Selective Reductions. 46. Effect of the Steric Requirement at the 2- Position of Apopinene on Chiral Reductions. B-Iso-2-n-Propylapopinocampheyl-9- Borabicyclo(3.3.1)Nonane as Improved Reagents for the Chiral Reduction of Alpha, Beta-Acetylenic Ketones and Alpha-Keto Esters

Abstract : 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 alpha, beta-acetylenic ketones and alpha-keto esters to the corresponding alcohols with significant higher optical induction than does Alpine-Borane. (-)- 2-n-Propylapopinene was synthesized by treating nopyl tosylate with dimethylcuprate prepared in situ from methyllithium and cuprous iodide. (+)-2-n- Propylapopinene was synthesized by Schlosser metallation of (+)-alpha-pinene followed by treatment with ethyl iodide. 4-Phenyl3-butyn-2-one was reduced to the corresponding propargylic alcohol in 89% ee and 96% ee by Eapine-Borane and Prapine-Borane, respectively, as compared to 82% ee with Alpine-Borane.