Sekgranaticin, a SEK34b-Granaticin Hybrid Polyketide from Streptomyces sp. 166.

Sekgranaticin (1), a novel hybrid polyketide with a complex 6/6/6/6/6/6/6 7-ring system, was isolated together with granaticins A (2) and B (3) and methyl granaticinate (4) from the culture broth of Streptomyces sp. 166#. Its structure was elucidated by spectroscopic analysis. The absolute configuration was determined on the basis of the calculated 13C NMR and electronic circular dichroism data. Compounds 1-4 exhibited potent cytotoxicity against cancer cell lines MCF-7, A549, P6C, and HCT-116 with IC50 values of 0.02-6.77 μM. The biosynthetic pathway of sekgranaticin (1) was proposed.