Selective reductions. 46. Effect of the steric requirement at the 2-position of apopinene on chiral reductions. B-(iso-2-ethylapopinocampheyl)- and B-(iso-2-n-propylapopinocampheyl)-9-borabicyclo[3.3.1]nonanes as improved reagents for the chiral reduction of .alpha.,.beta.-acetylenic ketones and .alpha.-keto esters

B-(Iso-2-ethylapopinocampheyl)-9-borabicyclo[3.3.1]nonane (Eapine-Borane), and B-(Iso-2-n-propylapopinocampheyl)-9-borabicyclo[3.3.1]nonane (Prapine-Borane), prepared via the hydroboration of 2-ethylapopinene or 2-n-propylapopinene, respectively, with 9-borabicyclo[3.3.1]nonane, reduce prochiral α,β-acetylenic ketones and α-keto esters to the corresponding alcohols with significantly higher optical induction than does Alpine-Borane