Activity-monitored fractionation of a CHCl3-soluble extract of Deprea subtriflora using a quinone reductase induction assay led to the purification of subtrifioralactones A-J (1-10), 10 novel C-18 norwithanolides based on a new C27 skeleton. These compounds were characterized by spectroscopic and chemical studies, and single-crystal X-ray diffraction analysis was used to confirm the structures of 1 and 4. Compounds 1-10 were evaluated for their cancer chemopreventive activity in terms of their ability to induce quinone reductase activity with cultured murine hepatoma cells, and compounds 1 and 6 were found to be highly effective.