(+/-)-7,7'-Dihydroxy-8,8'-biquinolyl (6) was resolved into its enantiomorphic atropisomers via reverse phase (C18) chromatographic separation of epimeric bismenthyl carbonates, (-)-lk-9 and (+)-ul-9, derived from 6 and (+)-menthyl chloroformate. The faster eluting diastereoisomer, (-)-lk-9, was revealed to possess an (aS)-configurated biaryl axis by X-ray crystallographic analysis. Saponification of the separated bismenthyl carbonates gave enantioenriched samples of biquinolyl 6, and absolute stereochemical configurations were assigned to the two optical isomers as (-)-(aS)-6 and (+)-(aR)-6 by correlation with their respective progenitors, (-)-lk-9 and (+)-ul-9. First-order rate constants for the enantiomerization of 6 in water were obtained over the temperature range 316-366 K, and activation parameters were determined as Delta H double dagger = 34.0 kcal mol(-1) and Delta S double dagger = 18.7 cal mol(-1) K-1 by Eyring plot analysis. A low level (AM1) computational study of the rotational dynamics of 6 showed excellent agreement with kinetic experimental data and suggested that enantiomerization occurs preferentially via a syn pathway. In common with (-)-(aS)-1,1'-bi-2-naphthol (BINOL), (-)-(aS)-6 showed positive exciton chirality in its electronic circular dichroism (CD) spectrum and gave a characteristic couplet composed of a positive maximum Cotton effect at 250 nm and a negative minimum at 234 nm (Delta Delta epsilon = +40 M-1 cm(-1) at 64% ee).
(±)-7,7'-二羟基-8,8'-二
喹啉基(6)通过反相(C18)色谱分离其表异构体双薄荷基
碳酸酯(-)-lk-9和(+)-ul-9,得到其对映异构的阿托皮素。通过X射线晶体学分析,较快洗脱的双薄荷基
碳酸酯(-)-lk-9显示了一个具有(aS)构型的二芳基轴。分离出的双薄荷基
碳酸酯经皂化后得到手性富集的二
喹啉基6样品。通过与相应的前体(-)-lk-9和(+)-ul-9相关联,分别将二者的绝对立体
化学构型确定为(-)-(aS)-6和(+)-(aR)-6。在316-366 K温度范围内,测定物质6在
水中的对映异构转变的第一级速率常数,并通过Eyring图分析确定活化参数为ΔH‡=34.0 kcal mol⁻¹和ΔS‡=18.7 cal mol⁻¹ K⁻¹。对物质6旋转动力学的低级别(
AM1)计算研究显示,其结果与动力学实验数据吻合良好,并表明对映异构化优先通过顺式方式发生。与(-)-(aS)-1,1'-双-
2-萘酚(BINOL)类似,(-)-(aS)-6在其电子圆二色(CD)光谱中显示出正的纠缠手性,并在250 nm处有一个正的最大 Cotton 效应,在234 nm处有一个负的最小值(ΔΔε=+40 M⁻¹ cm⁻¹,纯度为64% ee)。