Prapine-Borane

Brown and coworkers [1] have found that NB-Enantrane is effective only for the reduction of a,(3-acetylenic ketones. The reduction of other ketones is too slow to be of any practical use. The retarded rate is attributed to the steric bulk at the 2 position since no internal coordination has been detected by "B NMR (6 86 ppm) [2]. On the other hand, Alpine-Borane has proven to be versatile reagent for the asymmetric reduction of variety of ketones. Consequently, two reagents B-(iso-2-ethylapopinocampheyl)-9-borabicyclo[3.3.1]nonane (Eapine-Borane) and B-(iso-2- -propylapopinocampheyl)-9-BBN (Prapine-Borane) having increasing steric requirement at the 2 position, are prepared by the hydroboration [3] of 2-ethyl- and 2-n-propylapopinene. 

Prapine-Borane is prepared neat from (-)-2- -propylapopinene by hydrobo-ration with solid 9-BBN, using the same procedure as used for the preparation of Alpine-Borane and Eapine-Borane. (-)-2- -Propylapopinene is prepared in 84% yield from nopyltosylate by treatment with dimethylcuparate, prepared in situ [5] from methyllithium and cuprous iodide (Scheme 26.4). 

The rate of reduction of representative a,(3-acetylenic ketones with Prapine-Borane is slightly slower than Eapine-Borane but with improved chiral induction (Eq. 26.18 Table 26.18) [1]. 

Reaction of Prapine-Borane with representative a,(5-acetylenic ketones at 25 °C [1]... 

Ketone Reagent equiv Reaction time (h) With Prapine-Borane With Eapine-Borane With Alpine-Borane... 

Prapine-Borane obtained from (+)-2- -propylapopinene reacts with 4-phe-nyl-3-butyn-2-one and provides the (R)-propargylic alcohol in 96% ee (Eq. 26.19). Thus, both enantiomers of propargylic alcohols are synthesized in very high ee. 

However, unlike Eapine-Borane and Alpine-Borane Prapine-Borane fails to provide improved asymmetric induction in the reduction of a-keto esters (Table 26.19) [1]. 

For the preparation of chiral reducing agent Prapine-Borane, the required (+)-2- -propylapopinene, is prepared from (+)-a-pinene via Schlosser metala-tion, followed by treatment with ethyliodide. The undesired olefin is removed by its conversion to 9-BBN derivative and the desired (+)-2- -propylapopinene is distilled off (Scheme 29.2) [4] (vide supra). 

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