LiAlH cyclic ketones

Stereoselectivity in the reaction of acyclic ketone 270 is different from that of the cyclic ketone 256. The acetate in 271, prepared by reduction of the ketone 270 to alcohol with LiAlH and acetylation, was displaced with Me A1 from the exo side to give 272 with retention of the stereochemistry. No racemization of benzyl cation was observed. However, reaction of 270 with MeLi gave 274. The OH group of 274 was removed with hydride from the less hindered side as shown by 275 to give 276 with... 

Cyclic chiral a-amino ketones are reduced by BH3, NaBH , or LiAlH predominately to chiral /ran.r-a-amino alcohols (80 90% yield). 

The conversion reqnires reduction however, the conditions necessary (LiAlH ) would also reduce the ketone carbonyl. The ketone functionality is therefore protected as the cyclic acetal. 

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