To analyze the conformational features of 2 and 4, a series of computational search was conducted by Monte Carlo (MC) search. A total of 3000 MC steps were performed to confirm the reproducibility of calculation results. After theMC conformational search, each of the resulting conformations was subjected to the energy-minimization calculation by MMFF force field. Each of the lowest energy conformers belonging to two separate clusters are represented as 2a (330.58 kJ/mol) and 2b (346.83 kJ/mol) for isoplagiochin B, and three clusters 4a (360.80 kJ/mol), 4b (358.16 kJ/mol), and 4c (368.27 kJ/mol) for dihydroisoplagiochin B (Fig. 2). For isoplagiochin
B, 2a was abundant, whereas energy levels for 4a and 4b for dihydroisoplagiochin B were within a range of 1 kcal/mol. The presence of conformations 2a and 4a was consistent with the NOESY data as shown in Figure 3. These conformational characteristics of 2 were compatible with those observed in the crystal conformation of isoplagiochin B triacetate by X-ray analysis.