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

•The first example of nor-α-tocopheroid was isolated from Cirsium setosum.•The assignments of structure on the long side chain were achieved for the first time.•The biosynthetic pathway was postulated.•α-Tocospiro C showed significant cytotoxic activity against human colon cancer.