Convenient procedures for the asymmetric reduction of 1,4-diphenylbutane-1,4-dione and synthesis of 2,5-diphenylpyrrolidine derivatives

Asymmetric reduction of 1,4-diphenylbutane-1,4-dione (1) was carried out using the reducing agents NaBH4, BH3.THF, and PhNEt2.BH3 in combination with the chiral reagents (S)-(-)-alpha,alpha-diphenyl-2-pyrrolidinemethanol (4) or (S)-proline (5), in the presence of TMSCl or B(OMe)(3) under various conditions to obtain the corresponding 1.4-diol 2 in 52% to 97% ee. The chiral 1,4-diol 2 was converted to various C-2-symmetric (2S,5S)-2,5-diphenyl-pyrrolidine derivatives 3a-e (45 to 75% yield) via the corresponding dimesylate prepared using MsCl and Et3N.