The structure of an unknown yellow base from Menisperum dauricum DC. (Menispermaceae) was determined to be 5,6,9-trimethoxy-7H-dibenzo [de, h]quinolin-7-one (1) by spectral data and total synthesis. It was named menisporphine and the skeletal name “oxoisoaporphine” was proposed for this new type of alkaloid. The biosynthesis route of oxoisoaporphine-type alkaloids in plants is suggested. © 1982, The Pharmaceutical Society of Japan. All rights reserved.