Pachydictyols B and C: New Diterpenes from Dictyota dichotoma Hudson

Two new diterpenoids, pachydictyol B (1a/1b) and C (2), were isolated from the dichloromethane extract of the marine brown alga, Dictyota dichotoma, collected from the Red Sea coast of Egypt, along with the known metabolites, pachydictyol A (3a), dictyol E (4), cis-africanan-1α-ol (5a), fucosterol (6), tetrahydrothiophen-1,1-dioxide and poly-β-hydroxybutyric acid. GC-MS analysis of the nonpolar fractions also indicated the presence of β-bourbonene and nonanal, along with three hydrocarbons and five fatty acids or their simple derivatives, respectively. GC-MS analysis of the unsaponifiable algal petroleum ether extract revealed the presence of a further eight compounds, among them 2,2,6,7-tetramethyl-10-oxatricyclo[4.3.0.1(1,7)]decan-5-one (7), N-(4-bromo-n-butyl)-piperidin-2-one (8) and tert-hexadecanethiol. Structures 1–6 were assigned by 1D and 2D NMR, mass spectra (EI, CI, HREI and HRESI) and by comparison with data from related structures. The crude algal extract was potently active against the breast carcinoma tumor cell line, MCF7 (IC50 = 0.6 µg mL−1); pachydictyol B (1a) and dictyol E (4) showed weak antimicrobial properties, and the other compounds were inactive. Pachydictyols B (1a) and C (2) demonstrated a weak and unselective cytotoxicity against twelve human tumor cell lines with a mean IC50 of >30.0 µM.


Introduction
Brown algae belonging to the family, Dictyotaceae, are a rich source of biologically active isoprenoids [1,2]. About 200 diterpenoids, belonging to 15 chemical classes, have been isolated from Dictyota spp. [3][4][5]. Some of these compounds are reported to display significant cytotoxic, antiviral, feeding-deterrent and antifouling activities [3,[6][7][8][9][10] or were useful for chemotaxonomic and biogenic studies of the genus, Dictyota [11,12]. The production of secondary metabolites in other genera of benthic marine brown algae has also been reported and is often associated with protection against herbivores [13].
During our search for bioactive diterpenoids from marine sources, the brown alga, Dictyota dichotoma (Hudson) Lamouroux, from the Red Sea, was selected for further investigation on the basis of notable in vitro cytotoxicity of a crude extract against the breast carcinoma tumor cell line, MCF7 (IC 50 = 0.6 µg mL −1 ) and on the basis of chemical screening by TLC. Several UV-inactive bands ranging from low to high polarity were detected that turned pink or gave a blue-violet color after spraying with anisaldehyde/sulfuric acid, suggesting the presence of isoprenoids. Soxhlet extraction of the algae using dichloromethane, followed by a series of chromatographic steps, afforded three new diterpenes, cisand trans-pachydictyol B (1a/1b) and pachydictyol C (2), see Figure 1. Additionally, the known metabolites, pachydictyol A (3a) [3], dictyol E (4) [14,15], cis-africanan-1α-ol (5a) [16], fucosterol (6), poly-β-hydroxybutyric acid and tetrahydrothiophene-1,1-dioxide, were isolated. GC-MS analyses of the nonpolar fraction and of the unsaponifiable residue of the algal extract revealed 18 further components, among them 7-9 (Supplementary Material, Tables S1 and S2).

Structure Analysis and Characterization of Isolated Compounds
Separation of D. dichotoma extracts on silica gel delivered eight compounds with a wide range of polarities. Under TLC monitoring, four compounds of moderate to high polarity were especially conspicuous. They were not UV absorbing, but stained intensely violet when sprayed with anisaldehyde/sulfuric acid. The least polar compound and a moderately polar component were identified as pachydictyol A (3a) [3] and dictyol E (4) [14,15], respectively, by means of NMR and MS data. The other two compounds showed a close similarity to 3a and 4 and appeared to be new derivatives thereof.
Compound 1a was obtained as polar colorless oil, with a molecular weight of m/z 320 Dalton by DCI MS. EI MS showed two characteristic fragment ions resulting from the successive loss of two molecules of water. (+)-HRESI MS confirmed the molecular formula as C 20 H 32 O 3 , with the same number of double bond equivalents as in 3a, but with two more oxygen atoms.
The 13 C NMR/HMQC spectra of 1a confirmed the expected twenty carbon signals and pointed to a close structural similarity with 3a and 4. The olefinic carbons of 1a had nearly the same shifts as for 3a/4; however, they were assigned by HSQC to three olefinic methines, two sp 2 C q atoms and one exocyclic methylene group (3/2/1) instead of 2/3/1, as in 3a/4. Between δ C 70~76, there were three signals from oxygenated carbons visible in the spectrum of 1a, but only one for 3a and two for 4, respectively. This indicated a new dihydroxypachydictyol A, that we named pachydictyol B (1a).
The configuration at C-11 was estimated tentatively on the basis of the expected dominating conformation. By AM1 [17], about 5500 conformers were calculated using the Monte Carlo method, of which 99.3% in the Boltzmann distribution (19 of 22 molecules in a range of ~15 kJ/mol above the global minimum) all showed a hydrogen bridge between the hydroxy groups, 6-OH and 11-OH. Due to the restricted rotation around the C-7/C-11 bond resulting thereby, the (11S) configuration with the 11-methyl in β-orientation and an 11α chain (C12-C16) or the corresponding (11R) diastereomer might be differentiated by NOESY data. On this basis, the strong NOE between the double bond protons H-13/14 and both H-6 and H-7 was taken as a clear indication of the (11S) configuration.
In deuteriochloroform at 300 MHz, a 3 J coupling between H-13/14 was not visible, due to nearly identical shifts. Inspection of further 1a fractions revealed, however, a second isomer with slightly different 1 H and 13 C shifts in the region of C-13-C-16/20. In this compound, the olefinic proton, H-14, appeared as a doublet (J = 15.9 Hz), while H-13 gave a doublet of a triplet (J = 15.9, 6.4 Hz), clearly indicating an (E)-configuration of the side chain (Supplementary Material, Figure S7). As all 2D correlations of both isomers, along with the shifts of the chiral centers, were identical ( Figure 2 and Figure S11, Supplementary Material), 1a and 1b were determined to be cisand trans-pachydictyol B, respectively.

4
In contrast to 1a, 3a and 4, compound 2 showed a negative optical rotation ([α] 20 D = −15°), similar to that of the related 10-methyl derivative dictyol C (3b) ([α] 20 D = −16.6°) [14]. The configuration of 2 was assumed to be the same as in 1, 3a and 4, with regard to the common biosynthetic origin of all dictyols isolated here, because of the closely related shifts of the respective atoms in 3b and 4 and on the basis of similar NOESY correlations ( Figure 4). As H-6 showed a strong NOESY correlation with H-1 and a weaker one with CH 3 -18, an α-orientation with an equatorial position of this methyl must be assumed. This is confirmed by a clear correlation of H-10 with CH 2 -13 and CH-14, which both must be placed on the β-face, resulting in a (10R) configuration. Consequently, this compound was assigned as (1S,5S,6S,7R,10R,11S)-2.
Compound 5a did not absorb UV light as well as the other compounds isolated and gave a pink color with anisaldehyde/sulfuric acid. It was obtained as a nonpolar colorless oil with the molecular formula, C 15 H 26 O, and identified as cis-africanan-1α-ol, whose structure 5a had been reported previously [16], but was not completely characterized. We report herein the first full NMR assignment of 5a, based on 2D experiments (Table 3, Figure 5).
(Z)-Pachydictyol B (1a) displayed high antimicrobial activity in the agar diffusion test at 10 µg /paper disc against Mucor miehei (20 mm) and was weakly active against Candida albicans (11 mm) and Pythium ultimum (12 mm). Pachydictyol C (2) showed no antimicrobial activity, and both 1a and 2 were not toxic towards brine shrimp at 10 µg mL −1 . At the time of isolation of pure compounds, the MCF7 test was no longer available and has been substituted by other cell lines (Table 4); the in vitro examination demonstrated weak and unselective cytotoxicity against twelve human tumor cell lines, with a mean IC 50 of >30.0 µg mL −1 ; the high activity of the crude extract could not be reproduced. Table 4. Cytotoxic activities of pachydictyols A-C (3a, 1a, 2), Dictyol E (4), cis-africanan-1α-ol (5a) and fucosterol (6).

General Experimental Procedures
NMR shifts were referenced on the solvent signal of CDCl 3 (δ H = 7.27, δ C = 77.0; 300 or 600 MHz for 1 H and 125 Hz for 13 C). GC-MS spectra were measured on a Trace GC-MS Thermo Finnigan chromatograph, using EI ionization mode (70 eV) and a CP-Sil 8 CB capillary column for amines (length: 30 m; inside diameter: 0.25 mm; outside diameter: 0.35 mm; film thickness: 0.25 µm). The analysis was carried out using a temperature program. The initial temperature was 40 °C (maintained for 1 min), and the temperature was then ramped up at a rate of 10 °C/min to a final temperature of 280 °C (kept for 10 min). The injector and detector temperature were 250 °C, and He was used as the carrier gas at a flow rate of 1 mL min −1 . The total run time was 27 min, and the injection volume was 0.2 µL . For details see reference [26].

Collection and Taxonomy of the Marine Alga
The brown alga, Dictyota dichotoma (Huds) Lamour, was collected in the summer of 2007 at Ras Abu-Bakr, 65 km north of Ras Gharib on Suez-Gulf, Red Sea, Egypt. The identification was carried out by Abou-ElWafa according to Nasr's method [27,28]. A reference specimen of the alga is kept at the Department of Botany, Faculty of Science, Mansoura University, Egypt.
Samples of Dictyota dichotoma (Huds) Lamour were separated from epiphytes and the dead matrix in running water and rinsed several times in distilled water. The sample was then spread on string nets, allowed to dry in air, ground and stored in closed bottles at room temperature.

Extraction and Isolation of the Bioactive Constituents
The air-dried algal material (~360 g) was extracted in a Soxhlet apparatus for ~12 h using dichloromethane (DCM). The DCM extract was filtered and the solvent evaporated in vacuo at 40 °C, affording 14.3 g of a greenish brown crude extract. This extract was fractionated on a silica gel column, eluting with petroleum ether (boiling range 40-60 °C)-DCM and DCM-MeOH gradients, delivering five fractions: I (0.11 g), II (3.2 g), III (2.3 g), IV (2.6 g) and V (5.1 g). TLC monitoring was used, with anisaldehyde/sulfuric acid as the spraying reagent. The first nonpolar fraction I was submitted to GC-MS analysis, detecting the existence of tetrahydrothiophen-1,1-dioxide, β-bourbonene and nine further compounds (Supplementary Material, Table S1). A preparative separation of fraction I on silica gel (eluting with a cyclohexane-DCM gradient) afforded a pale yellow oil, which was further purified on Sephadex LH-20 (DCM/40% MeOH) to give the colorless, oily, tetrahydrothiophene-1,1-dioxide (9 mg, 0.06%). Fraction II was applied to a Sephadex LH-20 (DCM/40% MeOH) to afford two sub-fractions, IIa (0.7 g) and IIb (2.4 g). Sub-fraction IIa was not further investigated. Sub-fraction IIb (2.4 g) was washed with methanol to give the insoluble, colorless solid, poly-β-hydroxybutyric acid (1.47 g, 10.3%). The soluble part of the methanolic extract (0.88 g) was applied to a silica gel column and eluted with a cyclohexane/DCM gradient to deliver pachydictyol A (3a) (18.2 mg, 0.12%) and cis-africanan-1α-ol (5a) (13.1 mg, 0.09%) as colorless oils. Further purification of sub-fraction IIb afforded fucosterol (6, 30.2 mg, 0.21%) as a colorless solid. The eluates from fractions III and IV were combined (4.9 g) and purified on a Sephadex LH-20 column (MeOH) to give sub-fraction IIIa (2.5 g). The latter was further purified by silica gel column chromatography, eluting with DCM-MeOH gradients, to afford the colorless oily compound, dictyol E (4, 55.0 mg, 0.38%). The last polar fraction V was separated by column chromatography on silica gel, again eluting with DCM-MeOH gradients, to give sub-fractions Va (1. trans-Pachydictyol B (1b): The trans isomer was obtained as a colorless oil with similar chromatographic properties and mass spectra as found for 1a, but with a slightly lower polarity (R f = 0.45 (CH 2 Cl 2 /3% MeOH); NMR data, see Table 1.

Estimation of Phytosterols and Hydrocarbons
A powdered sample (10 g) of Dictyota dichotoma was extracted with petroleum ether (60-80 °C) at room temperature and concentrated in vacuo to give an oily residue (70 mg). This extract was then treated with 50 mL of 10% alcoholic KOH and refluxed in a water bath for 2 h. After cooling, 50 mL of water was added, and the solution was extracted with chloroform. The organic phase was washed with water until it became alkali free and was then dried over anhydrous Na 2 SO 4 . The solvent was evaporated to give the unsaponified fraction as oil, which was subsequently subjected to GC-MS analysis [29] (Supplementary Material, Table S2).

Biological Activity Study
Antimicrobial activity was determined according to Burkholder et al. [24]. The Brine Shrimp Microwell Cytotoxic Assay was performed according to Takahashi et al. and Sajid et al. [22,23]. The in vitro cytotoxicity test was carried out using the sulforhodamine B SRB assay according to Skehan et al. [30].
The antitumor activity testing was performed as follows: A modified propidium iodide assay was used to examine the antiproliferative activity of the compounds against human tumor cell lines. The test procedure has been described elsewhere [31]. Cell lines tested were derived from patient tumors engrafted as a subcutaneously growing tumor in NMRI nu/nu mice or obtained from American Type Culture Collection, Rockville, MD, National Cancer Institute, Bethesda, MD, or Deutsche Sammlung von Mikroorganismen und Zellkulturen, Braunschweig, Germany.

Conclusions
Three new pachydictyols, namely (Z)-and (E)-pachydictyols B (1a/1b) and C (2), along with the known pachydictyol A (3a), dictyol E (4), cis-africanan-1α-ol (5a), fucosterol (6), tetrahydrothiophene-1,1-dioxide and poly-β-hydroxybutyric acid, were isolated from the marine brown alga, Dictyota dichotoma. GC-MS analysis of the nonpolar fractions of the algal extract revealed the presence of ten further compounds, whilst the same analysis of the unsaponified petroleum ether extract of the algae detected a further eight compounds (Supplementary Material, Tables S1 and S2). The chemical structures of compounds 1-6 were assigned by 1D and 2D NMR spectroscopy, mass spectrometry (EI, CI, HREI, HRESI) and by comparison of the data with that of related structures. The algal extract exhibited no antimicrobial activity against a diverse range of microorganisms and no cytotoxicity against brine shrimp. In contrast to the high anticancer activity of the crude extract against the breast carcinoma tumor cell line, MCF7 (IC 50 = 0.6 µg mL −1 ), the purified components were only weakly active (Table 4).