In previous papers (1-4) we have reported ten new structures of the spirasine series, all C19-diterpene alkaloids, isolated from Spiraea japonica L. f. var. fortunei (Pl.) Rehd. (Rosaceae). We now report the structural elucidation of another new C19-diterpene alkaloid designated as spirasine X [1]. Spirasine X [1] has the molecular formula C19H29NO3 (molecular ion peak at m/z 311.1853, calculated 311.1885), melting point 224-227℃, and specific rotation [α]D+51.0° (c 1.5, CHCl3). It shows an exocyclic methylene group (δC 140.5 s, 112.4 t) and a methyl group (δH 1.01, 3H, s), suggesting a C19-diterpene alkaloid. The IR spectrum exhibits ketone absorption at 1714 cm-1, assigned to the C-15 position based on the UV absorption of a β,γ-unsaturated ketone (ca. 300 nm) and positive Cotton effect at 304 nm (Δε+2.95) in the CD spectrum. The use of CD data assumes a "normal" absolute configuration for the alkaloid, whereas a C-13 keto group would imply an unprecedented antipodal skeleton. However, the absolute configuration is justified by unequivocal 13C chemical shifts of C-9 and C-14 from 2D-NMR (see Table 1), which would require unreasonable reversal for an antipodal skeleton. The signals δC 65.1 (d) and δH 3.30 (1H, br. s) exclude a hydroxyl group at C-6 (compared to spiradine A). An α-hydroxy group at C-13 is evidenced by δC 67.7 (d) and δH 4.24 (1H, q). The 1H-NMR spectrum shows well-resolved peaks downfield of 1.50 ppm, attributable to olefinic, allylic, and α/β protons of oxygen and nitrogen functions. COSY spectrum (Figure 1) allows assignment of C-12, C-14, and C-20 protons via coupling with C-13 H, characterization of C-15 protons by allylic coupling, and discernment of C-6 H and C-7 H relationship. 2D-heteronuclear proton-carbon correlation spectrum (Figure 2) shows C-3 (25.7 ppm) and C-15 (33.7 ppm) chemical shifts distinct from those of known C19-diterpene alkaloids (e.g., nominine, C-3 at 34.2 ppm). The upfield shift of C-20 (ca. 5 ppm vs. spirasine IX) is explained by the γ-effect of C-13 α-OH. The C-13 OH is designated as α, with the molecular model held with the N-atom away from the viewer to avoid confusion.