CHIRAL SYNTHESIS VIA ORGANOBORANES .34. SELECTIVE REDUCTIONS .47. ASYMMETRIC REDUCTION OF HINDERED ALPHA,BETA-ACETYLENIC KETONES WITH B-CHLORODIISOPINOCAMPHEYLBORANE TO PROPARGYLIC ALCOHOLS OF VERY HIGH ENANTIOMERIC EXCESS - IMPROVED WORK-UP PROCEDURE FOR THE ISOLATION OF PRODUCT ALCOHOLS

The Midland reagent, Alpine-Borane (2a), is excellent for the asymmetric reduction of many acetylenic ketones, but it fails with hindered derivatives. On the other hand, B-chlorodiisopinocampheylborane (DIP-Chloride, 4) reacts with hindered alpha,beta-acetylenic ketones to provide the corresponding propargylic alcohols in 96 to greater-than-or-equal-to 99% ee. The reaction is in accordance with the tentative mechanism proposed earlier. While 4-phenyl-3-butyn-2-one is reduced in only 21% ee, 4 reduces 4-methyl-1-phenyl-1-pentyn-3-one in 53% ee and 4,4-dimethyl-1-phenyl-1-pentyn-3-one in greater-than-or-equal-to 99% ee. The generality of this observation is demonstrated by reducing a series of hindered acetylenic ketones with increasing steric requirements and differing electronic environments. Thus, 2,2-dimethyl-4-tridecyn-3-one, 1-cyclopentyl-4,4-dimethyl-1-pentyn-3-one, and 3,3-dimethyl-5-tetradecyn-4-one are all reduced to the corresponding alcohols in greater-than-or-equal-to 99% ee and 97% ee, respectively. A modified and operationally simpler workup procedure for obtaining the alcohols in high isolated yields is described. Comparison of reagent 4 with 2a is also made, making clear the range of applicability of each reagent. This development makes it possible to reduce asymmetrically any acetylenic ketone by a judicious choice of either 2a or 4.