Three new eunicellin-based diterpenoids from an Okinawan Cladiella species of soft coral

Applications of Norrish type I and II reactions in the total synthesis of natural products: a review

Natural products and their analogue have played a key role in the drug discovery and development process. In the laboratory, the total synthesis of secondary metabolites is very useful in ascertaining the hypothetical complex structure of molecules of natural origin. Total synthesis of natural products using Norrish type I and II reactions as a crucial step has been explored in this overview. Norrish reactions are important photo-induced transformations of carbonyl compounds in organic synthetic chemistry and are connected in numerous industrially and biologically relevant procedures and the processing of carbonyl compounds in the atmosphere. The present review tries to focus on the brilliant applications of Norrish type I and II photochemical reactions as a key step in the total synthesis of natural products and highlights on natural sources, structures, and biological activities of the promising natural products for the first time elegantly.

Cladieunicellin T, a New Eunicellin-based Diterpenoid Produced by the Octocoral Cladiella sp

A new eunicellin-based diterpenoid, cladieunicellin T (1), was produced by an octocoral identified as Cladiella sp. The structure of eunicellin 1 was established by extensive spectroscopic methods.

Eunicellin-Based Diterpenoids, Hirsutalins S-V, from the Formosan Soft Coral Cladiella hirsuta

Four new eunicellin-type hirsutalins S–V (1–4), along with a known compound (–)-6α-hydroxy polyanthellin A (5), were isolated from the soft coral Cladiella hirsuta. The structures of the metabolites were determined by extensive spectroscopic analysis. Cytotoxity of compounds 1–5 against the proliferation of a limited panel of cancer cell lines was measured. Anti-inflammatory activity of compounds 1–5 was evaluated by measuring their ability in suppressing superoxide anion generation and elastase release in fMLP/CB-induced human neutrophils.

Eunicellin-based diterpenoids, hirsutalins N-R, from the formosan soft coral Cladiella hirsuta

New eunicellin-type hirsutalins N–R (1–5), along with two known eunicellins, (6 and 7) were isolated from the soft coral Cladiellahirsuta. The structures of the metabolites were determined by extensive spectroscopic analysis. Cytotoxic activity of compounds 1–7 against the proliferation of a limited panel of cancer cell lines was measured. The invitro anti-inflammatory activity of compounds 1–7 was evaluated by measuring their ability in suppressing superoxide anion generation and elastase release in fMLP/CB-induced human neutrophils.

Antifouling eunicellin-type diterpenoids from the gorgonian Astrogorgia sp

Twelve new eunicellin-based diterpenoids, astrogorgins B-M (1-12), were isolated from a Chinese gorgonian Astrogorgia sp., together with ophirin, muricellin, astrogorgin, calicophirins A and B, and 14-deacetoxycalicophirin B. The structures of the new compounds were elucidated by extensive spectroscopic analysis and by comparison with data reported in the literature. Significant antifouling activity was observed for 14-deacetoxycalicophirin B against the larval settlement of the barnacle Balanus amphitrite at nontoxic concentrations with an EC(50) = 0.59 μg/mL, while the other analogues were effective within an EC(50) range of 5.14-17.8 μg/mL.

New 6-hydroxyeunicellins from a soft coral Cladiella sp

Two new 6-hydroxyeunicellin diterpenoids, cladieunicellin G (1) and 6-epi-cladieunicellin F (2), were isolated from an Indonesian octocoral Cladiella sp. The structures of eunicellins 1 and 2 were established by spectroscopic methods and 2 was found to be an epimer of the known eunicellin cladieunicellin F (3). Eunicellin 2 displayed inhibitory effects on the generation of superoxide anion and the release of elastase by human neutrophils.

The 2,11-cyclized cembranoids: cladiellins, asbestinins, and briarellins (period 1998-2010)

The 2,11-cyclized cembranoids are isolated from marine invertebrates of Octocorallia species. They are a very interesting class of natural products sharing a common oxatricyclo[6.6.1.0(2,7)]pentadecane core and carrying a varied substituent pattern. This review presents their structural diversity along with the reported biological activities. The 2,11-cyclized cembranoids were comprehensively reviewed previously in 1998, and this contribution will serve as an update of that work. Since 1998 a number of structural assignments of the isolated products have been revised, some as a result of total synthesis efforts. The chemical reactivity of several of the natural compounds has been studied, and the relevance of these findings to the biosynthesis or the generation of isolation artifacts is discussed. The wide range of biological activities displayed by the 2,11-cyclized cembranoids justifies the interest shown within the synthetic chemistry community and suggests that this class of natural products remains a fruitful area for future synthetic and biological research.

Hirsutalins A-H, eunicellin-based diterpenoids from the soft coral Cladiella hirsuta

Eight new eunicellin-base diterpenoids, hirsutalins A-H (1-8), were isolated from the soft coral Cladiella hirsuta. Their structures were elucidated by spectroscopic methods, particularly in 1D and 2D NMR experiments. The absolute configuration of 1 was determined by Mosher's method. Compounds 1, 5, and 6 have been shown to exhibit cytotoxicity toward several cancer cell lines. Compounds 2-4 and 8 were found to display significant in vitro anti-inflammatory activity in LPS-stimulated RAW264.7 macrophage cells by inhibiting the expression of the iNOS protein, with compound 2 also effectively reducing the level of COX-2 protein.

Pachycladins A-E, prostate cancer invasion and migration inhibitory Eunicellin-based diterpenoids from the red sea soft coral Cladiella pachyclados

Alcyonaria species are among the important marine invertebrate classes that produce a wealth of chemically diverse bioactive diterpenes. Examples of these are the potent microtubule disruptor sarcodictyins and eleutherobin. The genus Cladiella has proven to be a rich source of cytotoxic eunicellin-based diterpenoids. Five new eunicellin diterpenes, pachycladins A-E (1-5), were isolated from the Red Sea soft coral Cladiella pachyclados. The known sclerophytin A Cladiellisin, 3-acetylcladiellisin, 3,6-diacetylcladiellisin, (+)-polyanthelin A, klysimplexin G, klysimplexin E, sclerophytin F methyl ether, (6Z)-cladiellin (cladiella-6Z,11(17)-dien-3-ol), sclerophytin B, and patagonicol were also identified. The structures of the isolated compounds were elucidated by extensive interpretation of their spectroscopic data. These compounds were evaluated for their ability to inhibit growth, proliferation, invasion, and migration of the prostate cancer cells PC-3. Some of the new metabolites exhibited significant anti-invasive activity.

A unified strategy for enantioselective total synthesis of cladiellin and briarellin diterpenes: total synthesis of briarellins E and F and the putative structure of alcyonin and revision of its structure assignment

Enantioselective total syntheses of briarellin E (12) and briarellin F (13), as well as the structure originally proposed for the cladiellin diterpene alcyonin (10), have been realized. Comparison of the spectral data for synthetic 10, natural alcyonin, cladiellisin (33), and cladiellaperoxide (34), as well as chemical transformations of 10 and natural alcyonin, suggest that the structure of this coral metabolite is allylic peroxide 11. The unified approach detailed herein can be used to access both C4-deoxygenated and C4-oxygenated cladiellins and briarellins. The central step in these syntheses is acid-promoted condensation of (Z)-alpha,beta-unsaturated aldehydes 17 with cyclohexadienyl diols 18 to form intermediates 16 incorporating the hexahydroisobenzofuran core and five stereocenters of these marine diterpenes (Scheme 1 ).

A General Strategy for Synthesis of Both (6Z)- and (6E)-Cladiellin Diterpenes: Total Syntheses of (−)-Cladiella-6,11-dien-3-ol, (+)-Polyanthellin A, (−)-Cladiell-11-ene-3,6,7-triol, and (−)-Deacetoxyalcyonin Acetate

The first total synthesis of an (E)-cladiellin diterpene, (-)-cladiella-6,11-diene-3-ol (1), was accomplished featuring an intramolecular amide enolate alkylation-intramolecular Diels-Alder strategy. In addition, a highly stereo-, regio-, and chemoselective synthetic strategy for other members of the cladiellin diterpenes such as (+)-polyanthellin A (2), (-)-cladiell-11-ene-3,6,7-triol (3), and (-)-deacetoxyalcyonin acetate (4) was developed utilizing the synthetic (E)-cladiellin 1 as a common intermediate by taking advantage of the unique chemical properties of its C(6)-(E)-oxatricyclic skeleton.

Two new pregnane glycosides from soft coral Cladiella krempfi

Two new pregnane-type steroidal glycosides, pregna-5,20-dien-3-O-alpha-fucopyranoside (1) and pregna-20-en-3-O-alpha-fucopyranoiside (2), along with two known pregnane derivatives, were isolated from the soft coral Cladiella krempfi. Their structures were determined by spectroscopic data analysis.

Eunicellin-based diterpenoids, australins A-D, isolated from the soft coral Cladiella australis

The EtOAc extract of a Taiwanese soft coral, Caldiella australis, yielded four new eunicellin-based diterpenoids, australins A-D (1-4). The chemical structures of these metabolites were determined on the basis of extensive spectroscopic (including 1D and 2D NMR) analyses and by comparison of their NMR spectral data with those of related metabolites. Metabolite 2 was found to possess a moderate cytotoxic activity against selected breast and liver cancer cell lines.

Eunicellin diterpenes from two Kenyan soft corals

Six new eunicellin diterpenes designated klyxumines A and B (1, 2) and epoxycladines A-D (3-6) were isolated from Klyxum flaccidum and Cladiella kashmani, collected in Kenya. The structures of the compounds were elucidated by interpretation of MS, COSY, HMQC, HMBC, and NOESY data.

Toward a general route to the eunicellin diterpenes: the asymmetric total synthesis of deacetoxyalcyonin acetate

A novel approach to the hydroisobenzofuran core utilizing a TiCl4-mediated [4+3] annulation is reported. This [4+3] annulation protocol provides a short, general route to the hydroisobenzofuran core present in the eunicellin diterpenes. Using this annulation, a short synthesis (17 steps) of deacetoxyalcyonin acetate, a member of the eunicellin family, has been achieved.

A general strategy for the synthesis of cladiellin diterpenes: enantioselective total syntheses of 6-acetoxycladiell-7(16),11-dien-3-ol (deacetoxyalcyonin acetate), cladiell-11-ene-3,6,7-triol, sclerophytin A, and the initially purported structure of sclerophytin A

Enantioselective total syntheses of the cladiellin diterpenes, 6-acetoxycladiell-7(16),11-dien-3-ol (deacetoxyalcyonin acetate, 6), cladiell-11-ene-3,6,7-triol (1), sclerophytin A (8), and tetracyclic diether 7, have been achieved by differential elaboration of tricyclic allylic alcohol 57. The central step in these syntheses is acid-promoted condensation of alpha,beta-unsaturated aldehydes 45, 69 or 87, and cyclohexadienyl diol 44 to form, with complete stereocontrol, the hexahydroisobenzofuran core and five stereocenters of these cladiellin diterpenes. These syntheses also feature stereospecific photolytic deformylation of beta,gamma-unsaturated aldehydes 46, 70, and 71 to remove the extraneous carbon introduced in the Prins-pinacol step; chemo- and stereoselective hydroxyl-directed epoxidation of 49, 72, and 90 followed by regioselective reductive opening with hydride to install the C3 tertiary hydroxyl group; and a diastereoselective Nozaki-Hiyama-Kishi cyclization of iodoaldehyde 56 to forge the oxacyclononane ring and the C6 hydroxyl stereocenter. Other key transformations include chemo- and stereoselective hydroxyl-directed epoxidation of tricyclic allylic alcohol 57 followed by regioselective reductive opening with hydride to install the C7 tertiary hydroxyl center of 1 and 8; chemo-, regio-, and stereoselective intramolecular oxymercuration-reductive demercuration of dienyl diol 62 to form the bridging tetrahydropyran ring of tetracyclic diether 7; and photochemical isomerization of the endocyclic double bond of 92 and 1 to give exocyclic congeners 7 and 8. The absolute stereochemistry of the synthetic products originates from two chiral nonracemic starting materials, (S)-(+)-carvone and (S)-(-)-glycidol. These syntheses define a versatile and concise strategy for the total synthesis of cladiellin diterpenes and provide additional illustrations of the uncommon utility of pinacol-terminated cationic cyclizations for the stereocontrolled synthesis of complex oxacyclic products.

Cembrane diterpenes from the southern African soft coral Cladiella kashmani

Two known cembrane diterpenes, flaccidoxide (1) and (1Z,3E,7E,11S, 12S,14S)-11,12-epoxy cembra-1,3,7-trien-14-ol (2), and the new cembrane diterpene flaccidoxide-13-acetate (3) were isolated from specimens of Cladiella kashmani collected off Ponto do Oura, Mozambique. The modified Mosher's method established the previously unassigned absolute configuration of 1 as (1Z,3E,7E,11S,12S,13S, 14R)-14-acetoxy-11,12-epoxy cembra-1,3,7-trien-13-ol. Acetylation of 1 yielded 3 and thus confirmed the structure of 3 as (1Z,3E,7E,11S, 12R,13S,14R)-13,14-diacetoxy-11,12-epoxy cembra-1,3,7-triene. All three diterpenes were toxic to the brine shrimp Artemia salina.