Oxime ethers asymmetric reduction

Jacobsen epoxidation 359 -, Katsuki epoxidation 361 -, Mukaiyama-aldol reaction 367 f. -, oxime ether reduction 363 -, Sharpless asymmetric dihydroxyla-tion 361... 

Asymmetric reduction of oxime ethers,2 The complex (1) of (- )-norephedrine with BH3 (2 equiv.) reduces prochiral oxime ethers to optically active amines the... 

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. 

Chu, Y, Shan, Z., Liu, D. and Sun N. Asymmetric Reduction of Oxime Ethers Promoted by Chiral Spiroborate Esters with an O3BN Framework. J. Org. Chem. 2006, 71, 3998-4001. 

The in situ generated catalyst from ATBH and trimethyl borate has also been used in the stereoselective reduction of a-oxoketoxime ethers to prepare the corresponding chiral 1,2-amino alcohols. Thus the asymmetric borane reduction of buta-2,3-dione monoxime ether followed by acidic work-up and subsequent reaction with benzyloxycarbonyl chloride affords a 90% yield of 7V-(Z)-3-aminobutan-2-ol with excellent enantioselectivities (eq 5). A trityl group in the oxime ether is required for high enantioselectivity. This method has been successively applied to both cyclic and acyclic a-oxoketoxime ethers. 

In this model the sense of asymmetric induction is controlled by two principle factors (i) coordination of borane on the least-hindered face of the bicyclic ring system and (ii) coordination of the Lewis acid syn to the small group (R ). The latter point is in good agreement with the structural data that has been presented in this chapter, and is further supported by results from the asymmetric reduction of oxime ethers (Figure 51). As predicted by the model (75 and 78), (E) and (Z)-oxime ethers afford enantiomeric amines upon reduction by the reagent derived from (-)-norephedrine and borane (2 equiv.). Here, Lewis acid coordination is dictated by the ( )/(Z) stereochemistry of the oxime ether rather than by the rule of coordination syn to the small group. 

Asymmetric Reduction of Ketone Oxime Ethers with (-)-(25, 3 )-2-Aiiiino-3-methyl-l,l-diphenylpentanol-Borane Reagent General Procedure33 ... 

Cyclopropanation of the cinnamate 52 followed by oximinatlon gave rac-53. Asymmetric reduction of the oxime ether gave two diastereomers, one of which was taken on to 54 by oxidative cleavage of the furyl group. 

Asymmetric reduction of chiral oxime ethers with lithium... 

Very high ee values have been obtained in the asymmetric conjugate reduction of Q , unsaturated esters and amides with NaBH4 in the presence of a chiral semicorrin (a bidentate nitrogen ligand) cobalt catalyst. Good to excellent ee values were realized in the reduction of oxime ethers with NaBHa-ZrCU in the presence of a chiral 1,2-amino alcohol. ... 

Based on a comparative study for asymmetric reduction of oxime ethers of acetophenone and 2-hepitanone using different classes of OABs, such as la, 2a, and 6a, Cho and Ryu reported the 6a-induced borane reduction of ketoxime O-trimethylsilyl ethers in moderate to good enantioselectivities (Scheme 11.19) [83a, bj. Similarly, ben-zylic amine derivatives were obtained with high enantioselectivities by stoichiometric reduction of oxime ethers using ent-6a-BHj or la-BH (Scheme 11.20) [84-86],... 

It is interesting that Didier also studied the reduction of anti-acetophenone oxime methyl ether (Scheme 17.3)36 and again observed that m-aminoindanol yielded one of the highest selectivity (95% ee). However, a stoichiometric amount of amino alcohol was needed to achieve high degrees of asymmetric induction. Use of a catalytic amount of amino alcohol resulted in a considerable decrease in product enantioselectivity. 

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