α-Tocospiro C, a novel cytotoxic α-tocopheroid from Cirsium setosum

α-Tocospiro C ( 1 ), the first example of nor-α-tocopheroid possessing unusual 7,8-dimethyl-1-oxaspiro[4.4]non-7-ene-6,9-dione carbon skeleton, was isolated and characterized from Cirsium setosum . Its structure and absolute configuration were determined by extensive spectroscopic methods, especially 2D NMR and ECD data analysis. The biosynthetic pathway was postulated. α-Tocospiro C ( 1 ) exhibited significant selective cytotoxic activities against human colon cancer (HCT8) cells, with IC 50 values of 0.03 μM.