The effectiveness and limitations of density functional theory (DFT) calculations in the structural determination of complexed and conformationally flexible natural products were demonstrated using the cyclohelminthols CP-1 (1) CP-2 (2), CP-3 (3), and CP-4 (4) newly isolated from Helminthosporium velutinum yone96. Prior to DFT calculations, the structures were tentatively assigned using conventional spectroscopic analyses. The structures were verified with reference to DFT-derived ¹³C and ¹H NMR chemical shifts, ³J_HH and ⁿJ_CH values, and electronic circular dichroism (ECD) spectra. The ¹³C chemical shift calculations were very effective for verifying the ring-structure moieties but less effective for verifying the geometry of the side chain in which the juncture asymmetric carbon (C-16) was apart from the ring-structure moiety. However, ¹H chemical shift calculations compensated for the imperfection of the latter. ECD spectral calculations were used to determine the absolute configurations. Calculations for virtual simple model molecules enabled us to evaluate the reliability of the ECD spectral calculation and derive the chiral torsion responsible for the characteristic Cotton effects.

中文翻译：

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Cyclohelminthol CPs：基于密度泛函理论的天然产物结构阐明的范围和局限性

使用环蠕虫CP-1（1）CP-2（2），CP-3（3）和CP证明了密度泛函理论（DFT）计算在复杂和构象灵活的天然产物结构测定中的有效性和局限性-4（4）新分离自Helminthosporium v​​elutinum yone96。在进行DFT计算之前，使用常规的光谱分析方法初步确定了结构。参照DFT衍生的¹³ C和¹ H NMR化学位移，³ J _HH和ⁿ J _{CH验证了结构}值和电子圆二色性（ECD）光谱。的^13点Ç化学位移的计算是用于验证环结构的部分，但用于验证，其中接合处的不对称碳（C-16）是开的从环状结构部分的侧链的几何形状不太有效很有效。但是，¹ H化学位移计算可以弥补后者的不完善之处。ECD频谱计算用于确定绝对配置。虚拟简单模型分子的计算使我们能够评估ECD光谱计算的可靠性，并得出造成棉花特征效应的手性扭转。