17-Hydroxybrevianamide N and Its N1-Methyl Derivative, Quinazolinones from a Soft-Coral-Derived Aspergillus sp. Fungus: 13S Enantiomers as the True Natural Products
J Nat Prod. 2021 Mar 25. doi: 10.1021/acs.jnatprod.1c00098. Online ahead of print.
Under the guidance of MS/MS-based molecular networking and HPLC-UV, two new alkaloid racemates, (±)-17-hydroxybrevianamide N (1) and (±)-N1-methyl-17-hydroxybrevianamide N (2), featuring a rare o-hydroxyphenylalanine residue and an imide subunit, were isolated from a soft-coral-derived Aspergillus sp. fungus. The true natural products (+)-1 and (+)-2 were further monitored and obtained from the freshly prepared EtOAc extracts, while (-)-1 and (-)-2 are artifacts generated during extraction and purification processes. Simultaneously, the structures including absolute configurations of (+)-13S–1, (-)-13R–1, (+)-13S–2, and (-)-13R–2 were elucidated on the basis of comprehensive spectroscopic analysis, ECD calculations, and X-ray diffraction data. Interestingly, basic solution promotes the racemization of (+)-1 and (-)-1, whereas acidic solution suppresses the transformation. The current research was concerned with the true natural products and their artifacts, providing critical insight into the isolation and identification of natural products.