Asymmetric Reduction of Various Ketones with the Sodium Salts of α-Amino Acid Borane Complexes

The sodium salts of amino acid borane complexes (1) produced from equimolar amount of NaBH4 and optically active α-amino acids in tetrahydrofuran at room temperature reduced various ketones to the corresponding optically active alcohols (2-62% optical yield). Sodium prolinate borane complex (2) gave the best results. Asymmetric reduction of (3) with sodium L-prolinate borane complex (2) followed by catalytic hydrogenolysis of benzyl and carbobenzoxy groups gave (4), a cardiotonic agent, in 62% optical yield.