Fusiformines A and B: new indole alkaloids from Melodinus fusiformis

1 Introduction

Monoterpenoid indole alkaloids elaborated mainly by plants of Apocynaceae family have been attractive targets for chemists for several decades [1–3]. The genus Melodinus (Apocynaceae family) comprises about 50 species and were distributed mainly in tropical or subtropical Asia and Australia [4]. In recent years, a series of novel alkaloids with highly complex polycyclic skeletons and interesting biological activities have been isolated and identified from the genus Melodinus, such as melohenines A and B [5], melotenine A [6], and melodinines H–K [7]. In the current study, two new monoterpenoid indole alkaloids, fusiformines A and B (1 and 2), were isolated from the twigs and leaves of Melodinus fusiformis. Herein, we describe the isolation and structure elucidation of the new alkaloids 1 and 2.

2 Results and discussion

Fusiformine A (1) had the molecular formula of C₂₂H₂₆N₂O₄, as established by HR-ESI-MS (m/z = 383.1893, [M+H]⁺; calcd. 383.1893), with 11 degrees of unsaturation. IR absorptions at 3384, 1675, and 1615 cm⁻¹ as well as UV absorptions at 327 and 246 nm indicated the presence of hydroxyl and β-anilinoacrylate functionalities, respectively [8]. The distortionless enhancement by polarization transfer (DEPT) and ¹³C NMR spectra (Table 1) revealed 22 carbon signals, including 1 methyl and 1 methoxy groups, 6 methylenes (one oxygenated carbon atom), 6 methines (five olefinic carbon atoms), and 8 quaternary carbon atoms (five olefinic carbon atoms and one ester carbonyl group). Detailed analysis of the NMR data of 1 (Table 1) indicated that 1 was an aspidosperma-type monoterpenoid indole alkaloid and has a great similarity with the known compound 11-methoxytabersonine [9]. The striking difference was the presence of an oxygenated methylene group (δ_H = 3.51, m, 2H; δ_C = 58.8 ppm) in 1 instead of a methyl group in 11-methoxytabersonine. ¹H–¹H correlation spectroscopy (COSY) correlations of H₂-18 (δ_H = 3.51 ppm, m, 2H) and H₂-19 (δ_H = 1.30 ppm, ddd, J = 14.0, 8.0, 4.0 Hz; 1.21, ddd, J = 14.0, 8.0, 6.8 Hz), and the key heteronuclear multiple-bond correlations (HMBC) of H₂-18 with C-20 (δ_C = 40.4 ppm) and H-19 with C-15 (δ_C = 133.3 ppm) and C-17 (δ_C = 29.5 ppm) confirmed the oxygenated methylene group (δ_C = 58.8 ppm), which was assigned as C-18. Thus, the gross structure of fusiformine A was established as 1, as shown in Fig. 1a. The relative stereochemistry of 1 elucidated by a rotating frame Overhauser effect spectroscopy (ROESY) spectrum was identical with that of 11-methoxytabersonine (Fig. 1b).

Fig. 1:

(a) ¹H–¹H COSY (bold) and HMBC (arrow, H→C) correlations of 1. (b) ROESY correlations of 1.

Fusiformine B (2) was obtained as a light yellow amorphous solid; its molecular formula was assigned as C₂₁H₂₄N₂O₄ from HR-ESI-MS (m/z = 369.1815, [M+H]⁺; calcd. 369.1813), with 11 degrees of unsaturation. The DEPT and ¹³C NMR data (Table 1) of 2 revealed 21 carbon signals, which were classified as 1 methoxy group, 5 methylenes, 9 methines, and 6 quaternary carbon atoms. Detailed analysis of the NMR data of 2 and comparison with those of melodinine L [7] indicated that 2 was 19-hydroxy derivative of melodinine L. The oxygenated methine group (δ_H = 3.69 ppm, d, J = 8.4 Hz; δ_C = 71.9 ppm) was assigned as C-19 and was verified by HMBC of H-19 (δ_H = 3.69 ppm, d, J = 8.4 Hz) with C-2 (δ_C = 67.3 ppm), C-20 (δ_C = 41.5 ppm), and C-21 (δ_C = 83.0 ppm) (Fig. 2a). The coupling constant of H-19 (d, J = 8.4 Hz) and ROESY correlations of H-19 and H-21 indicated a β-OH group at C-19 (Fig. 2b). Detailed analysis of 2D NMR (heteronuclear single-quantum coherence, ¹H–¹H COSY, HMBC, and ROESY) data confirmed that the other parts were the same as those of melodinine L.

Fig. 2:

(a) ¹H–¹H COSY (bold) and HMBC (arrow, H→C) correlations of 2. (b) ROESY correlations of 2.

The absolute configurations of 1 and 2 could not be established by comparison of the calculated and experimental electronic circular dichroism (ECD) spectra due to the lack of sufficient amounts of substances. However, the absolute configurations might be identical with those of known compounds from a biogenetic point of view [10, 11].

The cytotoxic activities of 1 and 2 against the growth of human tumor cell lines (HL-60 and A-549) by using the 3-(4,5)-dimethylthiahiazo (-z-y1)-3,5-di-phenytetrazoliumromide (MTT) method were evaluated [12]. The results indicated that 1 showed a moderate cell growth inhibitory activity against all cancer cell lines with IC₅₀ values of 9.80 and 12.38 μm, respectively, while 2 were inactive against the above cancer cells (IC₅₀ > 40 μm).

3 Experimental section