Assignment of absolute configuration of cyclic secondary amines by NMR techniques using Mosher's method: a general procedure exemplified with (-)-isoanabasine.
A general procedure to determine the absolute configuration of cyclic secondary amines with Mosher's NMR method is demonstrated, with assignment of absolute configuration of isoanabasine as an example. Each Mosher amide can adopt two stable conformations (named rotamers) caused by hindered rotation around amide C--N bond. Via a three-step structural analysis of four rotamers, the absolute configuration of (-)-isoanabasine is deduced to be (R) on the basis of Newman projections, which makes it easy to understand and clarify the application of Mosher's method to cyclic secondary amines. Furthermore, it was observed that there was an unexpected ratio of rotamers of Mosher amide derived from (R)-isoanabasine and (R)-Mosher acid. This phenomenon implied that it is necessary to distinguish the predominant rotamer from the minor one prior to determining the absolute configuration while using this technique.
Kang
Chuan-Qing
CQ
The State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, P. R. China. kangcq@ciac.jl.cn
Guo
Hai-Quan
HQ
Qiu
Xue-Peng
XP
Bai
Xiao-Li
XL
Yao
Hai-Bo
HB
Gao
Lian-Xun
LX
D000588:Amines / Q000737:chemistry*; D000691:Anabasine / Q000031:analogs & derivatives* / Q000737:chemistry*; D007306:Insecticides / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D008968:Molecular Conformation; D015394:Molecular Structure; D017433:Protein Structure, Secondary; D013237:Stereoisomerism
D000588:Amines; D007306:Insecticides; D000691:Anabasine