DFT calculation–assisted stereo‐structural assignment of arundifungin

Triantaspirols A-C and Paraphaeolactone Cs from Paraphaeosphaeria sp. KT4192: Sensitivity of CP3 in Distinguishing Close NMR Signals

Hybridized spirobisnaphthalene derivatives, triantaspirols A-C (1-3) and paraphaeolactones C1 and C2 (4 and 5), were identified from the culture broth of the fungus Paraphaeosphaeria sp. KT4192. The NMR spectra of 2 and 3, as well as 4 and 5, closely resembled each other, indicating that these were pairs of diastereomers. Although this NMR spectral resemblance made it challenging to distinguish their relative configurations, detailed analysis of the electronic circular dichroism (ECD) spectra and NOE correlations allowed us to deduce them. The CP3 metric with the DFT-based NMR chemical shifts was found to distinguish configurations of diastereomers in a highly sensitive and accurate manner that DP4 could not account for because of the very close chemical shift differences in the experimental NMR spectra. The reliability of this method was assessed using 23 published examples which could not be distinguished by DP4 protocol.

Stereochemistries and Biological Properties of Oligomycin A Diels-Alder Adducts

Oligomycin A is a potent antibiotic and antitumor agent. However, its applications are restricted by its high toxicity and low bioavailability. In this study, we obtained Oligomycin A Diels-Alder adducts with benzoquinone and N-benzylmaleimide and determined their absolute configurations by combining ¹H and ROESY NMR data with molecular mechanics conformational analysis and quantum chemical reaction modeling. The latter showed that adduct stereochemistry is controlled by hydrogen bonding of the Oligomycin A side-chain isopropanol moiety with the carbonyl group of the dienophile. Biological studies showed that the Diels-Alder modification of the Oligomycin A diene system resulted in a complex antiproliferative potential pattern. The synthesized adducts were determined to be more active against the triple-negative (ERα, PR, and HER2 negative) breast cancer cell line MDA-MB-231 and lung carcinoma cell line A-549 compared to Oligomycin A. Meanwhile, Oligomycin A was more potent against myeloid leukemia cell line K-562 and breast carcinoma cell line MCF-7 than its derivatives. Thus, modification of the diene moiety of Oligomycin A is a promising strategy for developing novel antitumor agents based on its scaffold.