The semisyntheses of melampolides 1 and 2, previously found in Mikania minima, were carried out in order to confirm their chemical structures and to establish their absolute configurations. Their conformational analyses, performed using molecular mechanics, experimental (1)H NMR coupling constants, and NOE experiments, showed a preferred DU conformer in solution at room temperature. The cytotoxic activities of 1 and 2 against three tumor cell lines were also determined. Conjugated aldehyde 2 showed higher activity than alcohol 1.