Curviflorside and curviflorin, new naphthalene glycoside and flavanol from Plicosepalus curviflorus

1 Introduction

Naphthalenes are reported from fungi, plants, insects, and liverworts. Naphthalenes possess a wide range of bioactivities: antimicrobial, antioxidant, cytotoxic, anti-inflammatory, anti-platelet aggregation, and antiprotozoal [1]. Several naphthalene-containing drugs are available, such as nafacillin, naftifine, tolnaftate, and terbinafine, which play a vital role in controlling microbial infection [2]. Flavan-3-ols are the most common group of flavonoids in dietare 3-ring phenolic compounds having multiple hydroxyl groups on the A, B, and C rings. They are considered functional ingredients of fruits, beverages, vegetables, food grains, dietary supplements, herbal remedies, and dairy products [3]. Many studies indicated that they exhibited several health beneficial effects by acting as antioxidant, anti-diabetic, anti-proliferative, neuro- and cardio-protective, antimicrobial, and antiviral agents [3], [4], [5]. Family Loranthaceae plays an important and complex role in the biological system in which species of this family live by interacting with insects, birds, and mammals [6]. This family comprises four genera: Phragmanthera, Oncocalyx, Tapinanthus, and Plicosepalus, which grow naturally in Saudi Arabia. These genera include six species, Plicosepalus acacia, Plicosepalus curviflorus, Phragmanthera austroarabica, Oncocalyx schimperi, Oncocalyx glabratus, and Tapinanthus globiferus spread in the north, west, and south of the Saudi Arabia [7]. Earlier investigations on genus Plicosepalus reported various biological activities such as antioxidant [8], [9], [10], antihepatotoxic [11], anti-diabetic [12], [13], [14], antiviral [10], [15], antimicrobial [8], [10], [16], [17], and cytotoxic [10], [18], [19] activities. A literature survey revealed that very little phytochemical work has been carried out on the genus Plicosepalus. Flavonoids [8], [14], [19], phenolic acids [5], triterpenes, sterols [14], and sesquiterpene lactones [20] have been isolated from different Plicosepalus species. Flavane gallates, triterpenes, and sterols have been isolated from P. curviflorus Benth. [8], [15]. P. curviflorus is a parasitic plant which is generally known as “Enam ElTalh” (in Arabic). It is found in north East Africa, East Africa, Yemen, and Saudi Arabia [21]. The stems of P. curviflorus are used for the treatment of cancer in Yemen [10] and traditionally used in Saudi Arabia for increasing lactation in cattle [21]. Moreover, P. curviflorus is used for the treatment of diabetes in Saudi Arabia folk medicine [22], [23]. The present work reports the isolation and structural elucidation of two new compounds (3 and 4), along with two known ones (Figure 1) from the shoots of P. curviflorus. Their structures were assigned by extensive spectroscopic methods as well as comparison with the literature.

Figure 1:

Structure of isolated compounds 1–4.

2 Materials and methods

3 Results and discussion

Compound 3 was obtained as white amorphous powder, and its molecular formula was defined as C₁₆H₁₈O₈ by HRESIMS pseudo-molecular ion peak at m/z 339.1075 [M+H]⁺ (calcd for C₁₆H₁₉O₈, 339.1080), requiring eight degrees of unsaturation (see Supplementary Material, Figure S17). Seven of degrees of unsaturation were attributed to the naphthalene moiety and one for sugar moiety. The ESIMS revealed a fragment ion peak at m/z 190 [(M+H)-149]⁺, indicating the loss of a pentose moiety. Its IR spectrum exhibited absorption bands for hydroxyl (3425 cm⁻¹) and aromatic (1625 and 1518 cm⁻¹) functionalities. The ¹³C NMR and heteronuclear single quantum coherence (HSQC) data showed resonances for 16 carbon signals: 10 carbon signals in the range δ_C 108.1–152.6 for a naphthalene ring, five oxygen-bonded aliphatic carbons in the range δ_C 66.1–103.3 for pentose moiety, and methoxy group (δ_C 61.4) (see Supplementary Material, Figures S12 and S15). The ¹H NMR spectrum of 3 exhibited two pairs of ortho-coupled aromatic protons at δ_H 7.55 (d, J=8.2 Hz, H-3), 7.70 (d, J=8.2 Hz, H-4), and 7.47 (2H, d, J=8.5 Hz, H-6, 7), correlating to the carbons at δ_C 112.0, 112.8, 110.7, and 109.1, respectively, in the HSQC spectrum characteristic for a tetra-substituted naphthalene moiety (Table 1). The heteronuclear multiple bond correlations (HMBC) of H-3 to C-1 and C-10; H-4 to C-1, C-2, C-3, and C-5; H-6 to C-8 and C-10; and H-7 to C-5 and C-9 confirmed the presence of such moiety (Figures 2 and S16). Moreover, the ¹H NMR spectrum showed characteristic signals of a sugar moiety between δ_H 3.25 and 5.00, including an anomeric proton signal at δ_H 5.00 (d, J=7.5 Hz, H-1′). The sugar was assigned as xylopyranose according to ¹³C NMR data (Table 1), where signals belonging to the sugar moiety were three oxymethines at δ_C 69.8 (C-4′), 73.5 (C-2′), and 76.0 (C-3′), one oxymethylene at δ_C 66.1 (C-5′), and one anomeric oxymethine at δ_C 103.3 (C-1′) [1], [24]. The configuration at the anomeric center of the xylopyranosyl moiety (C-1′) was determined to be β based on the coupling constant value (J_1′,2′=7.5 Hz). The sugar moiety was placed at C-2 on the basis of the ³J HMBC correlation of the anomeric proton to C-2 (δ_C 140.1). Moreover, signals for methoxy group at δ_H 4.05/δ_C 61.4 were observed. Its attachment at C-8 was established by the HMBC cross peak of the methoxy group to C-8 (δ_C 148.9). On the basis of these data, compound 3 was identified as 1,5-dihydroxy-8-methoxynaphthalene-2-O-β-D-xylopyranoside. The trivial name curviflorside was given to it.

Figure 2:

Some key HMBC correlations of 3 and 4.

Compound 4 was obtained as yellow amorphous powder and gave positive tests for flavonoids [25], [26]. The HRESIMS spectrum of 4 gave a pseudo-molecular ion peak at m/z 427.1033 [M+H]⁺, corresponding to the molecular formula C₂₂H₁₈O₉. The ESIMS spectrum showed a significant fragment ion peak at m/z 290 [M + H-(3,4-dihydroxybenzoyl)]⁺. Its UV spectrum displayed absorption bands at 218 and 277 nm, which are characteristic for the presence of a flavan-3-ol moiety in 4 [27]. Its IR spectrum exhibited absorption bands for hydroxyl (3356 cm⁻¹), ester carbonyl (1732 cm⁻¹), and phenyl (1614, 1519, and 1463 cm⁻¹) moieties. The ¹³C, distortionless enhancement by polarization transfer, and HSQC spectra of 4 showed the presence of 22 carbons, consisting of methylene, 8 aromatic methines, 2 oxymethines at δ_C 81.6 (C-2) and 66.4 (C-3), and 11 quaternary carbons, including one carbonyl at δ_C 164.9 (C-7″) (see Supplementary Material, Figures S19 and S20). The ¹H NMR spectrum showed resonances for two meta-coupled protons at δ_H 6.13 (d, J=2.0 Hz, H-6) and 6.21 (d, J=2.0 Hz, H-8) (Table 2). These signals showed HSQC cross peaks to the carbons, resonating at δ_C 100.6 and 101.2, respectively, consistent with a 5,7-dioxygenated A ring of flavane [28]. The ¹H NMR and ¹H-¹H correlation spectroscopy (COSY) also displayed an ABX system for 1,3,4-tri-substituted ring B at δ_H 6.75 (brs, H-2′), 6.68 (d, J=7.8 Hz, H-5′), and 6.58 (brd, J=7.8 Hz, H-6′) [29]. They showed HSQC cross peaks to carbon signals at δ_C 114.9, 115.6, and 118.9, respectively (Table 2). Moreover, two oxymethine protons at δ_H 4.64 (d, J=7.2 Hz, H-2) and 3.94 (m, H-3) and methylene group at δ_H2.67 (dd, J=16.1, 5.2 Hz, H-4A) and 2.45 (dd, J=16.1, 8.2 Hz, H-4B) were observed, suggesting that 4 contained a catechin moiety. This moiety was confirmed by the observed ¹H-¹H COSY cross peaks and HMBC correlations of H-2 to C-4, C-9, C-1′, C-2′, and C-6′; H-3 to C-5, C-10, and C-1′; H-4 to C-2, C-3, C-5, and C-10; H-6 and H-8 to C-7 and C-10; H-2′ and H-6′ to C-2 and C-4′; and H-5′ to C-1′ and C-3′ (Figure 2). This was further secured by the ESIMS fragment ion peak at m/z 290 [M + H-(3,4-dihydroxybenzoyl)]⁺. The ¹HNMR spectrum of 4 also displayed three coupled proton signals for a tri-substituted phenyl moiety at δ_H 7.09 (d, J=2.2 Hz, H-2″), 6.95 (d, J=8.2 Hz, H-5″), and 7.06 (dd, J=8.2, 2.2 Hz, H-6″), indicating the presence of a 3,4-dihydroxybenzoyl moiety [30]. This was confirmed by ¹³C NMR signals at δ_C 139.7 (C-1″), 109.5 (C-2″), 145.9 (C-3″), 146.1 (C-4″), 108.0 (C-5″), 109.5 (C-6″), and 164.9 (C-7″) and further secured by the observed HMBC correlations (Figure 2). Moreover, the four singlet signals at δ_H 5.75, 9.75, 8.87, and 9.38 were assigned to 3, 5, 3′, and 4′-OH groups, respectively. The connectivity of 3,4-dihydroxybenzoyl moiety at C-7 was established by the HMBC correlations of H-6 and H-8 to C-7″ at δ_C 164.9. Upon the hydrolysis of 4, catechin and 3,4-dihydroxybenzoic acid were identified by co-TLC alongside authentic samples. On the basis of the above evidences, the structure of 4 was assigned as (+)-catechin-7-O-3″,4″-dihydroxybenzoate and named curviflorin.

The known compounds were identified as (+)-catechin (1) [31] and quercetin (2) [32], [33] by comparing their NMR spectral and physical data with the literature.

4 Conclusions

Four compounds (1–4) were isolated and characterized from the shoots of P. curviflorus; two of them are new natural products (3 and 4). Their structures were determined on the basis of extensive spectroscopic data analysis.