17-Hydroxybrevianamide N and Its N1-Methyl Derivative, Quinazolinones from a Soft-Coral-Derived <i>Aspergillus</i> sp. Fungus: 13<i>S</i> Enantiomers as the True Natural Products

Abstract

Under the guidance of MS/MS-based molecular networking and HPLC-UV, two new alkaloid racemates, (±)-17-hydroxybrevianamide N (<b>1</b>) and (±)-N1-methyl-17-hydroxybrevianamide N (<b>2</b>), featuring a rare <i>o</i>-hydroxyphenylalanine residue and an imide subunit, were isolated from a soft-coral-derived <i>Aspergillus</i> sp. fungus. The true natural products (+)-<b>1</b> and (+)-<b>2</b> were further monitored and obtained from the freshly prepared EtOAc extracts, while (-)-<b>1</b> and (-)-<b>2</b> are artifacts generated during extraction and purification processes. Simultaneously, the structures including absolute configurations of (+)-13<i>S</i>-<b>1</b>, (-)-13<i>R</i>-<b>1</b>, (+)-13<i>S</i>-<b>2</b>, and (-)-13<i>R</i>-<b>2</b> were elucidated on the basis of comprehensive spectroscopic analysis, ECD calculations, and X-ray diffraction data. Interestingly, basic solution promotes the racemization of (+)-<b>1</b> and (-)-<b>1</b>, 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.