Genome-based discovery and total synthesis of janustatins, potent cytotoxins from a plant-associated bacterium

Host-associated bacteria are increasingly recognized as underexplored sources of bioactive natural products with unprecedented chemical scaffolds. A recently identified example is the plant root-associated marine bacterium Gynuella sunshinyii of the chemically underexplored order Oceanospirillales. Its genome contains at least 22 biosynthetic gene clusters suggesting a rich and mostly uncharacterized specialized metabolism. In this work, in silico chemical prediction of a non-canonical polyketide synthase cluster led to the discovery of janustatins, structurally unprecedented polyketide alkaloids with potent cytotoxicity that are produced at minute quantities. A combination of MS and 2D NMR experiments, density functional theory calculations of (13)C chemical shifts, and semiquantitative interpretation of T-ROESY data were conducted to determine the relative configuration, which enabled the total synthesis of both enantiomers and assignment of the absolute configuration. Janustatins feature a previously unknown pyridodihydropyranone heterocycle and an unusual biological activity consisting of delayed, synchronized cell death at subnanomolar concentrations.