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首页> 美国政府科技报告 >Selective Reductions. 45. Asymmetric Reduction of Prochiral Ketones by Iso-2-methyl, Iso-2-Ethyl, and (Iso-2(2-(Benzyloxy)Ethyl)Apopinocampheyl)-tert-Butylchloroboranes. Evidence for a Major Influence of the Steric Requirements of the 2-Substituent on the Efficiency of Asymmetric Reduction.
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Selective Reductions. 45. Asymmetric Reduction of Prochiral Ketones by Iso-2-methyl, Iso-2-Ethyl, and (Iso-2(2-(Benzyloxy)Ethyl)Apopinocampheyl)-tert-Butylchloroboranes. Evidence for a Major Influence of the Steric Requirements of the 2-Substituent on the Efficiency of Asymmetric Reduction.

机译：选择性减少。 45.异-2-甲基，异-2-乙基和（异-2（2-（苄氧基）乙基）apopinocampheyl） - 叔丁基氯硼烷不对称还原前手性酮。 2-取代基的空间要求对不对称还原效率的主要影响的证据。

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The asymmetric reduction of prochiral aliphatic ketones with chiral organylboranes has not yet given desirable results although considerable success has been achieved for the reduction of prochiral aralkyl and related ketones. We have now developed more efficient chiral reducing agents for aliphatic ketones following a strategy based on a tentative hypothesis that the steric requirements of the substituent at the 2-position of apopinene may be a major factor in achieving successful asymmetric reduction. The adduct of nopol benzyl ether and tert-butylchloroborane was prepared and characterized. This compound reduces prochiral ketones to the product alcohols in very high enantiomeric excess.The slowness of the reaction is attributed to the coordination of the ether oxygen to the boron atom. The higher optical yields achieved over those realized previously with the related isopinocampheyl-tert-butylchloroborane are attributed to the greater steric requirement of the (benzyloxy)ethyl group as compared to the methyl group of the alpha-pinene unit. On this basis, the corresponding derivative, (iso-2-ethylapopinocampheyl)-tert-butylchloroborane was synthesized in the hope that the rates would be higher, because of the absence of a coordinating ether linkage, while the optical yields would be comparable, because of the larger steric requirements of the 2-ethyl substituent. Indeed, the rates are considerably faster and the enantiomeric excesses achieved are almost comparable. Reprints. (aw)

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