The structure of oxyterihanine (2c), a new phenolic benzo[c]phenanthridone alkaloid which was isolated from Formosan Xanthoxylum nitidum (roxb.) d. c.(Fagara nitida roxb.), was established as 8-hydroxy-9-methoxy-5-methyl-2,3-methylenedioxybenzo[c]phenanthridin-6(5H)-one by direct comparison with a synthetic specimen, which was derived from 2-(4-isopropoxy-3-methoxyphenyl)-6,7-methylenedioxy-3,4-dihydronaphthalen-1(2H)-one (8a) through the synthetic sequence shown in Charts 3 and 4. 9-Hydroxy-8-methoxy-5-methyl-2,3-methylenedioxybenzo[c]phenanthridin-6(5H)-one [oxyisoterihanine (2d)] and two phenolic quaternary benzo[c]phenanthridine alkaloids, terihanine (1c) and isoterihanine (1d), were also synthesized. © 1987, The Pharmaceutical Society of Japan. All rights reserved.