Reactions of Magnesium Hydride. 4. Stereoselective Reductions of Cyclic and Bicyclic Ketones by Lithium Alkoxymagnesium Hydrides

Article abstract of DOI:10.1021/jo01294a022

A series of lithium alkoxymagnesium hydrides were prepared and allowed to reduce 4-tert-butylcyclohexanone (I), 3,3,5-trimethylcyclohexanone (II), 2-methylcyclohexanone (III), and camphor (IV).It was found that very bulky secondary cyclic alkoxy groups such as 2,2,6,6-tetramethyl and 2,2,6,6-tetrabenzylcyclohexoxy were very stereoselective in the reduction of these ketones.For example, LiMgH2(2,2,6,6-Me4-c-HxO) reduced ketone I to provide 89 percent of the axial alcohol compared to 83 percent for HMg(2,6-i-Pr2PhO) which previously had provided the greatest degree ofselectivity among the new hydrides of magnesium.The dialkoxyhydrides, LiMgH(OR)2 , were also found to reduce ketones I, II, III, and IV stereoselectively but to a lesser extent than the monoalkoxyhydrides, LiMgH2(OR).These reactions were also accompanied by more enolization than observed for the monoalkoxy reagents.