime O-ethers to the corresponding amine (eq Unlike the ketone reduction described above, the... 

Table 2 Enantioselective Reduction of Prochiral Ketoxime O-Ethers 1-...

Enantiomerically pure boranes have a long history in the reduction of prochiral ketones/ Amongst the early results using stoichiometric oxazaborolidines, the work of Itsuno is of particular interest. For example, acetophenone (3.32) could be reduced with the oxazaborohdines (3.113) or (3.114) where the ratio of amino alcohol to borane was 1 2, implying that one equivalent of oxazaborolidine and one equivalent of borane were present in the transition state. Itsuno also reported that the oxazaborohdine reagent (3.114) could be used catalytically in the reduction of prochiral ketoxime ethers. ... 

In the asymmetric reduction of ketones, stereodifferentiation has been explained in terms of the steric recognition of two substituents on the prochiral carbon by chirally modified reducing agents40. Enantiomeric excesses for the reduction of dialkyl ketones, therefore, are low because of the little differences in the bulkiness of the two alkyl groups40. In the reduction of ketoxime ethers, however, the prochiral carbon atom does not play a central role for the stereoselectivity, and dialkyl ketoxime ethers are reduced in the same enantiomeric excess as are aryl alkyl ketoxime ethers. Reduction of the oxime benzyl ethers of (E)- and (Z)-2-octanone with borane in THF and the chiral auxiliary (1 R,2S) 26 gave (S)- and (R)-2-aminooctane in 80 and 79% ee, respectively39. 

Enantioselective Reduction of Imines and Ketoxime O-Ethers. In addition to the reduction of prochiral ketones, chiral oxazaborolidines have been employed as enantioselective reagents and catalysts for the reduction of imines (eq 11) and ketoxime O-ethers (eq 12) - to give chiral amines. It is interesting to note that the enantioselectivity for the reduction of ketoxime O-ethers is opposite that of ketones and imines. For more information, see 2-Amino-3-methyl-l,l-diphenyl-I-butanol. 

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