The residue was purified by basic silica gel column chromatography (ethyl acetate:hexane=7:3→1:0). The obtained residue was dissolved in methanol, 4N hydrogen chloride-ethyl acetate solution (1 ml) was added, and the mixture was crystallized from methanol-ethyl acetate. The precipitated crystals were collected by filtration, and recrystallized from methanol-ethyl acetate to give the title compound (0.06 g).