Cytotoxic and antibacterial polyketide-indole hybrids synthesized from indole-3-carbinol by Daldinia eschscholzii

Two skeletally undescribed polyketide-indole hybrids(PIHs), named indolchromins A and B,were generated from indole-3-carbinol(I3 C) in the fungal culture(Daldinia eschscholzii). The indolchromin structures were elucidated mainly by their 1 D and 2 D NMR spectra with the former confirmed by the single-crystal X-ray crystallographic analysis. Each indolchromin alkaloid was chirally separated into four isomers, whose absolute configurations were assigned by comparing the recorded circular dichroism(CD) spectra with the electronic CD(ECD) curves computed for all optional stereoisomers. Furthermore, the indolchromin construction pathways in fungal culture were clarified through enzyme inhibition, precursor feeding experiment, and energy calculation. The cascade reactions,including decarboxylative Claisen condensation catalyzed by 8-amino-7-oxononanoate synthase(AONS),C(sp3)-H activation, double bond migration, and Michael addition, all undergone compatibly during the fungal cultivation. In an MIC range of 1.3–8.6 μmol/L,(2 S,4 R)-and(2 R,4 S)-indolchromin A and(2 R,4 S)-indolchromin B are inhibitory against Clostridium perfringens, Clostridium difficile, Veillonella sp.,Bacteroides fragilis, and Streptococcus pyogenes.(2 R,4 S)-Indolchromin A and(2 S,4 S)-indolchromin B were cytotoxic against the human breast cancer cell line MDA-MB-231 with IC50 values of 27.9 and131.2 nmol/L, respectively, with the former additionally active against another human breast cancer cell line MCF-7(IC_(50)94.4 nmol/L).

Carbon-nitrogen bond formation to construct novel polyketide-indole hybrids from the indole-3-carbinol exposed culture of Daldinia eschscholzii

A plenty of cytochrome P450s have been annotated in the Daldinia eschosholzii genome.Inspired by the fact that some P450s have been reported to catalyze the carbon-nitrogen(C-N)bond formation,we were curious about whether hybrids through C-N bond formation could be generated in the indole-3-carbinol(I3C)exposed culture of D.eschscholzii.As expected,two skeletally undescribed polyketide-indole hybrids,designated as indolpolyketone A and B(1 and 2),were isolated and assigned to be constructed through C-N bond formation.Their structures were elucidated by 1D and 2D NMR spectra.The absolute configurations of 1 and 2 were determined by comparing the recorded and calculated electronic circular dichroism(ECD)spectra.Furthermore,the plausible biosynthetic pathways for 1 and 2 were proposed.Compounds 1 and 2 exhibited significant antiviral activity against H1N1 with IC_(50) values of 45.2 and 31.4μM,respectively.In brief,compounds 1 and 2 were reported here for the first time and were the first example of polyketide-indole hybrids pieced together through C-N bond formation in the I3C-exposed culture of D.eschscholzii.Therefore,this study expands the knowledge about the chemical production of D.eschscholzii through precursor-directed biosynthesis(PDB).