Wong Lawrence W-Y, Au Yeung Alex S-F, Tam Gemma S-S, Kan Jack W-H, Sung Herman H-Y, Sheong Fu Kit, Lin Zhenyang, Williams Ian D
Department of Chemistry, Hong Kong University of Science and Technology Clear Water Bay Kowloon Hong Kong China
RSC Adv. 2018 Jan 4;8(3):1451-1460. doi: 10.1039/c7ra11997f. eCollection 2018 Jan 2.
The chiral spiroborate anions [B (Sal)] and [B (Sal)], ( and subscripts indicate boron stereochemistry) have been isolated as 1 : 1 quininium and 1 : 2 sparteinium salts, [HQuin][B (Sal)] and [HSpa][B (Sal)] respectively, by either cation metathesis or a simple one-pot synthesis involving reaction of boric and salicylic acids with the alkaloid base. Circular dichroism (CD) spectroscopy shows that the B-based chirality is stable in polar aprotic media, such as DMF or DMSO, though labile in protic solutions. Enantiopure salts with achiral counter-cations such as [NBu][B (Sal)] may then be prepared by exchange, so these B-chiral anions may have use in metathesis-based resolutions. Due to a site disorder the anion in [HSpa][B (Sal)] is limited to 70% ee, however an enantiopure analogue [HSpa][B (5-Cl-Sal)] is readily formed using 5-chlorosalicylic acid. This also indicates a wide family of stable enantiopure B-chiral anions may be isolated by this approach.
手性螺硼酸根阴离子[B (Sal)]和[B (Sal)]（下标 和 表示硼的立体化学）已分别通过阳离子复分解反应或涉及硼酸和水杨酸与生物碱碱反应的简单一锅法合成，以1∶1的奎宁鎓盐和1∶2的鹰爪豆碱盐形式分离出来，即[HQuin][B (Sal)]和[HSpa][B (Sal)]。圆二色性（CD）光谱表明，基于硼的手性在极性非质子介质（如DMF或DMSO）中是稳定的，但在质子溶液中不稳定。然后可以通过交换制备具有非手性抗衡阳离子的对映体纯盐，如[NBu][B (Sal)]，因此这些基于硼的手性阴离子可用于基于复分解反应的拆分。由于位点无序，[HSpa][B (Sal)]中的阴离子对映体过量仅为70%，然而使用5-氯水杨酸很容易形成对映体纯类似物[HSpa][B (5-Cl-Sal)]。这也表明通过这种方法可以分离出一大类稳定的对映体纯硼基手性阴离子。
