The structure of brunonine (I), a new C20-diterpenoid alkaloid isolated from the whole plant of Delphinium brunonianum Royle, was established on the basis of spectroscopy and the result of X-ray crystallography. Brunonine (1) was shown to possess the atisine azomethine (k) skeleton, with an ethoxyl group at C(19) position. Delphinium brunonianum Royle, growing in the southwestern parts of China, is used in Tibet as folk medicine for influenza, itchy rash and snake-bite. Its alkaloidal constituents have now been investigated, a C20-diterpenoid alkaloid designated as brunonine (1) along with two known bases dictysine (2) and ajaconine (3) was isolated. Brunanine, C22H33NO3 (isi 319.2413, calc. 319.2461), m.p. 238.209°C. [α]D^20 +174° (c 0.067, MeOH), showed the presence of an exocyclic methylene group (115.7 ppm, s and 109.1 ppm, t) and a C-CH3 group (40.97, 3H, s), demonstrating that it belongs to a C20-diterpenoid alkaloids. ¹H-NMR signal of one hydrogen at δ6.01 and signal of a doublet at 165.5 ppm as well as IR absorption (1654 cm⁻¹) suggested the Presence of H-C=N moiety in the base. The resistance of an ether-linkage in brunonine was deduced from the finding that no hydroxyl group absorption was found in the IR spectrum of its diacetate (5) (at 463, amorphous). That the ether takes the form in which OCH₂CH₃ is linked to one of the skeleton carbons as a side chain is Justified by the following spectral data: ¹H-NMR δ1.27 (3H, t, J = 7.2 Hz) and ¹³C-NMR 15.2 ppm (q) for OCH₂CH₃; ¹H-NMR δ3.67 and 4.14 (1H each, dq, J = 7.2, 10.8 Hz) and ¹³C-NMR 64.6 ppm (t) for OCH₂CH₃. The ¹³C-NMR signal 94.7 ppm (d) pointed out that the ethoxyl group is bonded to C(19) or C(20) which in turn is linked to the nitrogen in the molecule. The location of OCH₂CH₃ group was then clarified by an NOE demonstration. When the signal δ0.97 for C(4)-CH₃ was irradiated. the one-proton peak at δ4.30 (broadened by a long-range coupling with H-C=N), attributable to C(19), increased by 12% in intensity, while the other one-proton peak at δ8.03, which may then be assigned to C(20), remained unchanged. Brunonine yielded a pair of isomeric methylcyclohexanones (IR 1705 and 1710 cm⁻¹) upon acid-catalyzed allylic alcohol rearrangement, indicating that the compound bears C(15)-α-OH. This is in accordance with the assignment of the signal δ5.45 (1H, t, J = 1.5 Hz) appeared in ¹H-NMR spectrum of brunanine diacetate (5). The chemical shift of C(15) in ¹³C-NMR (70.6 ppm, d) was found, however, to be 4.6 ppm upper field than that of an analogous compound atisine azomethine (4) (75.2 ppm) (see Table 1), implying that the second hydroxyl group could be situated at C(7). Thereby a release of the non-bonding interaction between C(7)-α-H and C(15)-α-H was manifested. The presence of C(7)-β-OH, but not C(16)-OH, was justified by ¹H-NMR signal δ4.75 (1H, dd, J = 4.8, 11.0 Hz) of diacetylbrunonine (2). Based upon the above reasoning. structure 1 was assigned to brunonine. Interestingly, the NaBH₄-reduced product (6) (m.p. 401) of 5 showed the elimination of ethoxyl group (the disappearance of δ4.14, 3.67 and 1.27 signals) simultaneously with the saturation of C=N double bond (the absorption at δ8.01 vanished). The ¹³C-NMR of structure was in reasonable agreement with that of the known compounds atisine azomethine (4), 7-β-hydroxylisoatisine (7) and ajaconine hydrochloride (8) (Table 1). Brunonine has the structure 1 was confirmed later by X-ray crystallography