Conformational Mobility of (-)-α-Phellandrene, Deduced from its Optical Rotatory Dispersion
Conformational Mobility of (-)-α-Phellandrene, Deduced from its Optical Rotatory Dispersion
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(-)-α-水芹烯的构象迁移率，由其旋光色散推导
DOI：
10.1021/ja00874a024
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发表时间：
1962
影响因子：
15
通讯作者：
U. Weiss
中科院分区：
文献类型：
--
作者：
H. Ziffer;E. Charney;U. Weiss
测定了（-）-α-水芹烯（I）和3，5-胆甾二烯（III）的旋光度为11，23和82。观察到的温度依赖性的旋转的I，而不是III，被解释为表明存在一个移动的平衡之间的两种构象的I。这些构象的人口和相对稳定性进行了讨论。本文的结论是，其中一种构象Ia具有较大的转动强度，但至少在室温下其浓度小于Ib。在最近的一份通讯中，从（-）-α-水芹烯的旋光色散曲线中推断出，该分子的优选构象是异丙基为准轴向的构象（IA）。这一结论是基于“偏斜二烯”规则2，该规则允许从二烯的偏斜方向或螺旋度预测含有非平面顺式二烯的分子的旋光度的符号：如果共轭体系的四个碳原子形成左手螺旋，则其在二烯吸收带的长波长侧的贡献产生左旋，而右手螺旋产生右旋。此外，由偏斜二烯产生的对旋光性的贡献比与例如不对称碳原子相关的贡献大得多。因此，在二烯吸收附近的旋光色散将在很大程度上反映发色团双键的相对取向。由此推论，在已知绝对构型的（-）-α-水芹烯的情况下，所观察到的负旋转表明在室温下主要构象是准轴向构象（IA）而不是准赤道构象
The optical rotations of (—)-a-phellandrene (I) and 3, 5-cholestadiene (III) have been measured at 11, 23, and 82. The observed temperature-dependence of the rotation of I, but not of III, is interpreted as indicating the existence of a mobile equilibrium between two conformations of I. The populations and relative stabilities of these conformers are discussed. It is concluded that one of these conformers la has the larger rotational strength, but it is present in smaller concentrations than lb, at least at room temperature.In a recent communication from this Laboratory1 it was deduced from the rotatory dispersion curve of (—)-a-phellandrene that the preferred conforma-tion of this molecule is that in which the isopropyl group is quasi-axial (IA). This conclusion was based on the “skewed diene’’rule2 which allows the sign of the optical rotation of a molecule con-taining a non-planar cisoiddiene to be predicted from the skew sense or helicity of the diene: if the four carbon atoms of the conjugated system form a left-handed helix, its contribution on the long-wave length side of the diene absorption band produces levorotation, whereas a right-handed helix produces dextrorotation. Furthermore, the contribution to the optical activity produced by the skewed diene is much greaterthan that associated with, eg, an asymmetric carbon atom. Conse-quently, the rotatory dispersion in the vicinity of the diene absorption will largely reflect the relative orientation of the double bonds of the chromophore. By this reasoning, in the case of (—)-a-phellan-drene of known absolute configuration, the observed negative rotation suggested that the predominant conformer at room temperature was the quasi-axial (IA) rather than the quasi-equatorial