Conformational analysis. CII. Levopimaric acid and related steroidal dienes.
Conformational analysis. CII. Levopimaric acid and related steroidal dienes.
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构象分析。
DOI：
10.1021/ja00825a021
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发表时间：
1974
影响因子：
15
通讯作者：
N. L. Allinger
中科院分区：
文献类型：
--
作者：
G. Lane;N. L. Allinger
力场计算表明，左旋海松酸和与其构型相关的甾体二烯如胆甾-2，4-二烯的构象最好用构象移动的体系来讨论，在该体系中，二烯环可以采用“类左旋海松酸”或“类胆甾-2，4-二烯”构象。这些构象的能量随取代基的数目和位置而变化，并讨论了一些模型化合物的平衡。以前报道的圆二色谱数据进行了讨论，这些构象计算的相对人口。取代基的影响在决定圆二色性曲线的振幅方面似乎是重要的。射线晶体学8表明，左旋海松酸（Ia）在结晶状态下以折叠构象存在，C9-C10键相对于二烯环呈准轴向取向。对左旋海松酸（1a）和胆甾-2，4-二烯（2a）旋光色散的研究表明，左旋海松酸在溶液中以这种折叠构象存在，而胆甾-2，4-二烯（2a）的C10-C9键是准赤道型的。支持后一结论的证据已从核磁共振4，5表面膜6和光化学7研究中获得。
Force field calculations suggest that the conformation of levopimaric acid and configurationally related steroidal dienes such as cholesta-2, 4-diene may bestbe discussed in terms of a conformationally mobile system in which thediene ring can adopt a “levopimaric like” or a “cholesta-2, 4-diene like” conformation. The energies of these conformations vary with the number and positioning of substituents, and the resulting equilibria are dis-cussed for a number of model compounds. Previously reported circular dichroism data are discussed in terms of relative populations calculated for these conformers. The effects of substituents appear to be important in deter-mining the amplitudes of the circular dichroism curves.-Ray crystallography8 has shown that levopimaric acid (la) exists in a folded conformation in the crystalline state, with the C9-C10 bondin a quasiaxial orientation with respect to thediene ring. In-vestigation of the optical rotatory dispersion4 of levopimaric acid (la) and cholesta-2, 4-diene (2a) had pre-viously led to the suggestion that levopimaric acid exists in this folded conformation in solution, in con-trast to the extended conformation deduced for the configurationally related cholesta-2, 4-diene (2a), in which the C10-C9 bond is quasi-equatorial. Sup-porting evidence for the latter conclusion had been ob-tained from nmr, 4, 5 surface film, 6 and photochemical7 studies.