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 13 C NMR and electronic circular dichroism data. Compounds 1 – 4 exhibited potent cytotoxicity against cancer cell lines MCF-7, A549, P6C, and HCT-116 with IC 50 values of 0.02–6.77 μM. The biosynthetic pathway of sekgranaticin ( 1 ) was proposed.