Highly enantioselective catalytic asymmetric synthesis of a (R)-sibutramin precursor.

The first highly enantioselective, catalytic asymmetric synthesis of di-des-methylsibutramine 3 is described. Dienamide 10, prepared by acetic acid anhydride quenching of the condensation product of nitrile 4 with a methallyl magnesium chloride, proved to be an excellent substrate for ruthenium-catalyzed asymmetric hydrogenation with atropisomeric diphosphine ligands. Hydrogenation with a ruthenium/(R)- MeOBiPheP catalyst at S/C = 500, gave the chiral amide (R)-9 in 98.5% ee in almost quantitative yield. After acidic amide hydrolysis the desired amine (R)-3 was obtained without erosion of enantioselectivity. It is anticipated that the overall process will be amenable to large-scale production.