The chemical investigation of the trunk wood of a specimen of Iryanthera juruensis Warb. (Myristicaceae) revealed the presence of sitosterol, (±)-2'-hydroxy-7-methoxy-4',5'-methylenedioxyflavan [1] and a lactone, C₂₇H₄₂O₅, designated juruenolide. The PMR spectra of the compound and of its acetate allowed expansion of the formula to 1a. This contains a piperonylethyl unit (τ 3.3–3.5, 3ArH; 4.16, O₂CH₂; 7.53, t, J 7.5 Hz, ArCH₂), a methylene chain (τ 8.2–8.8, 15 CH₂), whose signal covers an additional one H signal, and a hydroxy-γ-lactone unit (1a vₘₐₓ 1745 cm⁻¹; 1b vₘₐₓ 1770, 1730 cm⁻¹). The allocation of the hydroxyl to C-2 and of the methylene chain to C-4 of this lactone was based on the analysis of the signals associated with its 2 oxymethine and 2 methylene protons. The latter can only be situated on C-3. Double irradiation experiments revealed one of them (τ 8.7) to be coupled to H-4 (τ 5.52) and the other one (τ 7.44) to H-2 (τ 5.88). Further correlation of the τ 5.88 doublet with the carbinolic proton was based on the paramagnetic shift (Δ – 0.86 ppm) of this signal upon acetylation. The relatively high τ value (8.7) of one of the methylene proton signals indicated that the corresponding H-3 is situated well over the plane of the carbonyl. Coupling and absence of coupling was seen as evidence, respectively, for its cis-relation to H-4 and trans-relation to H-2. An identical conclusion about the relative stereochemistry of juruenolide was reached upon observing the coupling of the relatively deprotected (τ 7.44), and hence quasiequatorial, H-3 to H-2 and the absence of coupling to H-4. According to the modified Hudson rule [2], formula 1a represents also the absolute configuration of the lactone, since this is more dextrorotatory, [α]ᴰ²⁰ + 12.5 (MeOH), than its potassium salt [α]ᴰ²⁰ + 100 (MeOH).