The first naturally-occurring hexahydroimidazo-[1,2-a]-pyrimidines, alkaloids (Ia) and (IIIa), and the guanidines (IVa) and (IVb) have been obtained from Alchornea javanensis (Bl.) Muell.-Arg. Two hexahydroimidazo-pyrimidine, and two guanidine alkaloids have been isolated from Alchornea javanensis (Bl.) Muell.-Arg., a small rain-forest tree of the family Euphorbiaceae. Alchornine, C11H17N3O, m.p. 134-135°C; [α]D +74° (CHCl3); picrate, m.p. 275-278°C, has been shown to be (Ia). Vmax 1662, 1690 cm⁻¹ (CCl4); M+ 207; n.m.r. spectrum shows signals at δ 1.26 (s, 7-Me), 1.28 (s, 7-Me), 1.71 (m, with allylic couplings to the 10-CH2, 9-Me), 2.44 (s, 6-CH3), 4.84 and 4.98 (multiplets with allylic couplings to 9-Me and 2-H, 10-CH2), 4.41 (q, J2,3d 9.0 Hz, 3-H), and 6.90 (bs, exchangeable with D2O, NH). Catalytic reduction over Adams catalyst in ethanol gives dihydroalchornine (Ib), picrate, m.p. 263-267°C; M+ 209; three-proton doublets in n.m.r. spectrum at δ 0.85 and 0.92 (J 7.0 Hz, 9-methyls) and a one proton multiplet at 1.65 (9-H). Alchornine is unaffected by NaBH4 in ethanol at room temp., but with LiAlH4 in tetrahydrofuran under reflux it is converted into (II), picrate m.p. 163-165°C; M+ 193; triplet in n.m.r. spectrum at δ 1.87 (J 6.0 Hz, 6-CH2), and a triplet at 3.33 (J 6.0 Hz, 5-CH2). Alchornine on being heated with concentrated hydrochloric acid gives a two component mixture from which has been isolated the major component (Ic), m.p. 79-80°C; [α]D +67°; M+ 225; singlets at δ 1.13 and 1.25 (9-methyls), and a broadened singlet at 5.26, exchangeable with D2O (OH and NH). Alchornidine, C17H25N3O2, m.p. 96-97°C; [α]D -18° (CHCl3); M+ 289, has been shown to be (IIIa). The n.m.r. spectrum has signals at δ 1.19 (s, 7-Me), 1.29 (s, 7-Me), 1.64 (m, with allylic couplings to 10-CH2, 9-Me), 1.89 (d, Jallyl 2.0 Hz, 3'-Me), 2.12 (d, Jallyl 2.0 Hz, 3'-Me), 2.25 and 2.43 (AB doublets, JAB 16.0 Hz, 6-CH2), 4.67 (q, J2u,3 4.2 Hz, JZd,3 7.8 Hz, 3-H), 3.88 and 3.82 (multiplets, J2u,zd 11.8 Hz, 2-CH2), 7.33 (m, 2'-H), and 4.90 (m, 10-CH2). Hydrolysis of alchornidine in aqueous ethanolic KOH affords 2,2-dimethylacrylic acid and alchornine (Ia), while hydrolysis by heating in aqueous acetic acid affords an isomer, isoalchornine (IIIb), which is converted into alchornine on being warmed with dilute ethanolic NaOH. Isoalchornine (IIIb), m.p. 137-138°C; [α]D -84° (CHCl3); M+ 207; has n.m.r. signals at δ 1.30 (s, 7-methyls), 1.71 (m, with allylic couplings to 10-CH2, 9-Me), 2.46 (s, 6-CH2), 3.36 (q, J2u,2d 13.0 Hz, J2u,3 5.0 Hz, 2-H), 3.87 (q, J2d,3 10.0 Hz, 2-H), 4.72 (q, 3-H), 4.88 (m, 10-H), 4.93 (m, 10-H), and 4.37 (bs, exchangeable with D2O, NH). Conversion of (IIIb) into (Ia) occurs by opening the lactam ring at N-4 and recyclization to N-1. The respective structures (Ia) and (IIIb) can be assigned on the basis of the deshielding influence of the 5-oxo-group which leads to a relative downfield shift of the 3-H signal in (IIIb) compared with the 2-H signal in (Ia), and also of the 3-methylene signals in (Ia) compared with the 2-methylene signals in (IIIb). All n.m.r. spectra were measured at 100 MHz in CDCl3 solution, and chemical shifts are relative to SiMe4. In subscripts, "u" and "d" refer to the upfield and downfield multiplets of non-equivalent methylene protons. The other constituents isolated from A. javanensis (1H. Ulrich, J. N. Tilley, and A. A. R. Sayigh, J. Org. Chem., 1964, 29, 2401) include 2,2-dimethylacrylamide and two new guanidine alkaloids, N-1, N-2-di-isopentenylguanidine (IVa), M+ 195; hydrochloride, m.p. 139-141°C; acetate, m.p. 218-220°C; picrate, m.p. 156-157°C; and N-1, N-2, N-3-tri-isopentenylguanidine (IVb), M+ 263, m.p. (hydrate) 44-46°C; hexahydro-derivative, M+ 269, m.p. (hydrate) 180-181°C. The structure of (IVb) has been confirmed by synthesis of the hexahydro-derivative by a known method for the synthesis of N-1, N-2, N-3-trisubstituted guanidines.