Two new acridone alkaloids, natsucitrine-I (la) and natsucitrine-I1 (lb) were isolated from the root bark of Citrus natsudaidai and their structures were elucidated on the basis of the spectral and chemical studies.The chemical investigations of the alkaloid constituents of several species of the genus (Rutaceae) have indicated the presence of simple alkyl mines, quinolines and furoquinolines2), and recent studies have revealed the existence of many acridone alka~oids~-~). Wereport here the isolation of two new acridone alkaloids from the root bark of Citrus natsudaidai Hayata and their structure determination.Natsucitrinc-I (la): light yellow prisms, mp 292-293'~, C15H13N05. The UV spectrum (Amax 214, 258, 264, 291, 328 and 365 nm) and the IR spectrum (3350, 1650 and 1600 crn-') showed the characteristic absorption of the 1-hydroxy-9-acridone system7). The presence of two methoxyl groups was suggested by the CMR (6 60.4, 55.2) and PMR (6 3.93, 3.86) spectra8). The presence of one NH and two hydroxyl groups was suggested by thc three one-proton singlets at 6 14.35, 10.29 and 9.28 in the PMR spectrum. In the aromatic proton region, m2-coupled two-proton signals at 6 6.64 and 6.12 (Zxd, J=1.96 Hz), and ortho-coupled two-proton signals at 6 7.90 and 6.90 (2xd, J=8.79 Hz) were observed. The lower orthoorthocoupled signal at 6 7.90 was characteristic of C R in the 9-acridone system. Consequently, these four aromatic 8 protons were assigned to locate at C2, C4, C7 and C8 in the 9-acridone skeleton. Furthermore, the NOE experiment showed 17.3 and 21.9% enhancements of the signalsat S 6.64 and 6.12, respectively, on irradiation at the frequency corresponding to the 0-methyl proton signal at 6 3.93. On the other hand, no NOE enhancement between C7-N at 6 6.90 and the methoxyl protons at 6 3.86 suggested the location of a methoxyl and a hydroxyl groups at C-5 and C-6, respectively. The above data were in good accord with the structure la for natsucitrine-I.Natsucitrine-I1 (lb): light yellow needles, mp 246-247'c, C16tl15N05, UV Amax 215, 257, 266, 289 and 329 nm; 1R v:t: 3360, 1640 and 1605 cm-l. The UV and IR spectra indicated the characteristic absorption of the 1-hydroxy-9-acridones. The CMR studies revealed the presence of three methoxyl groups at 6 60.8, 56.2 and 55.3 8 1 . In the PNR spectrum, intramolecular hydrogen-bonded anc hydroxyl proton signal due to the C-l hydroxyl group in the 9-acridone skeleton was observed at 6 14.26. In addition, one NH proton singlet at 6 10.30, ms-coupled two-proton signals at 66.64 and 6.12 (Zxd, J=1.94 Hz), and e-coupled two-proton signals at 6 7.11 and 8.00 (2xd, J=9.28 Hz) were observed. Treatment of la with diazoaethane gave yellow needles, mp 244-245'~~ which were identified with & by comparisons of TLC and IR spectrum. On the basis of these data, the structure of natsucitrine-I1 was assigned unequivocally to l_b.The structures of natsucitrine-I and natsucitrine-I1 correspond to des N-methylated citpressine-I (2.3) and citpressine-I1 (2b13), respectively. Our present study indicates the first example of the isolation of tho Nil type arridonc alkaloids from Citrus species.