Croton tonkinensis GAGNEP belonging to Euphobiaceae grows wildly in Vietnam, and the leaves of this plant are prescribed for stomachache. 1) The crude drug from the leaves was previously shown to have an inhibitory effect on malaria parasites. 2) Later, Phan et al. 3) reported the isolation and structural elucidation of a new ent-kaurane diterpenoid, ent7b-hydroxy-15-oxokaur-16-en-18-yl acetate from the leaves. In the course of our investigation of the biologically active compounds from Vietnamese medicinal plants, we reinvestigated the chemical constituents of the leaves of C. tonkinensis and isolated six alkaloids. 4) Recently, we collected a large amount of the leaves and isolated a novel ent-kaurane-type diterpenoid (1). This paper describes the isolation and structural characterization of compound 1.Compound 1 was obtained as white needles, [a] D 20 Ϫ96.0°( cϭ0.80, CH 3 OH). The molecular formula was determined to be C 22 H 32 O 5 by high resolution electron impact (HR-EI)-MS. Its IR and UV spectra indicated the presence of a hydroxyl group (3272 cm Ϫ1 ), a double bond (1651 cm Ϫ1 ), an acetyl group (1727, 1243 cm Ϫ1 ), 3) and an absorption maximum at 230 nm, respectively. The 1 H-NMR spectrum of 1 (Table 1) showed the presence of three methyl groups, two exo-methylene protons and two hydroxyl groups. The 13 C-NMR spectrum of 1 (Table 1) showed resonances for 22 carbons including one conjugated ketone (d 207.1), an acetyl group (d 21.0, 169.7), a methine (d 72.3) bearing an acetoxyl group, an exo-methylene (d 116.8, 148.3), five methylenes and three quaternary carbons. The NMR spectral data of compound 1 were similar to those of ent-1a,11a-diacetoxy7b,14a-dihydroxy-kaur-16-en-15-on (rastronol A), 5) suggesting that compound 1 was a kaurane-type diterpenoid. The position of an acetyl group was determined at C-1 due to a long-range correlation between this group and H-1 in the heteronuclear multiple bond connectivity (HMBC) spectrum. 6) Further, two hydroxyl groups were determined at C-7 and C-14, respectively, by HMBC correlation between 7-OH and C-7 as well as the 14-OH and C-13, C-14 and C-15 in the HMBC spectrum. The stereochemistry of 1 was determined by nuclear Overhauser effect spectroscopy (NOESY) spectrum (Table 1), in which the NOEs were observed between (1) H-5 and H-9, H-18; (2) H-7 and H-5, H-9; (3) H-1 and H-19, H-20; (4) H-14 and H-13, H-20 indicating that H-5, H-7, H-9, H-18, 1-OAc and 14-OH were axial, and H-1, H-13, H-19, H-20 and 7-OH were equatorial.Acetylation of 1 with acetic anhydride in pyridine yielded a triacetate (2) with the molecular formula of C 26 H 36 O 7 by HR-EI-MS. The IR spectrum of 2 was similar to that of 1 except for the absence of absorption of the hydroxyl group. The A novel ent-kaurane diterpenoid, ent-1a a-acetoxy-7b b,14a a-dihydroxy-kaur-16-en-15-on has been isolated from leaves of Croton tonkinensis GAGNEP. Its structure was determined by a combination of spectroscopy, X-ray crystallographic analysis and the chemical reaction acetylation.