Streptospirodienoic acids A and B, 6,6-spiroketal polyketides from Streptomyces sp

Three new 6,6-spiroketal polyketides, streptospirodienoic acids A (1), B (2) and streptospirodienoate A (3), were obtained from the fermentation broth of Streptomycetes sp. (no. 0950134). Their structures were constructed by one 6,6-spiroketal core coupled with hexadienoic acid, in which the starter unit might be 2-methylpropionyl-CoA. Their chemical structures were determined by detailed analyses of UV, IR, HRESIMS, 1D, 2D-NMR spectroscopic data and the X-ray crystallography technique. The biosynthetic pathway for 1 and 2 was proposed. Compound 3 showed moderate cytotoxic activity against HCT-116 cells (IC50 = 5.2 μM), while compounds 1 and 2 showed no activity (IC50 > 10.0 μM).