Natural Product Chemistry of Gorgonian Corals of Genus Junceella—Part II

The structures, names, bioactivities, and references of 81 new secondary metabolites obtained from gorgonian corals belonging to the genus Junceella are described in this review. All compounds mentioned in this review were obtained from sea whip gorgonian corals Junceella fragilis and Junceella juncea, collected from the tropical and subtropical Indo-Pacific Ocean.


Introduction
This review describes 81 new natural products from gorgonian corals belonging to the genus Junceella (phylum Cnidaria, class Anthozoa, order Gorgonacea, family Ellisellidae) [1][2][3][4]. Extending from a previous review in 2004 [5], this review describes compounds reported from November 2003 to September 2011 and provides structures, names, bioactivities and references for all compounds in tabular form.

Junceella fragilis
Two new chlorinated briarane-type diterpenoids (3,8- (Table 1), along with five known briaranes, junceellin, praelolide, and junceellolides A, B and D, were isolated from the gorgonian J. fragilis, collected at the Pass Reef of Madang, Papua New Guinea [6]. The absolute stereochemistry of (−)-3-deacetyljunceellin (2) was determined by the application of a new method using a combination of proton chemical shifts and molecular dynamic calculation. Table 1. The new natural products from Junceella fragilis-I.
From the characteristics of the chemical shifts, it was shown that the briarane derivatives contained an exocyclic double bond between C-11/12. The proton chemical shifts were summed up for the olefin protons H 2 -20; these appear at δ H 4.95-5.30 and 4.85-5.15 ppm, respectively, when the cyclohexane rings are in a twist boat conformation. Likewise, the 1 H NMR data for H 2 -20 appear at δ H 4.95-5.10 and 4.40-4.75, if the cyclohexane rings were found to exist in a chair conformation [22].
Symbiotic algae (zooxanthella) exist throughout the life cycle of J. fragilis, while J. juncea is a gorgonian coral free of zooxanthellae [32]. Two known chlorine-containing briaranes, junceellin and praelolide, were isolated in the same proportions from both J. fragilis and J. juncea, and this observation suggests that junceellin and praelolide could be chemical markers that enable one to infer that the briarane-type compounds are originally synthesized by the host corals [28] and are not produced by their zooxanthella.
The known briaranes, gemmacolides A, B, and junceellolide D, were also found to exhibit an antifouling activity as potent as that of juncins R-ZII [38], and these three compounds were not cytotoxic towards the K562, A549, Hela and Hep2 cells. In addition, all the known briaranes showed medium antifeedant activity toward the second-instar larvae of Spodoptera litura at a concentration of 500 µg/mL [38].

Conclusions
The chemical class distribution of the natural products obtained from the organisms Junceella fragilis and Junceella juncea compiled in this review indicates that terpenoid derivatives, particularly briarane-type diterpenoids, are the major components of the natural products isolated. Of the 81 new metabolites, 74 compounds are briarane-type diterpenoids (91.4%). Of these briaranes, over 50% are chlorinated briaranes (38/74 = 51.4%), which are rarely found. Briarane-type compounds continue to attract attention owing to their structural novelty, complexity and interesting bioactivities, such as anti-inflammatory activity [48][49][50][51]. Terpenoid compounds are often present in large amounts in marine invertebrates, and as a major class represent the largest percentage of natural products isolated from marine organisms [52]. Over 500 naturally-occurring briarane derivatives have been isolated from various marine organisms [48][49][50][51]. However, owing to their structural complexity, it is difficult to obtain sufficient amounts of the bioactive metabolites, such as junceols B (75) and C (76), for further study of their potential medicinal usage. We have therefore begun to culture the potential useful gorgonian corals J. fragilis and J. juncea (Figure 1) in tanks using our highly developed aquaculture technology for extraction of natural products to establish a stable supply of bioactive materials, which also protects the natural population and habitats from over-exploitation.