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首页> 美国政府科技报告 >Chiral Synthesis via Organoboranes. 22. Selective Reductions. 44. The Effect of the Steric Requirements of the Alkyl Substituent in Isopinocampheylalkylchloroboranes for the Asymmetric Reduction of Representative Ketones
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Chiral Synthesis via Organoboranes. 22. Selective Reductions. 44. The Effect of the Steric Requirements of the Alkyl Substituent in Isopinocampheylalkylchloroboranes for the Asymmetric Reduction of Representative Ketones

机译：有机硼烷的手性合成。 22.选择性削减。 44.异丙基氨基烷基氯硼烷中烷基取代基的空间要求对代表性酮的不对称还原的影响

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Diisopinocampheylchloroborane, dIpc2BCl, is an excellent reagent for the chiral reduction of aralkyl ketones and alpha-tert-alkyl ketones. Modifications of this reagent to make new reagents applicable to additional classes of ketones were explored. Reagents with alkyl groups of varying steric requirements, such as dIpcBRCl, where R = Me, Et, i-Pr, Cyp, t-Bu, and Thx, were prepared, and their effectiveness in achieving chiral reduction of acetophenone and 3-methyl-2-butanone was examined. Whereas all of the reagents produced (S)-3-methyl-2-butanol in a modest range of 22-48% ee, the reagents with R = Me, Et, i-Pr, and Cyp produced (S)-1-phenethanol in the range of 15-84% ee, while the reagents with R = t-Bu and Thx gave (R)-1-phenethanol in 96% and 83% ee, respectively. Thus the reagent dIpcB-t-BuCl achieves high asymmetric reduction of acetophenone in the range achieved by dIpc2BCl, but the product possesses the opposite configuration, R, instead of S. The general effectiveness of the new reagent, dIpcB-t-BuCl, was explored by reducing the ten standard ketones. dIpcB-t-BuCl reduces acetophenone, 2-chloroacetophenone, and 3-acetylpyridine in 96%, 98%, and 96% ee, respectively, highly enriched in the R isomer. Methyl benzoylformate was reduced in 91% ee (S). trans-4-Phenyl-3-buten-2-one and cyclohexenone were reduced in 84% and 46% ee, respectively, enriched in the R isomer. 2,2-Dimethylcyclopentanone and 3-methyl-2-butanone were reduced in 34% ee (R) and 44% ee (S), respectively. Reprints. (aw)

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