A new class of sesquiterpene derivative useful for treating cancerous and inflammatory diseases are disclosed. These deoxyelephantopin derivatives are effective in suppressing proliferation, migration, mobility, invasion, growth, and/or metastasis of cancer cells in a patient, or useful for enhancing an anti-proliferative effect of another anti-cancer drug on cancer cells when treating a patient, or for sensitizing and/or enhancing an anti-cancer effect of a gluthathione synthesis blocker on inhibition of triple negative breast cancer cell activity, or for treatment and/or prophylaxis of lipopolysaccharide-stimulated inflammatory response in a patient, or for all of the above. Also disclosed are methods of preparing the deoxyelephantopin derivatives.