The structures and relative stereochemistries of rocaglamide, a novel antileukemic 1H-cyclopenta[b]benzofuran isolated from Aglaia elliptifolia, and dehydrorocaglamide, derived from rocaglamide, have been established from spectral and single-crystal X-ray analysis. The alcoholic extract of dried roots and stems of Aglaia elliptifolia Merr. (Meliaceae) has been reported to exhibit significant antileukemic activity against P388 lymphocytic leukemia in CDF1 mice and inhibitory activity in vitro against cells derived from human epidermoid carcinoma of the nasopharynx (KB cell). Examination of the chloroform-soluble fraction of this extract has now led to the isolation of a novel 1H-2,3,3a,5b-tetrahydrocyclopenta[h]benzofuran, rocaglamide (1), with significant antileukemic activity against P388 leukemia in CDF1 mice. Rocaglamide (1), C26H27NO7, m/e 505.2087 (M+); m.p. 118-119 °C (MeOH); [α]D25 -96° (c. 1.00, CHCl3), λmax (EtOH) 214 (log ε 4.38), 232 (sh, log ε 3.96), 273 nm (log ε 2.91). When (1) was treated with conc. HCl in diethyl ether solution it yielded dehydrorocaglamide (2), C26H25NO6, m/e 487.1980 (M+, found; 487.1972, calc.); m.p. 233-234 °C (MeOH); [α]D25 +435° (c 0.2, CHCl3). The similarity of the 1H n.m.r. spectra of (2) and (1) attested to the fact that the former had retained several of the functional groups originally present in the latter. The appearance of new absorptions at 1700 cm-1 in its i.r. spectrum and λmax (EtOH) 312 nm (log ε 3.71) in its U.V. spectrum confirmed the existence of an α,β-unsaturated cyclopentenone system and a stilbene chromophore, respectively, in (2). A single-crystal X-ray analysis of (1), as the methanol solvate, unequivocally defined its complete structure and stereochemistry. Crystal data: C26H27NO7·CH3OH, M= 537.62, monoclinic, space group P21, a = 14.260(6), b = 7.822(3), c = 12.323(5) Å, β = 98.01(2)°, U = 1361.1 Å3, Z= 2, Dc = 1.312 g cm-3. The structure was solved by direct methods. Least-squares refinement of atomic positional and thermal parameters (anisotropic C, N, O; fixed H contributions) converged at R 0.061 using 1498 reflections measured on an Enraf-Nonius CAD-3 automated diffractometer (Ni-filtered Cu-Kα radiation, λ = 1.5418 Å; θ-2θ scans, θmax 67°). KB-cell culture assay and P388 in vivo activity were conducted under the auspices of the National Cancer Institute by literature procedures. The extract showed ED50 = 4.4 × 10-3 mcg/ml in the KB assay. In the P388 system, rocaglamide (1) exhibited optimal values of T/C of ca. 156% at a non-toxic dose of 1.0 mg/kg. Satisfactory elemental analyses were obtained; i.r., 1H and 13C n.m.r. spectra were consistent with the structures shown.