Two structurally unidentified alkaloids (tentatively named bases III and IV), isolated from Menispermum dauricum DC. (Menispermaceae), were found to be dauriporphine (1), a known oxoisoaporphine-type alkaloid, and 2, 3-dihydromenisporphine (3), a new alkaloid of the same type, respectively. The structure of dauriporphine was confirmed by synthesis of 4, 5, 6, 9-tetramethoxy-7H-dibenzo [de, h] quinolin-7-one (1).