Opportunities and Limitations for Assigning Relative Configurations of Antibacterial Bislactones using GIAO NMR Shift Calculations

Four new bislactones, dihydroacremonol
(1), clonostachyone
(2), acremodiol B (3), and acremodiol C
(4), along with one known compound, hymeglusin (5), were isolated from cultures of two fungal strains (MSX59876
and MSX59260). Both strains were identified based on phylogenetic
analysis of molecular data as Clonostachys spp.;
yet, they biosynthesized a suite of related, but different, secondary
metabolites. Given the challenges associated with elucidating the
structures and configurations of bislactones, GIAO NMR calculations
were tested as a complement to traditional NMR and HRESIMS experiments.
Fortuitously, the enantiomer of the new natural product (4) was known as a synthetic compound, and the predicted configuration
from GIAO NMR calculations (i.e., for the relative configuration)
and optical rotation calculations (i.e., for the absolute configuration)
matched those of the synthesis product. These results engendered confidence
in using similar procedures, particularly the mixture of GIAO NMR
shift calculations coupled with an orthogonal technique, to predict
the configuration of 1–3; however,
there were important limitations, which are discussed for each of
these. The metabolites displayed antimicrobial activities, with compounds 1 and 4 being the most potent against Staphylococcus aureus with MICs of 1 and 4 μg/mL,
respectively.

从两株真菌菌株（MSX59876与MSX59260）的发酵培养物中，分离得到4个全新的双内酯类化合物（bislactones）：二氢顶头孢霉醇（dihydroacremonol，1）、棒座孢酮（clonostachyone，2）、顶头孢霉二醇B（acremodiol B，3）与顶头孢霉二醇C（acremodiol C，4），以及1个已知化合物黑麦草素（hymeglusin，5）。两株菌株均通过分子数据的系统发育分析鉴定为棒座孢属（Clonostachys）菌株，但二者所合成的次级代谢产物虽结构相关却存在显著差异。鉴于双内酯类化合物的结构与构型解析存在诸多难点，本研究尝试将GIAO NMR计算作为传统核磁共振（NMR）与高分辨电喷雾电离质谱（HRESIMS）实验的补充手段。幸运的是，新天然产物4的对映异构体此前已作为合成化合物被报道，通过GIAO NMR计算得到的相对构型预测结果，与旋光计算得到的绝对构型预测结果，均与该合成产物的构型完全吻合。上述结果为后续采用类似分析策略——尤其是GIAO NMR位移计算与正交技术联用的方法——预测化合物1~3的构型提供了坚实支撑，但该方法仍存在重要局限性，本文将针对各化合物逐一探讨其局限性。所分离得到的代谢产物均表现出抗菌活性，其中化合物1与4对金黄色葡萄球菌（Staphylococcus aureus）的抑菌活性最强，最低抑菌浓度（MIC，Minimum Inhibitory Concentration）分别为1 μg/mL与4 μg/mL。