Asymmetric reduction of prochiral cyclic imines with chiral sodium acyloxyborohydrides (5 a-i), which are easily prepared by the reaction of NaBH
4 with various
N-acyl α-amino acids, has been investigated. Of these new reducing agents, triacyloxyborohydrides (5 c-f), derived from NaBH
4 (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 CH
2Cl
2 or CHCl
2CH
3 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.
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