efficient resolution with BINOL, and assignment of absolute configuration by Mosher’s method.pdf

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efficient resolution with BINOL, and assignment of absolute configuration by Mosher’s method.pdf

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efficient resolution with BINOL, and assignment of absolute configuration by Mosher’s method

Tetrahedron: Asymmetry 16 (2005) 2141–2147 Tetrahedron: AsymmetryThe natural alkaloid isoanabasine: synthesis from 2,3 0-bipyridine, efficient resolution with BINOL, and assignment of absolute configuration by Moshers method Chuan-Qing Kang, Yan-Qin Cheng, Hai-Quan Guo, Xue-Peng Qiu and Lian-Xun Gao* The State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, PR China Received 18 April 2005; accepted 10 May 2005Abstract—A highly efficient and practical resolution of racemic 1-benzylisoanabasine, which was synthesized by reduction of the benzyl salt of 2,3 0-bipyridine, has been achieved through molecular complexation with (R)-BINOL or (S)-BINOL to afford pure enantiomers (100% ee). The two enantiomers of the natural alkaloid isoanabasine have been obtained by debenzylation of the cor- responding enantiomeric 1-benzylisoanabasine. Using Moshers method by NMR techniques, the absolute configuration of ()-iso- anabasine has been assigned as the (R)-configuration for the first time. Moreover, an unexpected rotamer ratio of Moshers amide was observed. The syn-form of two rotamers of (R)-MTPA-(R)-isoanabasine was predominant over the anti-form. 2005 Elsevier Ltd. All rights reserved.N N R 1 R = H 2 R = Bn OH OH OH OHRO CO2H RO CO2H 5 R = H 6 R = p-Toluoyl (R)-7 (S)-7 Figure 1. Chiral reagents bearing double acidic groups for resolution.1. Introduction The rarely investigated minor natural alkaloid isoanaba- sine 1 (Fig. 1) has been discovered only in low quantities in the desert plant Anabasis aphylla L., which can be found in the northwest of China and Central Asia.1 Re- cently, much attention has been paid to the synthesis and biological activities of its skeleton analogues includ- ing natural and synthetic alkaloids, such as anabasine, anatabine, cytisine, and some other pyridyl piperi- dines.2,3 These analog