Prenylated 9,10-dihydrophenanthrenes from Macaranga javanica

1 Introduction

The plants belonging to the genus Macaranga have been shown to produce a variety of terpenylated phenolic compounds [1]. They include flavonoids and stilbenes bearing prenyl, geranyl, or farnesyl groups, or a combination of these groups. Some of these compounds have also been tested for a number of biological properties, including anticancer, antioxidant, and anti-inflammatory activities [1]. Recently, our laboratory has succeeded to isolate a number of these phenolic compounds, including monoprenylated dihydrochalcones and flavanones from M. trichocarpa [2, 3]; geranylated and farnesylated dihydroflavonols from M. gigantea, M. pruinosa, and M. rhizinoides [4–6]; prenylated dihydroflavonols from M. lowii and M. recurvata [7, 8]; and phenolic derivatives with an irregular sesquiterpenyl side chain from M. pruinosa [9]. As further work on the phytochemistry of this genus, in this paper we report the presence of prenylated dihydrophenanthrenes, trivially named macajavanicins A–C (1–3) (Fig. 1), along with two known prenylated dihydrostilbenes 4, 5 [10] from the leaves of M. javanica (Blume) Mull. Arg. Structure elucidation of these compounds and their chemotaxonomic significances will be discussed. The antibacterial properties of compounds 1–5 will be briefly described. This is the first report on the occurrence of prenylated dihydrophenanthrenes in Macaranga.

Fig. 1:

Structures of compounds isolated from M. javanica.

2 Results and discussion

Macajavanicin A (1), isolated as yellowish solid, has the molecular formula C₂₄H₂₈O₄ from electrospray ionization-high resolution mass spectrum (ESI-HRMS) ([M–H]^–: found m/z = 379.1917, calcd. 379.1909). Its UV absorption (λ_max = 211, 271, and 305 nm) indicated a typical 9,10-dihydrophenanthrene structure [11], which is confirmed by the presence of a pair of multiplets of two methylene protons at δ_H = 2.59 and 2.55 ppm (Table 1). These data suggested that compound 1 is either a geranylated or a diprenylated 9,10-dihydrophenanthrene. In the ¹H NMR spectrum (Table 1), the presence of four methyl singlets (δ_H = 1.77, 1.75, 1.65, and 1.63 ppm), two methylene doublets (δ_H = 3.44 and 3.34 ppm), and two vinylic methine multiplets (δ_H = 5.13 and 5.09 ppm) indicated that 1 is a diprenylated derivative. The four oxygen atoms in 1 were deduced to be in the form of four phenolic –OH groups, as is shown by the presence of four –OH signals (δ_H = 8.03, 8.01, 7.75, and 6.39 ppm) and four oxyaryl carbon signals (δ_C = 154.3, 153.4, 142.2, and 141.8 ppm) in the NMR spectra. Two phenolic –OH groups are ortho-oriented (δ_C = 142.2 and 141.8 ppm) and the other two (δ_C = 154.3 and 153.4 ppm) are in a meta-position to each other. Two singlet signals of aromatic protons were observed at δ_H = 7.75 and 6.39 ppm, indicating that rings A and C of 9,10-dihydrophenanthrene are both trisubstituted. Structure 1, therefore, can be assigned to macajavanicin A. Heteronuclear single-quantum coherence and heteronuclear multiple-bond correlation (HMBC) correlations confirmed the structure of 1 as is shown in Table 1 and Fig. 2. Thus, important HMBC correlations were shown by each methylene signal (δ_H = 2.59 and 2.55 ppm) of the 9,10-dihydrophenanthrene to the quaternary aromatic carbon signals attached to the prenyl groups (δ_C = 117.5 and 125.0 ppm, respectively), confirming the position of the prenyl groups at C-2 and C-8. The HMBC spectrum also displayed ¹H–¹³C long-range correlations between the methylene signals at δ_H = 3.45/3.34 ppm of the prenyl groups and oxyaryl carbon signals at δ_C = 141.8/154.3 ppm, indicating that the ortho-hydroxy groups are at C-3 and C-4, while the meta-hydroxy groups are located at C-5 and C-7.

Fig. 2:

HMBC correlations in compounds 1–3.

Macajavanicin B (2) was isolated as a yellowish gum and showed similar UV absorptions to those by macajavanicin A (1). Its molecular formula was deduced to be C₂₉H₃₆O₄ from ESI-HRMS ([M–H]^–: found m/z = 447.2526, calcd. 447.2535), suggesting that 2 is a prenyl derivative of compound 1. The NMR data of 2 (Table 1) were very similar to those of 1, except that 2 showed only one singlet signal of an aromatic proton at low field (δ_H = 7.46 ppm). The disappearance of another aromatic proton singlet was replaced by the NMR signals of a third prenyl group (¹H NMR: δ_H = 3.48 [2H], 5.24 [1H], 1.67 [3H], and 1.79 [3H] ppm; ¹³C NMR: δ_C = 23.7 [t], 124.3 [d], 132.0 [s], 18.0 [q], and 25.9 [q] ppm), suggesting that the third prenyl group is at C-6. This was confirmed by the HMBC correlations as shown in Fig. 2. Therefore, structure 2 is assigned to macajavanicin B.

Macajavanicin C (3), isolated as yellowish solid, has the molecular formula C₂₉H₃₄O₄ from its ESI-HRMS ([M–H]^–: found m/z = 445.2374, calcd. 445.2379). The ¹H NMR spectrum of 3 was essentially very similar to that of 2. However, it showed signals for only two prenyl groups (Table 1), a pair of doublets (δ_H = 6.78 and 5.63 ppm, J = 9.9 Hz) of a cis-1,2-disubstituted ethene, two methyl signals at the same chemical shift as a singlet (δ_H = 1.39 ppm), and an aromatic proton singlet (δ_H = 7.51 ppm). These MS and ¹H NMR data suggested that 3 is a didehydro derivative of 2, in which one of the prenyl groups is in the form of a dimethylpyran moiety. From its HMBC correlations (Fig. 1), this dimethylpyran moiety is deduced to originate from the prenyl group attached to C-6, which can cyclize either with 5-OH or with 7-OH. However, the appearance of the –OH groups in 3 as broad singlets prevented the observation of their HMBC correlations to directly determine the position of this moiety. Changing a free prenyl group to a dimethylpyran moiety, in a structural system such as in 3, will reduce the chemical shifts of the oxyaryl carbons at C-5 and C-7 differently [12]. The chemical shift of the oxyaryl carbon conjugated with the dimethylpyran’s double bond will be more affected than the oxyaryl carbon that is part of the dimethylpyran. HMBC correlations of 3 (Fig. 2) allowed us to determine the carbon chemical shift of C-7 to be at δ_C = 150.2 ppm (1.9 ppm lower than those in 2) from its correlation with a methylene proton signal (δ_H = 3.32 ppm) of one of the prenyl groups. Consequently, the chemical shift of C-5 is 2.7 ppm lower than those in 2 (δ_C = 147.9 ppm). Therefore, the dimethylpyran moiety in 3 is formed from the prenyl group at C-6 with the –OH group at C-7, and structure 3 was assigned to macajavanicin C.

9,10-Dihydrophenanthrene derivatives are rare natural compounds that are limited in their distribution in organisms [13]. Several groups of plants, such as Combretaceae, Dioscoreaceae, Orchidaceae, and Juncus species of Juncaceae, are prominent in the presence of 9,10-dihydrophenanthrenes. In the first three families, structural variations occur by the number of oxygenated functionalities, O-methylation and/or O-glycosylation. However, in the Juncus species, the structural variation of 9,10-dihydrophenanthrenes is further extended with the attachment of C-methyl and C-vinyl groups, as well as their derivatives. In the family Euphorbiaceae, a number of 9,10-dihydrophenanthrene derivatives have also been reported from a few unrelated species. All of them have one or more C-methyl groups [13]. The only example of a 9,10-dihydrophenanthrene derivative with a prenyl group (in the form of a dimethylpyran ring) was a metabolite of Clusia paralycola (Gutiferae) [14]. The presence of compounds 1–3 in M. javanica was not only unknown previously in the genus Macaranga, but they also represent further examples of natural 9,10-dihydrophenanthrene derivatives containing prenyl groups.

Compounds 1–5 were tested for antibacterial properties against clinical isolates of pathogenic bacteria, including Bacillus subtilis, Enterobacter aerogenes, Escherichia coli, Pseudomonas aeruginosa, Salmonella typhi, Shigella dysenteriae, Staphylococcus aureus, and Vibrio cholerae. The tests were performed using a twofold microdilution method as described previously [3]. All compounds showed weak activities against the tested bacteria (minimum inhibitory concentration [MIC] values were 70–329 μm), except that for compounds 4 and 5 that showed significant activities against S. aureus (both MIC values were 35 μm).

3 Experimental section