Purpurascenines A–C, Azepino-Indole Alkaloids from Cortinarius purpurascens: Isolation, Biosynthesis, and Activity Studies on the 5-HT2A Receptor

Three previously undescribed azepino-indole alkaloids, named purpurascenines A–C ( 1 – 3 ), together with the new-to-nature 7-hydroxytryptophan ( 4 ) as well as two known compounds, adenosine ( 5 ) and riboflavin ( 6 ), were isolated from fruiting bodies of Cortinarius purpurascens Fr. (Cortinariaceae). The structures of 1 – 3 were elucidated based on spectroscopic analyses and ECD calculations. Furthermore, the biosynthesis of purpurascenine A ( 1 ) was investigated by in vivo experiments using 13 C-labeled sodium pyruvate, alanine, and sodium acetate incubated with fruiting bodies of C. purpurascens . The incorporation of 13 C into 1 was analyzed using 1D NMR and HRESIMS methods. With [3- 13 C]-pyruvate, a dramatic enrichment of 13 C was observed, and hence a biosynthetic route via a direct Pictet–Spengler reaction between α-keto acids and 7-hydroxytryptophan ( 4 ) is suggested for the biosynthesis of purpurascenines A–C ( 1 – 3 ). Compound 1 exhibits no antiproliferative or cytotoxic effects against human prostate (PC-3), colorectal (HCT-116), and breast (MCF-7) cancer cells. An in silico docking study confirmed the hypothesis that purpurascenine A ( 1 ) could bind to the 5-HT 2A serotonin receptor’s active site. A new functional 5-HT 2A receptor activation assay showed no functional agonistic but some antagonistic effects of 1 against the 5-HT-dependent 5-HT 2A activation and likely antagonistic effects on putative constitutive activity of the 5-HT 2A receptor.