SELECTIVE REDUCTIONS .46. EFFECT OF THE STERIC REQUIREMENT AT THE 2-POSITION OF APOPINENE ON CHIRAL REDUCTIONS - B-(ISO-2-ETHYLAPOPINOCAMPHEYL)-9-BORABICYCLO[3.3.1]NONANES 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, 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 alpha,beta-acetylenic ketones and alpha-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 ethyllithium and cuprous iodide. (+)-2-n-Propylapopinene was synthesized by Schlosser metalation of (+)-alpha-pinene followed by treatment with ethyl iodide. 4-Phenyl-3-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. 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 alpha-keto esters by Eapine-Borane. For example, while Alpine-Borane produced methyl and ethyl lactate in 92% and 91% ee, respectively, Eapine-Borane gave these alcohols in 97% and 96% ee, respectively. Unfortunately, Prapine-Borane shows no improvement in percent ee for the reduction of alpha-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.