New Asymmetric Reducing Agents for Cyclic Imines Chiral Sodium Triacyloxyborohydrides

Abstract

Asymmetric reduction of prochiral cyclic imines with chiral sodium acyloxyborohydrides (5 a-i), which are easily prepared by the reaction of NaBH4 with various N-acyl α-amino acids, has been investigated. Of these new reducing agents, triacyloxyborohydrides (5c-f), derived from NaBH4 (1 eq.) and (S)-N-acylproline (3 eq.),were found to reduce 3, 4-dihydropapaverine (1) in tetrahydrofuran to (S)-norlaudanosine (2) in 60 % optical yield. The N-benzyloxycarbonyl derivative (5 c) could be isolated as a powder and characterized. The effect of solvents on this asymmetric reduction has been examined by the use of the isolated reagent (5 c); halogenated alkane solvents such as CH2C12 or CHC12CH3 gave a better optical yield of compound (2) (70 % e. e.). The reagent (5 c) also reduced other cyclic imines (6 a-c) and (8) to the corresponding alkaloids (7 a-c) and (9) in excellent optical yields (70-86 % e. e.), providing an effective route to the asymmetric synthesis of these alkaloids. The asymmetric reduction of the imines (10 and 13) also proceeded smoothly to furnish the compounds (11 and 14), the precursors of TMQ and TA-073 (bronchodilating agents), in high optical yields. A possible reaction path for this reduction is also presented. © 1983, The Society of Synthetic Organic Chemistry, Japan. All rights reserved.