Asymmetric reduction of cyclic imines with chiral sodium acyloxyborohydrides

Publisher Website
Google Scholar
Abstract
Asymmetric reduction of prochiral cyclic imines with chiral sodium acyloxyborohydrides (5a–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 equiv.) and (S)-N-acylproline (3 equiv.), were found to reduce 3,4-dihydropapaverine (2) in tetrahydrofuran to (S)-norlaudanosine (3) hydrochloride in 60% optical yield. The N-benzyloxycarbonyl derivative (5c) 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 (5c); halogenated alkane solvents such as CH2Cl2 or CHCl2CH3 gave a better optical yield of compound (3)(70% e.e.). The reagent (5c) also reduced other cyclic imines (6a–c) and (8) to the corresponding alkaloids (7a–c) and (9) in excellent optical yields (70–86% e.e.), providing an effective route to the asymmetric synthesis of these alkaloids. A possible reaction path for this reduction is also presented.