Process for preparing Ezetimibe intermediate by enantioselective CBS catalyzed ketone reduction with BH3-DEA prepared in situ

The (S) alcohol in the benzylic position of compound 2, a key intermediate in the synthesis of the cholesterol lowering agent Ezetimibe, was introduced by the (R)-MeCBS catalyzed asymmetric carbonyl reduction of ketone 1 using borane diethylaniline complex (BDEA) as the reducing agent. The latter was prepared in situ from sodium borohydride (NaBH₄), diethylaniline (DEA) and dimethylsulfate (DMSO₄). BDEA prepared in situ offers considerable advantages from the industrialization standpoint (cost and stability on storage of the reagents) over commercial solutions of BH₃-THF (BTHF) or BH₃-DMS (BMS). The effect of critical reaction parameters such as addition mode of reagent, temperature, solvent, reaction quenching as well as LiCl addition on the selectivity has been examined. This reaction has been successfully applied in the process for the preparation of key intermediate 2 for Ezetimibe.