As a part of our studies on Vietnamese plants of biological and medical interest, we have investigated Solanum hainanense Hance, used in Vietnamese folk medicine as an antiphlogistic. We now wish to report the isolation and structure of the new steroidal alkaloid solasodenone (1) from this plant. Al₂O₃ chromatography of the chloroform extracts of dried roots or leaves yielded 0.03% of the new alkaloid C₂₇H₄₁NO₂ (M+ 411.3131), mp 178°, showing the presence of a spiroaminoketal system [1] (883, 913, 965, 978 cm⁻¹) as well as an α, β-unsaturated ketone (1621, 1680 cm⁻¹) in its IR spectrum. The UV data with λₘₐₓ (ε) at 242 (15000) and 310 (138) indicated also an enone chromophore. The measured ORD curve was in agreement with the Cotton effect, well known for Δ⁴-3-keto steroids (Δε₃₁₅ - 1.1) [2]. The 100 MHz ¹H NMR spectrum gave signals at δᵀᴹˢ^CDCl₃ 0.83 (d, J = 6 Hz, 27-H₃)[3], 0.86 (s, 18-H₃), 0.94 (d, J = 7 Hz, 21-H₃) [3], 1.20 (s, 19-H₃), 2.63 (m, 26-H₂), 4.30 (dd, J = 14 Hz, J' = 7 Hz, 16α-H), 5.74 (s, 4-H). Mass fragmentation pattern indicates that 1 is a steroidal alkaloid having spirosolane skeleton [4,5], high resolution diagnostic ion fragments being discernible at m/e 396 (C₂₆H₃₉NO₂, 6%), 383 (C₂₅H₃₆NO₂, 9%), 298 (C₁₉H₂₈NO, 11%), 269 (C₁₈H₂₃NO, 10%), 138 (C₈H₁₂N, 100%), 125 (C₇H₁₁N, 10%), 114 (C₇H₉NO, 78%) and 98 (C₆H₁₀N, 5 %). The alkaloid was characterized by the N-acetyl derivative 2, M+ 453, mp 124°, N-chloroamine 3, mp 168-170° (dec.), and N-nitrosamino compound 4, mp 159°, whose molecular rotation differences (Δ[M], 1 → 2 = +59°, 1 → 3 = - 186°, 1 → 4 = +13°) are in good agreement with a (22R:25R) spirominoketal side chain stereochemistry [6]. All these data suggested solasodenone as (22R:25R)-spirosol-4-en-3-one (1); this structure was finally confirmed by a partial synthesis of 1 via Oppenauer oxidation of solasodine [6] leading to a product identical in all respects with solasodenone (1) isolated from S. hainanense. Solasodenone (1) may be a suitable starting material for pharmaceutically interesting pregnane derivatives of the progesterone type.