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-borabicyclononanes as Improved Reagents for the Chiral Reduction of α,β-Acetylenic Ketones ...

Article abstract of DOI:10.1021/jo00313a020

B-(Iso-2-ethylapopinocampheyl)-9-borabicyclo<3.3.1>nonane (Eapine-Borane, 7), and B-(Iso-2-n-propylapopinocampheyl)-9-borabicyclo<3.3.1>nonane (Prapine-Borane, 9), prepared via the hydroboration of 2-ethylapopinene (6) or 2-n-propylapopinene (8), 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 (1). (-)-2-n-Propylapopinene was synthesized by treating nopyl tosylate with dimethyl cuprate prepared in situ from methyllithium and cuprous iodide. (+)-2-n-Propylapopinene was synthesized by Schlosser metalation of (+)-α-pinene followed by treatment with ethyl iodide. 4-Phenyl-3-butyne-2-one was reduced to the corresponding propargylic alcohol in 89percent ee and 96percent ee by Eapine-Borane and Prapine-Borane, respectively, as compared to 82percent ee with Alpine-Borane.Similar improved results were realized in the reduction of other acetylenic ketones by Eapine-Borane and Prapine-Borane.Similar improvements in the optical yields were realized in the reduction of α-keto esters by Eapine-Borane.For example, while Alpine-Borane produced methyl and ethyl lactate in 92percent and 91percent ee, respectively, Eapine-Borane gave these alcohols in 97percent and 96percent ee, respectively.Unfortunately, Prapine-Borane shows no improvement in percent ee for the reduction of α-keto esters.The increase in the percent ee realized is tentatively attributed to the increased steric requirements of the alkyl group at the 2-position of apopinene.