Abstract
The enantioselective hydrogenation of ( E)-2-(4-methoxy-3-(3-methoxypropoxy)-benzylidene)-3-methylbutanoic acid ( 1) to ( R)-2-(4-methoxy-3-(3-methoxypropoxy)-benzyl)-3-methylbutanoic acid ( 2)—a key intermediate in the synthesis of the pharmacologically important renin inhibitor Aliskiren—is described. The stereochemistry of the catalytic transformation has been studied using a number of homogeneous chiral Rh(I) and Ru(II) complexes bearing ferrocene-based phosphine ligands. The highest enantioselectivity for the homogeneous hydrogenation of 1 (up to 95% ee) was achieved with a [Rh(NBD) 2]BF 4 pre-catalyst (substrate/catalyst ratio 100:1, 10 bar H 2, 40 °C, in MeOH). To bring the enantioselectivity to perfection an effective method for the isolation of the enantiopure carboxylic acid is suggested likewise.
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