Terpenoids of Sinularia.: Chemistry and Biomedical Applications

Abstract

The soft coral genus Sinularia. is one of the most widely distributed soft corals. It constitutes a dominant portion of the biomass in the tropical reef environment. Sinularia. elaborates a rich harvest of secondary metabolites including sesquiterpenes, diterpenes, polyhydroxylated steroids, and polyamine compounds. These metabolites were recently shown to possess a range of biological activities such as antimicrobial, anti-inflammatory, and cytotoxic activities. During the past decade, Sinularia. has yielded many new structures with novel skeletons. Several of the previously published secondary metabolites have been reexamined for their pharmacological properties, and the results strongly support further investigations. The current article reviews the terpenoids of the soft coral genus Sinularia. and their pharmacological significance.

Keywords:

• Bioactive terpenes
• marine natural products
• octocorals
• Sinularia
• soft corals
• terpenoids

Introduction

In recent years, the current status of marine natural product bioprospecting has been a popular subject for reviews (Kelecom, 1999; Proksch et al., 2002; Alejandro & Gustafson 2003; Haefner, 2003), as it has resulted in a considerable number of drug candidates. The study of marine natural product chemistry began in 1960 with a conference entitled “Biochemistry and Pharmacology Derived from Marine Organisms” organized by the New York Academy of Sciences. Beginning in 1978, Scheuer (1978) and Faulkner (2002) provided comprehensive reviews on the chemical and biological perspectives of marine natural products and, therefore, played a central role in shaping the field of marine natural product chemistry.

Coral reefs represent an extraordinary diverse biota in tropical environments, and soft corals often constitute a dominant part of the reef biomass. Soft corals attracted considerable attention because of the wide range of bioactive secondary metabolites generated by these marine invertebrates. The interest began with the isolation of the prostaglandin [(15R.)-PGA₂] from the gorgonian Plexaura homomalla. Esper, 1792 (Weiheimer & Spraggins, 1969). This was followed, in the early 1970s, with the examination of the Alcyonacean octocorals. Chemists came to the conclusion that terpenoid chemistry predominates across the Octocorallia and Tursch et al. (1978) surveyed the known distribution of terpenoids across the Octocorallia. Coll (1992) reviewed the chemistry and chemical ecology of Octocorals and made several contributions to the ecological reasons for the structural chemical diversity. More recently, Venkateswarlu et al. (2001) gave an account on the novel bioactive metabolites from soft corals.

Sinularia. is a soft coral belonging to the phylum Cnidaria, class Alcyonaria, and family Alcyoniidae. Its taxonomic identification is largely based on the examination of the spicules, and a fairly reliable taxonomic key is available to guide species identification (Verseveldt, 1980). The genus Sinularia. consists of almost 90 species of which more than 50 have been chemically examined. Many bioactive metabolites that have been reported from soft corals and species of the genus Sinularia. have been studied, and the isolated components display potential bioactivities such as antimicrobial, anti-inflammatory, and cytotoxic activities (Venkateswarlu et al., 2001).

The study of Sinularia. has been dominated by Australian, Indian, and Japanese groups who have described a wide range of secondary metabolites including sesquiterpenes, diterpenes, polyhydroxylated steroids, and polyamine compounds. A number of questions were raised in the past regarding the origin of terpenes in symbiotic associations between marine invertebrates and algae (Zooxanthellae). Although Kobbe et al. (1984) came to the conclusion that the zooxanthellae do not make terpenes and that the coral polyp was the active organelle in terpenoid biosynthesis, the subject is still in question. Anjaneyulu and Venkateswalu (1995) reviewed the chemical components of the soft coral genus Sinularia.. Since 1995, the number of research papers investigating the chemical constituents of the soft coral genus Sinularia. has exceeded 100, the majority reporting new and novel terpenoids. In addition, many of the previously published terpenoids were reexamined for their pharmacological activities, and the results strongly promoted further investigations. The purpose of this review is to focus on the terpenoid constituents of Sinularia., highlighting their novel chemistry, pharmacological activities, and biomedical applications, and to examine the future perspectives for soft coral research.

Sesquiterpenes

The first sesquiterpene reported from the genus Sinularia. was a furano-sesquiterpenoid acid (1) (Fig. 1) isolated from S. gonatodes. Ehrenberg, 1834 by an Australian group (Coll et al., 1977). This was followed by a new furanoquinol (2) isolated from S. lochomodes. Kolonko, 1926 (Coll et al., 1978). Twelve related sesquiterpene furans (3–14) were subsequently reported from two Sinularia. species, S. capillosa. Tixier-Durivault, 1970, and S. firma. Tixier-Durivault, 1970 (Bowden et al., 1983). In 1994, the furanoic acid (1) was found to inactivate bee venom phospholipase A₂ (bvPLA₂) in vitro. with an IC₅₀ of 0.5 µM (Grace et al., 1994). The study of its kinetics of inactivation suggested that it may interact with multiple binding sites on bvPLA₂. Phospholipase A₂ (PLA₂), which is involved in the pathogenesis of a variety of inflammatory diseases, is a promising target for the development of anti-inflammatory drugs. Intrigued by the anti-inflammatory activity of this simple compound, Williams and Faulkner (1996) synthesized its geometrical isomer (5), which inhibited bee venom PLA₂ (46% at 0.8 µg/ml). They-designed two practical syntheses, which were both based on Claisen rearrangements. The geometrical isomeric compounds 7 and 9 showed a minimum inhibitory concentration (MIC) of 32 µg/ml against Mycobaterium tuberculosis. Lehman & Neumann, 1896 (TB), when combined (Kamel et al., unpublished data).

Figure 1 structures of the soft coral genus Sinularia..

A sesquiterpene (15) having a non-farnesolic skeleton (i.e., cannot be derived by direct cyclization of a farnesol precursor) was isolated from S. mayi. Luttschwager, 1914, in 1978 and named sinularene (Beecham et al., 1978ab). Having a unique skeleton, sinularene has attracted considerable attention as a synthetic target, and several total syntheses of sinularene have been reported using different approaches (Collins & Wege 1979; Oppolzer et al., 19821984; Antczak et al., 1987) such as intramolecular Diels-Alder reaction (Antczak et al., 1987) and intramolecular magnesium-ene reaction as the key steps (Oppolzer et al., 19821984).

The most commonly reported sesquiterpene from the genus Sinularia. is Δ^9,(15) africanene (16). It is a tricylic sesquiterpene belonging to the rare africanane skeleton, which was first found in africanol (17), a tertiary alcohol isolated from the octocoral Lemnalia africana. Gray, 1868 (Tursch et al., 1974). Africanene (16) was first isolated by Braekman et al. (1980) and Kashman et al. (1980) independently from S. polydactyla. Ehrenberg, 1834, and S. erecta. Tixier-Durivault, 1945, respectively. The absolute configuration was assigned by an X-ray diffraction study of derivatives. Recently, the pharmacological activities of africanene have been investigated (Reddy et al., 1999). It showed mild CNS depressant activity and dose-dependent hypotensive activity (Table 1). Furthermore, it exhibited anti-inflammatory activity against carrageenan-induced rat edema with a 60% reduction at a dose of 10 mg/kg body weight as compared to ibuprofen, which showed 49% reduction at a higher dose of 100 mg/kg body weight orally (Reddy et al., 1999). This anti-inflammatory effects suggests that it is worthy of further characterization studies.

Table 1.. Biological activities of terpenoids from the soft coral genus Sinularia..

	Metabolite	Activity	Mechanism of action and pharmacology	Ref.
1		Anti-inflammatory	Inactivate bv PLA2	Grace et al. (1994)
5		Anti-inflammatory	Inactivate bv PLA2	Williams & Faulkner (1996)
7 + 9		Antituberculosis	Minimum inhibitory concentration (MIC) = 32 µg/ml
16	Africanene	CNS depressant hypotensive Anti-inflammatory Cytotoxic	Against the EAC and DLAT cells in vitro. and in vivo.	Reddy et al. (1999)
35	13α-Acetoxypukalide	Nicotinic ACH receptor antagonist		Abramson et al. (1991b)
39	Sinulariolide	Algicidal		Sui-jan et al. (2002)
		Cytotoxic	ED50 against P-388 and L1210 are 8.5 and 10.0 µg/ml, respectively	Aceret et al. (1996)
		Antimicrobial	Inhibiting the growth of Gram-positive B. subtilis. and S. aureus.	Aceret et al. (1998)
		Cardiac vasorelaxant
51	Flexibilide	Anti-inflammatory	Inhibitory activity after oral administration in acute and chronic animal models of inflammation	Buckle et al. (1980)
		Cytotoxic	IC50 against Hela, BGC, 95-D, CNE, HepG2, and L-02 are 8.2, 13.5, 3.7, 4.5, 15.1, and 16 µg/ml, respectively. And an ED50 of 1.5 and 3.0 µg/ml against P-388 and L1210, respectively	Aceret et al. (1996)
		Antimicrobial	Inhibiting the growth of Gram-positive bacteria B. subtilis., S. aureus., S. faecalis., S. sanguis., E. faecalis., and M. phlei.	Aceret et al. (1998)
		Cardiac vasorelaxant
52	Dihydroflexibilide	Cytotoxicity	IC50 against Hela, BGC, 95-D, CNE, HepG2 and L-02 are 5.6, 11.2, 2.5, 3, 11.2, and 11 µg/ml, respectively	Sui-jan et al. (2002)
		Cardiac vasorelaxant		Aceret et al. 1996
53 54	Sinularin and dihydrosinularin	Cytotoxic	ED50 against KB and PS cell lines are 0.3 and 0.3 µg/ml for sinularin and 16 and 1.1 µg/ml for dihydrosinularin, respectively	Weinheimer et al. (1977)
77	Sinuflexolide	Cytotoxic	ED50 against A549, HT-29, KB and P-388 are 0.68, 0.39, 0.46, and 0.16 µg/ml, respectively	Duh et al. (1998b)
78	Dihydrosinuflexolide	Cytotoxic	ED50 against A549, HT-29 and P-388 are 16.8, 32.4, and 3.86 µg/ml, respectively	Duh et al. (1998b)
79	Sinuflexibilin	Cytotoxic	ED50 against A549, HT-29, KB and P-388 are 0.72, 0.22, 1.73, and 0.27 µg/ml, respectively	Duh et al. (1998b)
81	Sinugibberol	Cytotoxic	ED50 against HT-29 and P-388 cells are 0.5 and 11.7 µg/ml, respectively	Hou et al. (1995)
82–85		Cytotoxic	ED50 against A549, HT-29, KB and P-388 are 1.03, 0.64, 0.63, and 0.1 for compound 82., > 50, 0.36, > 50, and 3.26 for compound 83., 20.16, 1.29, 14.92, and 1.54 for compound 84., and 1.70, 1.81, 17.12, and 0.23 for compound 85., respectively	Duh & Hou (1996)
89	Capillolide	Cytotoxic	ED50 against P-388 and L1210 are 15 and 18.5 µg/ml, respectively	Su et al. (2000)
90		Cytotoxic	ED50 against P-388 and L1210 are 2.5 and 5.0 µg/ml, respectively	Su et al. (2000)
80	Sinuflexin	Cytotoxic	ED50 against P-388 is 1.32 µg/ml	Duh et al. (1998a)
101		Cytotoxic	ED50 against P-388 and L 1210 are 15.0 and 18.5 µg/ml, respectively	Lin et al. (2002)
102		Cytotoxic Anti-inflammatory	ED50 against KB and Hepa59T/VGH are 2.3 and 2.4, respectively	Sheu et al. (2002) Takaki et al. (2003)
104	Sinuleptolide	Cytotoxicity Anti-inflammatory	ED50 against KB and Hepa59T/VGH are 2.5 and 2.6, respectively Inhibit TNF-α production	Takaki et al. (2003)
110	Singardin	Cytotoxic	ED50 against P-388, A-549, HT-29, and MEL-28 are 1, 2.5, 5, and 5 µg/ml, respectively	Elsayed & Hamann (1996)
118	Scabrolide C	Cytotoxic	ED50 against KB and Hepa59T/VGH are 11.9 and 17.8 µg/ml, respectively	Sheu et al. (2002)
111	Leptocladolide A	Cytotoxic	ED50 against KB and Hepa59T/VGH are 5.9 and 2.6 µg/ml, respectively	Ahmed et al. (2003)
113	7E-leptocladolide	Cytotoxic	ED50 against KB and Hepa59T/VGH are12 and 3.2 µg/ml, respectively	Ahmed et al. (2003)
112	1-epi-leptocladolide A	Cytotoxic	ED50 against KB and Hepa59T/VGH are 15.1 and 14.5 µg/ml, respectively	Ahmed et al. (2003)
108	Ineleganolide	Cytotoxic	ED50 against P-388 is 3.82 µg/ml	Duh et al. (1999)
124	Ineleganene	Cytotoxic	GI50 against A549 and P388 are 3.63 and 0.20 µg/ml, respectively	Chai et al. (1992)
125–128	Sinulobatins A–C	Cytotoxic	IC50 against L1210 are 3.0, 4.8, and 3.2 µg/ml, respectively and against KB are 5.1, 7.7, and 4.5 µg/ml, respectively	Yamada et al. (1997)

Moreover, africanene exhibited cytotoxic activity against the Ehrlich ascites carcinoma (EAS) and Dalton's lymphoma ascites tumor (DLAT) cells. In an in vivo. cytotoxic assay, africanene increased the life span of mice with EAS by 22% and 53% at the concentrations of 10 and 20 mg/kg body weight, respectively (Reddy et al., 1999). Chemical and biocatalytic structure activity relationship (SAR) studies have been initiated for africanene (Venkasterwarlu et al., 1999a; Reddy et al., 2000). Δ^9,10-Africanene (18), leptographiol (9β-hydroxy africanane) (19), and O.-formyl leptographiol (O.-formyl 9β-hydroxy africanane) (20) were obtained by means of acid catalyzed rearrangement of africanene (Reddy et al., 2000), and 10α-hydroxy-9(15)-africanene (21) and 9α,15-epoxyafricanane (22) were obtained by microbial oxidation of africanene using Aspergillus niger. Teigh, 1867, and Rhizopus oryzae. Went. & Prinns. Geerl., 1895 (Venkateswarlu et al., 1999a). Compounds 21 and 22 were reported earlier as natural products along with a new oxygenated derivative, 9α,15-dihydroxyafricanane (23) from S. dissecta. Tixier-Durivault, 1945 (Ramesh et al., 1999). Later, the same group reported 15-hydroxy-Δ⁹-africanene (24) from this species (Ramesh et al., 2001). Sinularia intacta. Tixier-Durivault, 1945, of the Indian ocean has yielded three new derivatives, 9(R.)-africanane-9,15-diol (25), 9(R.)-9-methoxyafricanan-15-ol (26), and 9(S.)-africanane-9,15-diol-15-monoacetate (27) (Anjaneyulu et al., 1999). Recently, several secoafricanene derivatives (with the five membered-ring cleaved) have been isolated from different Sinularia. species. The diketosesquiterpenoid 8,9-secoafricanene-8,9-dione (28) was reported from S. intacta. Tixier-Durivault, 1945, of the Indian Ocean (Anjaneyulu & Gowri, 1999), and 4-acetoxy-3,15-dinor-2,3-seco-2-africanone (29) was reported from S. dissecta. of the Mandapam coast (Ramesh & Venkateswarlu, 2000). None of these africanene derivatives has been pharmacologically evaluated. However, the pharmacological activities reported for africanene are promising and encourage further investigations, and the biological testing of its derivatives should yield an optimized drug lead and future SAR.

Recently, spathulenol (30), a rare sesquiterpene with an aromadendrane skeleton found in the plant Eucalyptus spathulata. Hook, 1844, was isolated from S. kavarattiensis. Aldersalde & Shirwaiker, 1991 (Goud et al., 2002). Aromadendrene (31), 4α,7β-aromadendranediol (32), and 4α,7α-aromadendranediol (33) were identified from S. mayi. in 1978 (Beechan et al., 1978b). Several aromadendrane diterpenoids have been isolated from Sinularia. species (see next sections).

Cembranoid diterpenes

Cembranoids are a class of diterpenes possessing a 14-membered ring skeleton. Pukalide (34) was first isolated from S. abrupta. Tixier-Durivault, 1970, in 1975 by the Scheuer group (Missakian et al., 1975). The derivatives 13α-acetoxy pukalide (35) and 13α-acetoxy-11β,12β-epoxypukalide (36) were later obtained from an Australian S. polydactyla. Ehrenberg, 1834 (Bowden et al., 1989). Pukalide and its derivatives are produced by a variety of octocorals in the orders Gorgonacea and Alcyonacea. They are structurally similar to the gorgonian-derived compound lophotoxin and to bipinnatins, both of which are irreversible nicotinic acetylcholine receptor antagonists (Abramson et al., 1991a). In a structure-activity study of the lophotoxin family, 13α-acetoxypukalide (35) was found to be active (Abramson et al., 1991b). This study defined the C7-C8 epoxide and the acetate ester at C13 to be essential for the activity. The interaction of cembranoids with the acetylcholine receptors was characterized earlier. It was shown that the irreversible inhibition results from a specific covalent reaction between the cembranoid and Tyr¹⁹⁰ in the α-subunit of the receptor. Recently, two bis.-pukalide diterpenes were reported from S. erecta. Tixier-Durivault, 1945, named mayotolides A and B (37, 38) (Rudi et al., 1998). Pukalide, having a furanocembranolide skeleton, has been a challenge to synthesize. Generally, constructing the furanocyclized structure that typifies many of these compounds has been a problem. Although a total synthesis has not been reported to date, recently the synthesis of the C₁-C₁₈ segment of pukalide was accomplished in 11 steps with a 10% overall yield (Peter & Michael, 2002).

Sinulariolide (39) was first isolated from S. flexibilis. Quoy & Gaimard, 1833 (Tursch et al., 1975), and was later reported from other Sinularia. species. The structure was determined by spectroscopic and chemical methods, and the absolute configuration was established by X-ray diffraction by Karlsson (1977). Sinulariolide (39) demonstrates a broad range of biological activities, having been reported as an algicidal molecule responsible for antifouling properties (Tursch, 1976) and showing marginal cytotoxic activities against a number of cell lines (Sui-jian et al., 2002). Several sinulariolide derivatives were later reported including 11-episinulariolide acetate (40) and 11-dehydrosinulariolide (41) from Red Sea species (Kashman et al., 1977) and 11-epi-sinulariolide (42) and 11-epoxysinulariolide (43) from S. flexibilis. Quoy & Gimard, 1833 (Mori et al., 1983). A series of analogues (44–50) were prepared from sinulariolide and 11-epi-sinulariolide acetate, and their cytotoxic activities were evaluated (Hsieh et al., 2003). It was shown that the bioactivities diminish in compounds with hydrogenated α-methylene-lactone system, epoxidized double bond C-7, 8, and/or an ether linkage between C-8 and C-11.

Flexibilide (51) and dihydroflexibilide (52) were reported from the Australian species of S. flexibilis. (Kazlauskao et al., 1978). Flexibilide was reported to possess anti-inflammatory and antiarthritic activities in rats (Buckle et al., 1980). It reduced paw edema induced by carrageenan with a similar potency to phenylbutazone, and both (51 and 52) showed cytotoxic activities in vitro. (Sui-jian et al., 2002). Flexibilide, dihydroflexibilide, and sinulariolide were shown to be cardioactive, producing vasorelaxant responses in the isolated rat tissues (Aceret et al., 1996). Flexibilide and sinulariolide also exhibited antimicrobial activity and inhibited growth of Gram-positive bacteria Bacillus subtilis. Cohn, 1872, and Staphylococcus aureus. Rosenbach, 1884 (Aceret et al., 1998). It was speculated that the antibacterial activity is associated with the α-methylene-lactone moiety present in flexibilide and sinulariolide, but not in dihydroflexibilide. The cytotoxic cembranolides sinularin (53) and dihydrosinularin (54) were isolated from the same species, and their structures and absolute configurations were determined by X-ray diffraction analysis (Weinheimer et al., 1977). Dihydrosinularin and 11-episinulariolide were both isolated from the marine mollusk Planaxis sulcatus. Born, 1778, and it was suggested that they were of dietary origin in the mollusk (Sanduja et al., 1986).

Sinularia mayi. Luttschwager, 1914, is a common species in the coral reefs of southern Japan and has been subjected to intensive chemical studies by Japanese groups. The parent hydrocarbon cembrene (55) was reported from S. mayi. along with cembrenene (56) and the monooxycembratetraene mayol (57) (Uchio et al., 1981). Later, the first secocembrane type diterpenoid, mayolide A (58), was reported from the same species (Kobayashi, 1988). A secocembranoid has a unique skeleton in which the bond between the C-12 and C-13 positions of the 14-membered carbocyclic skeleton of usually encountered cembranolides is cleaved. The cembranoids mayolide B, C and D (59–61) were also reported from S. mayi. Luttschwager, 1914. The stereoselective total synthesis of mayolide A has been achieved starting from d-mannitol (Nagaoka et al., 1992). Sinularia mayi. also yielded sinulariol A–D (62–65), sinularial A (66), sinularic acid A (67), and sinularones A and B (68, 69) (Kobayashi et al., 1987; Kobayashi & Hamayuchi, 1988). Sinulariol B has been synthesized from E.-geraniol with a ∼ 10% overall yield (Lan et al., 1999; Yue et al., 1999). Mayolacetate (70) and a hydroxycembranolide γ.-lactone (71) were obtained from S. hirta. Pratt, 1903 (Anjaneyulu et al., 1996).

Mandapamate (72) is a diterpene related to the cembranoid series but with a more complex tetracyclic carbon skeleton and has been isolated from S. dissecta. Tixier-Duricvault, 1945 (Venkateswarlu et al., 1994). The diastereomeric, isomandapamate (73), was reported from S. maxima. Verseveldt, 1970 (Anjaneyulu et al., 1995b), and its homologue, bishomoisomandapamate (74), was reported from an undescribed Sinularia. sp. (Anjaneyulu & Sarada, 1999). It was postulated that the tetracylic framework of mandapamate could be derived by an intramolecular Diels-Alder reaction. Anjaneyulu et al. (2000) provided a possible biogenetic sequence for mandapamate and havellockate (75), which represent a novel seco. and spiro. lactone diterpenoid reported from S. granosa. Tixier-Durivault, 1970, of the Indian Ocean (Anjaneyulu et al., 1998). The structures were determined by a detailed study of their spectral data including X-ray analysis (Anjaneyulu et al., 1998). A related diterpene with an unusual 5-7-6 tricyclic skeleton, rameswaralide (76), was isolated from S. dissecta. (Ramesh et al., 1998).

Bioactivity guided fractionation of the extract of S. flexibilis. yielded sinuflexolide (77), dihydrosinuflexolide (78), and sinuflexibilin (79) (Duh et al., 1998b). Compounds 77 and 79 exhibited significant cytotoxicity. A biscembranoid diterpene, sinuflexin (80), was reported from S. flexibilis. (Duh et al., 1998a) and showed cytotxicity against the P-388 cell culture system. Sinugibberol (81) is a cytotoxic cembranoid isolated from S. gibberosa. Tixier-Durivault, 1970 (Hou et al., 1995). The same species yielded four new cytotoxic cembranoids: 11,12-epoxy-1(E.),3(E.),7(E.)-cembratrien-15-ol (82), 3,4:11,12-diepoxy-15-methoxy-1(E.),7(E.)-cembradiene (83), 1(E.),3(E.),7(E.),11(E.)-cembratetraene-14,15-diol (84), and 3,14-epoxy-1(E.),7(E.),11(E.)-cembranetriene-4,15-diol (85) (Duh & Hou, 1996).

Three nitrogen-containing cembranolide diterpenes, sinulamine I (85), II (86), and III (87), each possessing a dimethylamino group, were isolated from an Okinawan species, and their structures were elucidated by spectroscopic analysis and chemical transformations (Iguchi et al., 1992). Later, Kobayashi and Ishizaka (1992) revised the stereochemistry of these dimethylamine adducts and concluded that they should be regarded as formed in the organism during storage. A similar diterpene with a dimethylamino group isolated from the soft coral Lobopytum. exhibited moderate HIV-inhibitory activity (Rashid et al., 2000).

Capillolide (89) was isolated from S. capillosa. along with flexibilide, sinulariolide, and (1R.,13S.,12S.,9S.,8R.,5S.,4R.)-9-acetoxy-5,8:12,13-diepoxycembr-15(17)-en-16,4-olide (90) (Su et al., 2000). Capillolide showed moderate cytotoxic activity. Sinulariolone (91), a new highly oxygenated cembranoid, was obtained from a Philippine collection of S. flexibilis. (Guerrero et al., 1995), and the trihydroxy cembranolide lactones flexibiolide (92) and dihydroflexibiolide (93) were isolated from an Indian collection of S. flexibilis. (Anjaneyulu & Sagar, 1996). This same species has yielded new cembranoilde €-lactones named sandensolide monoacetate (94) and flexibolide (95) (Anjaneyulu et al., 1997a). Sandensolide (96) was isolated from S. sandensis. Verseveldt, 1977, of the Indian Ocean (Anjaneyulu et al., 1995a). Sinularia maxima. Verseveldt yielded the new furanocembranoid derivatives named sethukarailin and sethukarailide (97, 98) (Venkateswarlu et al., 1999b). Seco-sethukarailin (99) was recently obtained from another species, S. dissecta. (Reddy et al., 2002). Sinulariadione (100), an uncommon 1,3-diketone cembranoid, was isolated from an unidentified Sinularia. from the Mandapam coast (Anjaneyulu & Raju, 1995). More recently, the crystal structure of a new cytotoxic cembranolide diterpene (101) was reported from S. tenella. Li, 1982, by a Chinese group (Lin et al., 2002).

Norcembranoid diterpenes

The first norcembranolide (102) (which lacks a C-18 carbon atom at C-4) was isolated from S. leptoclados. Ehrenberg, 1834, by an Australian group (Bowden et al., 1978) and was reported later from several other species. The relative stereochemistry, derived by NMR spectroscopic data, was confirmed by X-ray diffraction analysis (Turner et al., 1979). However, the structure was drawn in error showing the configuration at C-11 opposite to what had been established by the crystal structure (Turner et al., 1979). Misinterpretation of the X-ray data and of the stereochemistry at C-11 has led to confusion in the literature, and this error has been propagated in many of the later papers (Lakshmi & Schmitz, 1986; Duh et al., 1993) and reviews (Wahlberg & Eklund, 1992) and is still repeated as late as 2003 (Takaki et al., 2003). Sato et al. (1985) later reported the C-11 epimer (103) while Shoji et al. (1993) reported the C-5 epimer (104a) and named it sinuleptolide, and Ramesh and Venkateswarlu (2000) reported 11-epi sinuleptolide (104b). Compounds 102 and 103 showed cytotoxic activities (Sheu et al., 2002), and compounds 102 and 104a showed an inhibitory effect on LPS-induced TNF-α production (Takaki et al., 2003).

Several other norcembrane diterpenes have been isolated from different Sinularia. species. Yonarolide (105) and sinulariadiolide (106) were both isolated from an Okinawan Sinularia.. Yonarolide possesses a novel tricyclo [7.5.0.0^3,7]-tetradecane skeleton (Iguchi et al., 1995), whereas sinulariadiolide is characterized by a new carbon skeleton with two lactone moieties, five- and nine-membered rings; the nine-membered system constitutes a β-hydroxy-α, β-unsaturated ester group (Iguchi et al., 1996). Dissectolide (107), isolated from the Indian Ocean S. dissecta., is shown to have a highly strained tetracyclic ring system with cyclopentane, cyclohexene, cycloheptane, and α,β-unsaturated γ.-lactone ring systems (Kobayashi et al., 1995). The norditerpenoid ineleganolide (108) was isolated from a Formosan S. inelegans. Tixier-Durivault, 1970, and exhibited moderate cytotoxicity (Duh et al., 1999). Horiolide (109) was isolated from a Sinularia. species and was shown to possess a new carbon skeleton having one six-membered cyclohexane ring bearing an isopropylene moiety, a carbonyl group, and one seven-membered ring attached to a five-membered lactone moiety (Radhika et al., 2002). The Red Sea S. gardineri. Pratt, 1093, yielded a novel cytotoxic cembranoid dimer, singardin (110), which was identified as a dimer of 5-epi-sinuleptolide and sinuleptolide with an epoxide replacing the Δ¹² olefin (Elsayed & Hamann, 1996). Sinularia leptoclados. and S. parva. Tixier-Durivault, 1970, yielded five new norcembranoids, leptocladolides A–C (111, 114, and 115), 1-epi.-leptocladolide (112), and 7E-leptocladolide (113) (Ahmed et al., 2003) with 114 and 115 exhibiting cytotoxic activities. Scabrolides A–D (116–119) were isolated from a Taiwanese S. scarab. Tixier-Durivault, 1970 (Sheu et al., 2002).

Non-cembranoid diterpenes

The first non-cembrane based diterpene, alcyonin (120), had a cladiellane skeleton and was isolated from an Okinawan species of S. flexibilis. (Kusumi et al., 1988). The same species was later reported to yield three lobane diterpenes (121–123) (Hamada et al., 1992). Recently, a cytotoxic lobane diterpene, ineleganene (124), was isolated from the Formosan S. inelegans. and showed cytotoxic activity (Chai et al., 2000). The amphilectane-type diterpenoids, Sinulobatins A–D (125–128), were reported from the Japanese S. nanolobata. Verseveldt, 1977 (Yamada et al., 1997), and exhibited cytotoxicity. The first report of aromadendrane diterpenoids (129–132) in marine systems was made from a new species of Sinularia. of the Indian Ocean whose extract showed moderate larvicidal activity (Anjaneyulu et al., 1997b).

Unusual chemistry of a Sinularia. hybrid

Polymaxenolide (133) is a novel metabolite isolated from the hybrid soft coral Sinularia maxima. x S. polydactyla. (Kamel et al., 2004). It is composed of a cembrane-africanane joined skeleton via a C,C-linkage. It was hypothesized that polymaxenolide represents a new mechanism of qualitative variation in hybrid secondary chemistry. It is formed by the combination of the basic skeleton of one parent (africanane sesquiterpene) with a basic skeleton from the other parental species (cembrane diterpene). To date, no bioactivity has been reported for this compound.

Conclusions

Marine natural products chemistry, and in particular, Alcyonaria chemistry has had a major impact on drug discovery and development over the past 25 years and will continue to inspire the chemistry of the future. The biological and pharmacological properties associated with soft coral terpenoids have only recently been investigated but are shown to be highly promising and support further investigations.

Cembranoid diterpenes isolated from soft corals are known to exhibit cytotoxic and antitumor properties, and future studies investigating the differential activity of these metabolites as well as their mechanism of action will be extremely valuable. The limited data available support the need for more extensive structure-activity relationship studies of cembranoid diterpenes in order to develop lead anticancer compounds. In addition, the anti-inflammatory properties of some Sinularia. metabolites such as africanene and flexibilide require a further evaluation of the mechanism of action. The reasonable yield of these natural products makes them intriguing for semisynthetic and biocatalytic modifications aimed at the reduction of toxicity while enhancing the activity. However, this is not always the case; because one major reason for the absence of biological data of Sinularia. terpenoids in the literature is the low yield. In addition, many researchers do not have access to relevant bioassays.

The discovery of a novel metabolite, polymaxenolide, from a hybrid coral represents an intriguing piece of data that implicates that new gene hybrids have the potential to generate new interesting molecules. Future research should investigate coral hybrid chemistry.

Acknowledgments

The authors are grateful to Dr. Mark Hamann for his thorough reading of the manuscript and Dr. Frank Fronszek for his helpful discussion.