Absolute configuration of azaphilones from Monascus kaoliang KB9 and solvent effects on their keto and enol forms

Abstract

Monascus fermented rice, also known as red yeast rice, exhibits a broad spectrum of biological activities due to its chemical constituents, such as monacolins and azaphilone pigments. Here, we cultured Monascus kaoliang KB9 in a liquid malt medium instead of on rice as a carbon source. Eleven known compounds (1–11) containing azaphilones and their early intermediate were isolated and identified. However, this was the first time that angular tricyclic azaphilones, monasfluols A (4) and B (7), acetyl-monasfluol A (5) and monasfluore A (6), were isolated from this species. Interestingly, all isolated tricyclic azaphilones existed exclusively in enol form in CD₃OD, as evidenced by NMR spectroscopy. The absolute configuration of compounds 4–7 was also first experimentally identified based on ECD spectroscopy combined with conformational analyses using computational techniques. The assigned stereochemistry of Monascus azaphilones in this work provides essential structural information that will benefit future biological and pharmaceutical investigations.

Graphical Abstract

Keywords:

• azaphilone
• Monascus kaoliang
• red yeast rice
• solvent effect

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the Center of Excellence for Innovation in Chemistry (PERCH-CIC), Ministry of Higher Education, Science, Research and Innovation. Computational studies were partially supported from the Higher Education Research Promotion and National Research University Project of Thailand, Office of the Higher Education Commission. We also thank Assoc. Prof. Prasat Kittakoop for allowing us to use the polarimeter and Dr Colin Lazarus for critically reading this manuscript.