LAUSR

Although Plectranthus species are known to produce varieties of important phytomolecules and considered to be a rich source of essential oils [1, 2], chemical studies of many Plectranthus species have not been performed yet. Hence, as part of our research interest on characterization of chemical compounds from medicinal and aromatic plants [3–5], in this study we report the isolation and identification of 9 secondary metabolites from two unexplored Plectranthus species, Plectranthus arabicus E. A. Bruce and P. asirensis J.R.I. Wood growing in Saudi Arabia. It is important to mention here that these plants have never been studied before for their phytochemical investigation except for a few short reports dealing with characterization of volatile components [6, 7]. Chemical investigation on various extracts of P. arabicus and P. asirensis led to the identification of nine phytomolecules. Analyzing the acquired spectral data of each purified compound and their comparison with previously reported spectral values, the structure of compounds 1–9 were characterized as β-sitosterol (1) [8], carveol (2) [9, 10], oleanolic acid (3) [11], 5-hydroxy-7,4′-dimethoxyflavone (4) [12], naringenin (5) [13], isopropyl palmitate (6) [14], phytol (7) [15, 16], naringin (8) [17], and myricetin (9) [18]. General. The melting points of the isolated compounds were determined on a Gallenkamp device and were uncorrected. A JEOL ECP-400 spectrometer was used for the determination of the NMR spectra of the purified compounds. The compounds were dissolved in CDCl3–DMSO-d6 for NMR analysis. TMS was employed as internal standard. The chemical shifts and coupling constants (J) are represented in δ (ppm) and Hz, respectively. MS spectra were obtained using an Agilent single-quadrupole mass spectrometer. The thin-layer chromatography (TLC) was performed on precoated silica gel 60 F254 (0.2 mm, Merck) plates. For the purification of organic compounds, column chromatography (CC) was applied using silica gel (60–120 and 200–300 mesh, Rankem, India) as stationary phase. Plant Material. The aerial parts of P. arabicus and P. asirensis were collected when the plants start flowering in the month of March 2012 from Jazan and Abha and identified by a botanical taxonomist from King Saud University, Riyadh, KSA. The specimens of P. arabicus and P. asirensis received are retained in our research herbarium. Extraction and Isolation of Secondary Metabolites from P. asirensis. Shade air-dried and ground plant materials of P. asirensis (3.0 kg) were first extracted with n-hexane in order to remove fatty parts of plant materials. The defatted plant material was then extracted with ethyl acetate (EtOAc) and then with methanol. Extractions with each solvent were done in triplicate at 25°C for 3 days each. Extracts were combined and dried under vacuum on a rotary evaporator at 40°C to give crude n-hexane (115.0 g), EtOAc (239.9 g), and MeOH (109.0 g) extracts of P. asirensis. Part of the n-hexane extract (30 g) was loaded on a glass column using SiO2 as stationary phase. Petroleum ether, benzene, and ethyl acetate were used as eluent solvents to collect 150 fractions (100 mL each). TLC was used to monitor each fraction. Fractions showing identical spots on TLC were combined. Fractions 22–46 collected from petroleum ether–benzene (9:2) as eluents formed a light brownish precipitate, which upon filtration gave 4.82 g of precipitate. This precipitate was further subjected to CC using SiO2 as stationary phase and employing petroleum ether–benzene (2:1) as eluting solvents to give pure molecule 1 (805 mg). On the other hand, fractions 84–92 eluted with benzene–ethyl acetate (1:1) were pooled together and chromatographed again on a glass column using silica gel as stationary phase and benzene–ethyl acetate (3:1) as eluents to produce pure compound 2. For the isolation of compound 3, the ethyl acetate extract (30.0 g) was applied on a glass column using silica gel as stationary phase and benzene, chloroform, and EtOAc as