Briaexcavatins M-P, Four New Briarane-Related Diterpenoids from Cultured Octocoral Briareum excavatum (Briareidae)

Abstract

Gorgonian corals are recognized as rich sources of diterpenoid derivatives with the well-known briarane carbon skeleton, a group of diterpenoids having a highly oxidized bicyclo [8.4.0] system with a g-lactone group. Compounds of this type exist only in marine organisms, particularly in octocorals.1,2) In our screening for novel natural products from the marine invertebrates collected in Taiwanese waters, we have discovered a series of terpenoid and steroid metabolites from the octocorals Alcyonium sp., 3,4) Briareum sp., 5)Briareum excavatum, 6-10) Ellisella robusta, [11][12][13][14] Junceella fragilis, 10,[15][16][17][18][19][20][21][22] Junceella juncea, 17,23) Rumphella antipathies, [24][25][26][27][28] and the tunicate Eudistoma sp. 29); and some of these metabolites have been reported to exhibit interesting biological activity such as anti-inflammatory, 8,9,20,22) cytotoxicity, 8,29) and antibacterial activity. 11,24,26,27) In this paper, we describe the isolation and structure determination of four new briarane-type diterpenoids, briaexcavatins M-P (1-4), from the cultured octocoral Briareum excavatum (Briareidae). The structures of briaranes 1-4 were established by spectroscopic methods. Results and DiscussionSpecimens of the octocoral B. excavatum, collected from the culturing tank in the National Museum of Marine Biology and Aquarium (NMMBA), Taiwan, were minced and extracted with a mixture of MeOH and CH 2 Cl 2 (1 : 1). The extract was further partitioned with EtOAc and H 2 O. The EtOAc layer was successively subjected to silica gel gravity column chromatography and re-purified by normal phase HPLC guided by the signals for the interesting structures found in the proton NMR experiments to afford briaranes 1-4 (see Experimental).Briaexcavatin M (1) . Thus, from the above NMR data, six degrees of unsaturation were accounted for, and compound 1 must be tetracyclic. The presence of a tetrasubstituted epoxide containing a methyl substituent was elucidated from the signals of two oxygenated quaternary carbons at d C 68.2 (s, C-8) and 63.1 (s, C-17), and further confirmed by the proton signal of a methyl singlet resonating at d H 1.57 (3H, s, H 3 -18) ( Four new briarane-related diterpenoids, designated as briaexcavatins M-P (1-4), were isolated from the cultured octocoral Briareum excavatum. The structures, including the relative configurations of natural products 1-4 were established on the basis of extensive spectral data analysis and by comparison with the spectral data from other known metabolites featuring a briarane carbon skeleton.

Briarenols I—K, New Anti-inflammatory 8,17-Epoxybriaranes from the Octocoral Briareum excavatum (Briareidae)

“…The Z-configuration of C-5/C-6 double bond was elucidated by a correlation between H-6 and H 3 -16. The NMR data of 2 were found to be similar to those of a known briarane, briaexcavatin P (5) [9]. It was found that the 2-acetoxy substituent in 5 was replaced by a hydroxy group in 2.…”

“…The compounds of this type are only found in marine invertebrates, particularly in octocorals and demonstrated a wide spectrum of bioactivities, such as anti-inflammatory activity [6] and cytotoxicity [7]. In our continuing research into the chemical constituents of an octocoral B. excavatum (Nutting 1911), which was distributed extensively in the coral reefs of Taiwan, have resulted in isolation of three previously unreported 8,17-epoxybriaranes-briarenols I-K (1-3) along with two known analogues, briaexcavatolide P (4) [8] and briaexcavatin P (5) [9],…”

Briarenol B, a New Polyoxygenated Briarane from the Octocoral Briareum excavatum

“…Therefore, based on the above findings, the s-trans diene moiety in 1 was elucidated. The NMR data of 1 were found to be similar to those of briarane derivatives with a 3E,5Z-diene system, such as briaexcavatin M [9], briaranolides I and J [10], excavatolide F [11] and briarenolide ZII (2) ( Figure 1) [12]. Thus, briarane 1 was identified as the 12-O-debutyryl derivative of 2.…”

Briarenolides F and G, New Briarane Diterpenoids from a Briareum sp. Octocoral

“…The compounds of this type were suggested to be of marine origin and the octocorals belonging to the genus Briareum have been proven to be the most important source of briarane-type compounds [4,5,6,7]. In previous studies, a series of interesting terpenoid derivatives, including briarane [8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35], cembrane [36] and carotenoid [37], had been isolated from the octocorals belonging to the genus Briareum that were distributed in the waters off Taiwan, at the intersection point of the Kuroshio current and the South China Sea surface current. In a continuation of our search for new substances from the Formosan marine invertebrates, the chemical constituents of a specimen octocoral identified as Briareum sp.…”