Cytotoxic Vobasine, Tacaman, and Corynanthe-Tryptamine Bisindole Alkaloids from Tabernaemontana and Structure Revision of Tronoharine Dawn Su-Yin Sim, † Kam-Weng Chong, † Choy-Eng Nge, † Yun-Yee Low, † Kae-Shin Sim, ‡ and Toh-Seok Kam* ,† † Department of Chemistry and ‡ Institute of Biological Sciences, University of Malaya, 50603 Kuala Lumpur, Malaysia * S Supporting Information ABSTRACT: Seven new indole alkaloids (1-7) comprising four vobasine, two tacaman, and one corynanthe-tryptamine bisindole alkaloid were isolated from the stem-bark extract of a Malayan Tabernaemontana. Two of the new vobasine alkaloids (1, 3), as well as 16-epivobasine (15) and 16-epivobasenal (17), showed appreciable cytotoxicity toward KB cells (IC 50 ca. 5 μg/mL). The structure of the known Tabernaemontana alkaloid tronoharine (8) was revised based on newly acquired NMR data, as well as X-ray diffraction analysis. T he genus Tabernaemontana (Apocynaceae) incorporates a large number of species, which are distributed over the tropical and subtropical regions of the world, including parts of the Americas, Africa including Madagascar, Asia, Oceana, and Australia. 1 The taxonomical aspects associated with this genus have long presented a formidable challenge to botanists, due in part to the large number of species, the wide geographical distribution, and the large number of synonyms. The latest and most comprehensive review of this difficult genus (the Old World species) is that by Leeuwenberg, whose revision has resulted in a significant reduction in the number of the Old World species. 1 Plants of this genus are prolific producers of alkaloids, in particular indole and bisindole alkaloids, and are well known to elaborate alkaloids with structurally novel molecular skeletons and useful biological activities. 2-5 About 13 species are known to occur in Malaysia (Peninsular Malaya and Malaysian Borneo), with T. corymbosa being the most widely distributed, being encountered in a number of different locations. 1 A number of the Malaysian Tabernaemontana have been the subject of thorough chemical studies, with the discovery of many new and biologically active alkaloids. 4-11 We now report the isolation, structure determination, and bio- logical activity of seven new alkaloids, comprising vobasine, tacaman, and a heterodimeric alkaloid, as well as the structure revision of the known hexacyclic alkaloid tronoharine. 12 ■ RESULTS AND DISCUSSION Compound 1 (vobasidine A) was obtained as a light yellowish oil with [α] 25 D -35 (CHCl 3 , c 0.2). The IR spectrum showed bands due to NH (3311 cm -1 ), ester, and conjugated carbonyl functions (1727 and 1647 cm -1 ), while the UV spectrum showed absorption maxima at 207, 226, 238, and 314 nm, indicative of a 2-acylindole chromophore. 13 The ESIMS showed an [M + H] + peak at m/z 369, and 13 C NMR and HRESIMS data established the molecular formula as C 21 H 24 N 2 O 4 . The 1 H and 13 C NMR data (Tables 1 and 2) showed the presence of an unsubstituted indole chromophore from the presence of four aromatic resonances (δ H 7.17-7.73), an indolic NH (δ H 8.94), a methyl ester group (δ H 2.66; δ C 170.9, 50.4), an N4-Me (δ H 2.68; δ C 43.1), and a conjugated carbonyl as part of a 2-acylindole (δ C 188.9) moiety. The 13 C NMR data (Table 2) showed a total of 21 carbon resonances, including three methyl, three methylene, eight methine, and two quaternary carbon atoms. The COSY spectrum showed, in addition to the four aromatic hydrogens and an isolated aminomethylene (C-21, δ C 46.8), a CH 3 CH-O and a CH 2 CHCHCHCH 2 fragment (Figure 1). One of the two methylenes was deduced to be α to the ketocarbonyl function from its 13 C NMR resonance at δ 42.5. The shifts for the two contiguous carbons at δ C 58.7 and 62.8 suggested the presence of an epoxide functionality. On the basis of these observations and analysis of the HMBC data (Figure 1), a vobasine-type alkaloid incorporating an epoxide moiety at C-19, C-20 was indicated, as shown in structure 1. The chemical shift of the ester methyl attached to C-16 was relatively shielded at δ 2.66, which was consistent with a C-16 configuration that places the Received: July 24, 2014 Published: October 21, 2014 Article pubs.acs.org/jnp © 2014 American Chemical Society and American Society of Pharmacognosy 2504 dx.doi.org/10.1021/np500589u | J. Nat. Prod. 2014, 77, 2504-2512