Nopol, reduction

Reduction of a, -acetylenic ketones with chiral borane NB-Enanthrane prepared by addition of 9-borabicyclo[3.3.1]nonane to the benzyl ether of nopol yielded optically active acetylenic alcohols in 74-84% yields and 91-96% enantiomeric excess [770]. Another way to optically active acetylenic alcohols is reduction with a reagent prepared from lithium aluminum hydride and (2S, 3R)-( -I- )-4-dimethylamino-3-methy 1-1,2-dipheny 1-2-butanol. At —78° mainly R alcohols were obtained in 62-99% yield and 34-90% enantiomeric excesses [893]. 

Asymmetric Reductions. NB-Enantrane is an analog of B-3-Pinanyl-9-borabicyclo[3.3.1 Jnonane (Alpine-Borane ). Since the absolute configuration of nopol is the opposite of (+)-a-pinene, the reagent provides the opposite mode of asymmetric induction. The reagent provides higher asymmetric induction than Alpine-Borane for cases where the two groups flanking the ketone are relatively small, such as methyl or ethyl alkynyl ketones (eq 2). 

Trialkylborohydrides prepared from optically active alkenes may be used for asymmetric-reduction. For example, reaction of fcrt-butyllithium with Alpine-borane, prepared by hy-droboration of ( + )- or ( —)-a-pinene with 9-borabicyclo[3.3.1]nonane (see Section D.2.3.5.1.), provides Alpine-hydride. 1 his reagent reduces 1-phenylethanone in 17% ee72. A more effective reagent is NB-Enantride, prepared in a similar manner by treatment of the product from hydroboration of the benzyl ether of 2-(2-hydroxyethyl)-6,6-diinethylbicyclo[3.3.1]hept-2-ene (nopol. see Section D.2.3.5.1.) with toV-butyllithium73. 

The reduction of a,p-acetylenic ketones is accomplished in 24-48 h at room temperature by using twofold excess of NB-Enantrane and running the reaction without solvent. Both chemical and enantiomeric yields are high (Table 26.14) and provide (S)-propargyl alcohols. The reduction fits the steric model proposed for Alpine-Borane reduction [3]. Nopol benzyl ether liberated after the reduction maybe easily isolated during purification of the product and recycled. 

Other Unsaturated Alcohols. An improved procedure that has been reported recently for the preparation and utilization of alkali-metal acetylides in liquid ammonia allows the ethynation of sensitive carbonyl compounds to be carried out, in good yield, to give propargylic alcohols. The reduction of ap-acetylenic ketones to (S propargyl alcohols of high enantiomeric purity can be performed with NB-Enantrane (27) [cf. (6) earlier], which is derived from nopol benzyl ether as a low-cost alternative to (—)-a-pinene. a-Allenic alcohols have been prepared by the new routes shown in Schemes... 

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