Terpenoids from Octocorals of the Genus Pachyclavularia

In this paper, we reviewed natural compounds isolated from octocorals belonging to the genus Pachyclavularia, including 20 cembrane-, 39 briarane-, and eight briarellin-related diterpenoids, and one secosterol. The chemical constituents of these 68 secondary metabolites, and their names, structures, and bioactivities, along with studies of their biological activities, are summarized in this review. Based on the literature, many of these compounds possess bioactivities, including anti-inflammation properties, cytotoxicity, and ichthyotoxicity, suggesting that they may have the potential to be developed into biomedical agents for treatment.


Introduction
Octocorals of the genus Pachyclavularia (phylum Cnidaria, class Anthozoa, order Alcyonacea, family Briareidae) [1], which originally belonged to the family Tubiporidae [2], distributed in the subtropical and tropical waters of the Indo-Pacific Ocean, have been investigated. Since the initial study in 1979 discovered a cembranoid diterpene, pachyclavulariadiol (1), from the Australian octocoral P. violacea [3], subsequent studies over the past few decades have yielded a series of interesting secondary metabolites, particularly diterpenoids originally synthesized from cyclized cembranoids, including briarane-and briarellin-related analogues (Scheme 1) [4]. In this article, we have listed the different types of compounds isolated from two Indo-Pacific octocorals, Pachyclavularia violacea (Scheme 1) and Pachyclavularia sp., and summarized their chemical constituents and research into the biological activities of these metabolites. Scheme 1. Octocoral P. violacea and the proposed biosynthetic pathways of the cembrane, briarane, and briarellin carbon skeletons [4].

Cembranoids from Pachyclavularia violacea (Quoy and Gaimard, 1833) (=Clavularia violacea, Briareum violaceum)
The Pachyclavularia genus includes one very common species, Pachyclavularia violacea [2,3]. Bowden and colleagues [3] isolated several compounds, including three new cembrane-type diterpenes, pachyclavulariadiol (1), and its naturally-derived mono-and di-acetylated derivatives (2 and 3), from octocoral P. violacea collected in Australia ( Figure 1). The study also established the structures, including the relative configurations, of new furanocembranoids 1-3 and determined the configuration of 1 by spectroscopic/chemical methods and X-ray diffraction, respectively. In 1989, Inman and Crews [5] reported the isolation of a new cembranolide from P. violacea collected from Vanuatu waters, and named the compound pachyclavularolide (4) (Figure 2). Later, the structure of 4, including the relative configuration, was determined using spectroscopic methods and molecular mechanics calculations, and further confirmed by single-crystal X-ray analyses in a study by Sheu et al. [6]. The former study was the first to present a detailed conformational analysis of a 14-membered cembrane ring [5]. Two new cembranes, pachyclavulariolides E (5) and F (6) (Figure 2), were isolated from P. violacea samples collected from waters off Madang in Papua New Guinea in 2000 [7]. The structures of 5 and 6, including the relative configurations, which were elucidated from spectroscopic data [7], and the structure, including the relative configuration of 5, was further confirmed from single-crystal X-ray analysis by Sheu et al. [6]. A cytotoxic assay showed that compound 6 had an IC 50 value of 1.0 µg/mL in the treatment of murine leukemia P388 cells [7], though this compound was claimed to be an artifact due to methanol used in the extraction procedure [7].
A study performed by Cheng et al. revised the structure of solenolide D (57) [31], and this compound was later found to be a potent anti-inflammatory agent with an efficacy comparable to that of indomethacin [27]. Potency in excess of a 70% reduction of edema was observed at a concentration of approximately 15 µg/per ear (mouse ear assay) [27].

Briarellins from Pachyclavularia violacea
Eight novel briarellins, including pachyclavulariaenones A-G (60-66) [32,33] and secopachyclavulariaenone A (67) [6] (Figure 12), were isolated from P. violacea collected off Kenting Coast, Taiwan. Pachyclavulariaenone B (61) was also isolated from P. violacea specimens collected in the Togian Islands near Sulawesi Island, Indonesia [34]. The structures, including the relative configurations, of briarellins 60-67 were established by spectroscopic methods, and the structures, including the relative configurations, of briarellins 62 (pachyclavulariaenone C) [32] and 65 (pachyclavulariaenone F) [33] were further confirmed by single-crystal X-ray diffraction analyses. Briarellins 60-67 were found to be unprecedented examples of this type of compound, as they possess a six-membered carbocyclic ring cis-fused to both the ten-membered carbocyclic ring and seven-membered ether ring, making three ring-junction protons cis to each other [6,32,33]. Cytotoxic assays revealed that pachyclavulariaenone G (66) exhibited cytotoxicity towards P388 and HT-29 tumor cells, with ED 50 values of 0.2 and 3.2 µg/mL, respectively [33]. It is obvious to note that the hydroxy groups at C-4 and C-6 in 66 caused this compound to be more cytotoxic than its analogues 62, 64, and 65.

Conclusions
Ever since pachyclavulariadiol (1) was obtained from a specimen of the octocoral P. violacea collected in Australia [4], 68 interesting secondary metabolites, including 20 cembranes (29.41%), 39 briaranes (57.35%), eight briarellins (11.76%), and one secosterol (1.47%), have been isolated from octocorals belonging to the genus Pachyclavularia. Many studies have reported that compounds obtained from Pachyclavularia spp. possess various biologic activities, such as cytotoxicity (seven cembranes, six briaranes, and one briarellin), anti-inflammatory activity (three briaranes), and ichthyotoxicity (one briarane). It is interesting to note that all compounds from Pachyclavularia spp. reported as having been isolated between 1979 and 2014 were collected from octocorals distributed in the Indo-Pacific Ocean. The briarane-type natural products act as a chemical marker of octocorals belonging to the family Briareidae. Up to date, 379 briaranes [35][36][37][38][39][40], 13 eunicellins, 34 asbestinane, one cembrane, and one pyranone [4], have been isolated from the octocorals belonging to the family Briareidae, including the genera Briareum and Erythropodium. As more than 50% of the compounds obtained from the Pachyclavularia genus are briarane-type diterpenoids, we support that the taxonomic position of the Pachyclavularia genus should be readjusted and placed under the family Briareidae. Based on above findings, these results suggest that continuing investigation of new terpenoid analogues with the potential bioactivities from this marine organism are worthwhile for further development. The octocoral Pachyclavularia violacea had been transplanted to culturing tanks located in the National Museum of Marine Biology and Aquarium, Taiwan, for extraction of additional natural products to establish a stable supply of bioactive material.