Newly discovered naturally occurring organohalogens

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Introduction
From fewer than 50 known naturally occurring organohalogens in 1968, 1 the number today -fifty years lateris more than 5,000.Three comprehensive reviews are available, [2][3][4] as are more recent separate compilations of halogenated alkaloids, 5 marine natural products, 6 and halogenated heterocyclic compounds that occur naturally. 7A recent review of naturally occurring organoiodides has also appeared, 8 and the annual review of marine natural products routinely covers new halogenated marine compounds for that year. 9It should be noted that previously known organohalogens and non-halogenated natural products discovered in the cited papers are not included in this review.

Sponges
Like most marine plants, but unlike fish and marine animals, sponges are anchored to the reef and must employ chemical defense for survival against predation.As we will see, the structural diversity of sponge metabolites is astonishing.It must be noted that the actual origin of some of these metabolites may be bacteria or microalgae associated with the sponge.This point will be visited later.0][21] Also reviewed last year were the metabolites from the sponge genus Agelas, 22 and the cyclic azole-homologated peptides from sponges. 23 common structural motif adopted by marine sponges is the pyrrole heterocycle.For example, 32 bromopyrrole alkaloids were isolated from the sponge Stylissa massa collected in Hainan Island, China.Of these metabolites five are new, stylisines A -E (50 -54), which include the enantiomeric pair 53 and 54 (Figure 8).The sponge Agelas sp. is a fruitful producer of bromopyrrole alkaloids and a recent collection of Agelas sp. from the South China Sea afforded four new dimeric bromopyrrole metabolites, hexazosceptrin (55), agelestes A and B (56, 57), and (9S,10R,9'S,10'R)-nakamuric acid (58) (Figure 9). 25 The absolute configuration of all four metabolites was established.None of these compounds show activity against lymphoma U937 and lung cancer PC9 cells.A deep-water (140 m) Palau Topsentia sp.sponge contains two novel brominated indoles, tulongicin A (59) and dihydrospongotine C (60), in addition to two known analogues (Figure 10). 26Their absolute configuration was determined, and both metabolites display strong antimicrobial activity against Staphylococcus aureus (MIC = 1.2 -3.7 µg/mL).An examination of the Indonesian sponge Oceanapia sp.yielded the indole metabolite 6-bromo-8-ketoconicamin A (61), which displays strong activity against the human pancreatic cancer cell line PANC-1 (IC50 = 1.5 µM). 27Four novel thiazole containing biakamides A -D (62 -65) were isolated from another Indonesian sponge, Petrosaspongia sp (Figure 11).These unique polyketides are also active against PANC-1 (IC50 = 0.5 -4.0 µM). 28Total syntheses of all possible stereoisomers from the optically pure monoprotected 2,4-dimethyl-1,5-diol established the absolute configurations of the two secondary methyl groups.Sponges have a proclivity for incorporating the bromotyrosine unit in their metabolites 2,3 and several new examples were reported in 2017.The Madagascan sponge Amphimedon sp.contains amphimedonoic acid (66) and psammaplysene E (67), along with the known 3,5-dibromo-4-methoxybenzoic acid. 29Neither compound is active against human epidermoid carcinoma KB cells (IC50 > 10 µg/mL).A sponge from Western Australia, Pseudoceratina cf.verrucosa, yielded pseudoceratinamide A (68) and B (69) and the enantiomer 70 of a previously known bromotyrosine (Figure 12). 30The Indonesian sponge Iotrochota cf.iota yielded seven new halogenated tyrosines, enisorines A -E (71 -75), (+)-1-O-methylhemibastadinol 2 (76), and (+)-1-O-methylhemibastadinol 4 (77) (Figure 13). 31All seven metabolites inhibit T3SS-dependent YopE secretion, which is a virulence factor employed by many Gramnegative pathogens that injects bacterial effector proteins into host cells to negate host cell defenses.

Corals
Whereas a few hard (stony) corals liberate organohalogen metabolites, soft corals (octocorals, sea fans, gorgonians) are extraordinarily generous producers of halogen-containing metabolites. 2,3These stunning marine animals, gently swaying in the reef currents, are both a delight for the scuba diver and a treasure trove for the marine natural products chemist seeking new metabolites.
3][4] A collection of the octocoral Briareum excavatum from Taiwanese waters furnished three new briarenols, one of which, briarenol E (78), contains chlorine. 32The gorgonian coral Junceella fragilis from Hainan Island, China, contains four pairs of chlorinated briarane diterpenes, five of which are new (79 -83) (Figure 14). 33Interestingly, these pairs of isomers undergo acetyl migration (i.e., 80 ⇄ 81, and 82 ⇄ 83), which was observed for the first time.The acetyl migration of 79 yields the previously known fragilide J (2-deacetylpraelolide), which is also present inn this coral.All of these metabolites inhibit the production of nitric oxide in RAW 264.7 cells.In addition to several known diterpenoids, another collection of Junceella fragilis from Hainan Island by this same research group yielded seventeen new diterpenoids, ten of which contain chlorine (84 -93) (Figure 15). 34Fragilolides D (86), G (89), and P (93) are inhibitory towards hepatitis B e antigen (HBeAg), but not active against the expression of hepatitis B surface antigen (HBsAg).None of these metabolites are cytotoxic in a panel of tumor cell lines.

Tunicates
Tunicates (ascidians, sea squirts) belong to subphyllum Urochordata (or Tunicata) of the phyllum Chordata, Class Ascidiacea.Like sponges, tunicates are filter feeders and rely on chemical defense for survival.They may be solitary or colonial marine animals.Some 3,000 species of tunicates have been described, 35 and a recent review is available. 36he Korean tunicate Pseudodistoma antinboja yielded four new cadiolides J-M (94-97) (Figure 16), 37 the only halogenated tunicate metabolites described in 2017.

Marine fungi
A relatively new source of metabolites is marine fungi and several examples were reported in 2017.

Marine bacteria
Bacteria live everywhere, including in the marine environment.A few halogenated bacterial metabolites from the oceans were described in 2017.Cultures of the marine bacterium Pseudovibrio denitrificans Ab134, which were isolated from the sponge Arenosclera brasiliensis, yielded several known bromotyrosine-derived alkaloids that were previously only isolated from marine sponges. 47This work shows for the first time that bromotyrosine-derived alkaloids can be biosynthesized by a marine bacterium.

Other marine organisms
Marine brittle stars (Ophiuroidea) are a very large group of reclusive, solitary echinoderms that inhabit the world's oceans.They are typically found hiding under rocks.
A brittle star, Ophionereis reticulata, from the coast of Brazil contains two known chamigrene sesquiterpenes in addition to the novel acetyl isoobtusadiene (139). 63The known metabolites found in this animal are elatol and isoobtusadiene, which are common in Laurencia red algae, suggesting a dietary origin for these compounds.The halogenated diterpene dolabellol A (140) was characterized from the opisthobranch Dolabella auricularia found living off the Japanese coast (Figure 28). 64The absolute configuration of 140 was established by a combination of spectroscopy, chemical degradation, and X-ray crystallography.As in the previous study, it is suggested that this animal acquires dolabellol A from its algae diet. 65Accordingly, dolabellol A is structurally similar to the metabolites obtusadiol, rogioldiol A, laurenditerpenol, and 14bromoobtus-1-ene-3,11-diol, all of which are found in the algae on which Dolabella auricularia feeds.

Terrestrial plants
Lacking the huge concentration of halide (i.e., chloride, bromide) in the oceans, terrestrial organisms manufacture far fewer halogen-containing metabolites than their marine counterparts. 2,3Nevertheless, last year several new examples of organochlorine compounds in terrestrial plants, fungi, and bacteria were reported.An excellent review of chlorinated plant steroids and their biological activities has appeared. 66A series of steroidal withanolides, including eleven new examples and two known chlorine-containing ones, were isolated from the plant Physalis peruviana L. (Solanaceae), and were evaluated for their cytotoxicity against prostate and renal cancer cells. 67The two known chlorinated examples (not shown) physalolactone and 4-deoxyphysalolactone 2 are inactive.
The small shrub Uvaria alba Merr.from Luzon Island, Philippines, and reported to have anti-infective and cytotoxic activities, 68 contains two novel chlorine-containing polyoxygenated seco-cyclohexenes, albanols A (141) and B (142). 69Both albanols show modest activity against Mycobacterium tuberculosis H37Rv (MIC = 26 -38 µM), and albanol A exhibits cytostatic activity towards HeLa cells.A collection of the traditional medicinal plant Cleistochlamys kirkii from Tanzania yielded 13 new metabolites including two chlorinated cyclohexenes, cleistenechlorohydrins A (143) and B (144) (Figure 29). 70This plant is a member of the genus of the family Annonaceae and is native to several eastern and southern African countries.For example, in Mozambique this plant is used to treat wound infections, rheumatism, and tuberculosis. 71Chinese agarwood (Aquilaria sinensis Lour.)Gilg.(Thymelaeaceae) is the source of three new chlorinated 2-(2-phenylethyl)chromones, 145 -147, in addition to two non-chlorinated analogues and 11 known compounds. 72The absolute configurations were determined and these compounds exhibit strong inhibition of nitric oxide production in RAW 264.7 cells (IC50 = 3.8-7.3µM).A collection of Seidlitzia rosmarinus from the Sinai desert shoreline of the Gulf of Aqaba, Egypt, yielded the novel, isomeric -chloroferuloylamides 148 and 149 (Figure 30). 73The traditional Chinese medicine plant Curculigo orchioides ("Xianmao") collected from Yunnan Province, China, afforded five new chlorinated phenolic glycosides, curculigines J -N (150 -154). 74Three related glycosides were isolated from Przewalskia tangutica (Solanaceae), przewatangosides A -C (155 -157), a plant found in the Tibet region of China (Figure 31). 75Only 155 shows (weak) activity against SMMC-7721 (liver carcinoma) (IC50 = 38.1 µM).The Chinese herb Valeriana jatamansi (Caprifoliaceae), which is an important traditional Chinese medicine for the treatment of nervous disorders, epilepsy, insanity, snake poisoning, and skin diseases, 76 furnished three new chlorinated iridoids, chlorovaltrates P -R (158 -160), in addition to five previously known chlorinated iridoids. 77A collection of Phlomis likiangensis (Lamiaceae) from Yunnan, China, yielded six new iridoids including the two new chlorine-containing phlolosides E (161) and F (162) (Figure 32). 78Like Valeriana jatamansi (vide supra), the Phlomis genus is used as a herbal tea to cope with gastrointestinal diseases and other disorders. 78Chlorine-containing iridoids are probably the largest collection of organochlorines present in the terrestrial environment.Another traditional Chinese medicine plant for pain relief dating back to the 1 st century is Rhododendion molle G.Don.A gathering of the fruits of this plant from Guangxi Province, China, led to the discovery of three new chlorinated diterpenoids rhodomollein XXXI -XXXIII (163 -165) in a group of 12 new compounds isolated in this study (Figure 33). 79All three metabolites show significant antinociceptive activity, especially 164 and 165 at a very low dose (2 mg/kg).In contrast to chlorinated terpenoids (vide supra), chlorinated plant alkaloids are exceedingly rare. 2,3,5wo examples were identified in 2017.The plant Ficus fistulosa var.tengerensis (Moraceae) from Malaysia contains the novel tengechlorenine (166) as a pair of phenanthroindolizidine enantiomers. 80This alkaloid shows strong cytotoxicity against three breast cancer cell lines, MDA-MB-468, MDA-MB-231, and MCF-7 (IC50 = 0.038 -0.91 µM).The widely distributed plant Rauvolfia vomitoria (Apocynaceae) contains the unusual alkaloid rauvomine A (167). 81This plant is found in the tropical regions of Africa and Asia, and has been used to treat fever, gastrointestinal and liver diseases, pain, and some cancers.

Terrestrial fungi
Terrestrial fungi are prodigious fabricators of natural products, many of which contain halogen.New examples are discovered yearly. 2,3he genus Colletotrichum includes a large group of fungal plant pathogens that cause disease to many crops, and this fungus is extremely detrimental to agriculture. 82An examination of the fungus Colletotrichum higginsianum afforded the novel colletochlorins G (168) and H (169) along with five known related metabolites. 83The absolute configuration of the colletochlorins could not be determined.Five new and five known metabolites were isolated from the ubiquitous fungus Aspergillus unguis that includes the novel aspergillusethers C (170) and D (171) (Figure 35). 84This fungal sample was collected from Surat Thani province in Thailand.The dichlorinated 171 is 4 to 8 times more active than the monochlorinated 170 in antifungal activity towards Candida albicans, Cryptococcus neoformans, and Penicillium marnefeii (MIC = 16, 8, 16 µg/mL, respectively).Fermentation broths of Penicillium concentricum, an endophytic fungus of the Liverwort Trichocolea tomentella (Trichocoleaceae), produced an array (more than 20) of metabolites including the novel 172 -175. 85Metabolites 172 and 173/174 are cytotoxic to MCF-7 breast cancer cells (IC50 = 8.4 and 9.7 µM, respectively).This liverwort was collected in Newport, Virginia.The mangrove Bruguiera sexangula var.rhynchopetala from the South China Sea is host to the fungus Penicillium citrinum HL-5126, and the latter produces the chlorinated xanthone 176 and anthraquinone 177 (Figure 36). 86Metabolite 177 is antibacterial against Vibrio parahaemolyticus (MIC = 10 µM).The fungal pathogen Cochliobolus australiensis was isolated from infected leaves of the invasive weed "buffel grass" Pennisetum ciliare (syn.Cenchrus ciliaris), which is a major problem in southern Arizona.From this pathogen were isolated the new metabolites chloromonilinic acids C (178) and D (179), in addition to some previously known metabolites. 87The stem bark of the Chinese tree Melia azedarach Linn.from Jiangsu Province, China, is associated with the fungus Pestalotiopsis sp.Extraction of this fungus led to the novel pestalachloride G (180) as a racemate. 88It shows strong antimicrobial activity against several pathogenic bacteria (Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, and Bacillus subtilis; MIC50 = 4.1, 15.0, 13.5, 16.5 µg/mL, respectively).Both enantiomers of 180 show similar bioactivities.Cultures of the fungus Helminthosporium velutinum yone96, which was isolated from dead twigs of a woody plant from Yakushima Island, Japan, led to cyclohelminthol X (181). 89This complex metabolite shows activity against COLO 201 (colon) and (especially) HL-60 (leukemia) cells (IC50 = 16 and 0.35 µM, respectively), and displays proteasome inhibition.Some fungal metabolites are structurally simple, such as 8-chloroxylarinol A (182) isolated from Malbranchea flavorosea (Figure 37). 90Metabolite 182 is a strong inhibitor of -glucosidase.The genus Malbranchea (Myxotrichaceae) is a worldwide soil-based fungus.A classic antifungal agent is griseofulvin and a new derivative of it was isolated from Penicilium griseofulvum CPCC 400528, 4'-demethoxy-4'-N-isopentylisogriseofulvin (183), along with several other metabolites including griseofulvin. 91This new metabolite is active against HIV (IC50 = 33.2µM).The mangrove plant Sonneratia caseolaris in Hainan Province, China, is associated with the endophytic fungus Penicillium janthinellum HDN13-309 and the latter yielded a series of alkaloids, including the chlorine-containing penicisulfuranols A (184) and D (185) along with four non-chlorinated analogues (Figure 38). 92Metabolite 184 shows potent cytotoxicity towards HeLa and HL-60 cells (IC50 = 0.5 and 0.1 µM, respectively).These novel compounds possess a rare 1,2-oxazadecaline core and the unusual spiro-furan ring.The legume-infesting fungus Diaporthe toxica causes fatal liver disease in lupin-fed sheep.The major responsible toxin is phomopsin A. The present study discovered a new metabolite of this fungus, phomopsin F (186), the N-methylated derivative of phomopsin A. 93 The new dichlorinated dehydrocurvularin 187 was characterized from Alternaria sp.AST0039, which is a fungal endophyte of Astragalus lentiginosus (Fabaceae, "spotted locoweed"), collected in central Arizona. 94The chlorinated lepistatins A -C (188 -190) were isolated from the culture broth of the basidiomycete Lepista sordida (Figure 39). 95No significant antibacterial and antiproliferative activities of these lepistatins are observed at 25 µg/mL.Botrysphaeria laricina is a fungus associated with the moss Rhodobryum umgiganteum living in Yunnan Province, China.This fungus produces the new chlorinated cyclohexenones botrysphones A (191) and C (192), along with a suite of other metabolies, including the known chlorosphaeropsidone. 96 The novel cosmochlorins D (193) and E (194) were characterized from endophytes associated with the shrub Ficus ampelas (Moraceae) (Figure 40).The producing organism is Phomopsis sp.N-125. 97Both 193 and 194 are cytotoxic towards HL-60 cells (IC50 = 6.1 and 1.8 µM, respectively).

Terrestrial bacteria
No other organism produces metabolites more complex than those produced by terrestrial bacteria.Some of these natural products are life-saving antibiotics, like the glycopeptide vancomycin.

Slime mold
Once classified as a fungus, slime mold (slime mould) is the name given to several unrelated organisms that either live freely as single cells or as unified structures.Some 900 species are known worldwide.
The bacteria-eating slime mold Dictyostelium monochasioides produces eight chlorinated alkylresorcinols, monochasiols A-H (212-219), the structures of which were confirmed by synthesis (Figure 44). 104onochasiol A (212) inhibits the concanavalin A-induced interleukin-2 production in Jurkat cells, which is a human T lymphocyte cell line.

Miscellaneous
Two other new, and quite surprising, natural sources of organohalogens were described in 2017.
As do termites, 105 red wood ants produce CH3Cl, CHCl3, CCl4, and CHBr3. 106For example, the average concentrations in the nests of Formica rufa and Formica polyctena are up to three-fold higher than the atmospheric background and as much as 70-fold higher than volcanic emissions, which are generally considered as one of the main geogenic sources of CHCl3.Bromoform was up to 20-fold higher than atmospheric background.
An astounding finding was the interstellar detection of CH3Cl and CH3F in the gas surrounding the protostar IRAS 16293-2422, and the presence of CH3Cl in the coma of the comet 67P/Churyumov-Gerasimenko. 107 The authors of this discovery calculate that approximately 600 tons/year of CH3Cl could have been delivered to the young earth based on the cometary CH3Cl abundance and during the heavy bombardment over 80 million years.This amounts to 50 gigatons of CH3Cl.
Halocarbon emissions from marine phytoplankton as influenced by climate change was reviewed in 2017. 108,109Some 40 low-molecular weight organohalogens were evaluated in these studies.Also evaluated were the formations of haloacetic acids and trihalomethanes that are produced by peracetic acid and chlorine drinking water disinfection processes. 110No new organohalogens were described in either of these three investigations.

Conclusions
Contrary to the widespead belief -pervasive prior to 1980 -that "nature would never make halogencontaining natural products," the past several decades have shown that literally thousands of organohalogen natural products have now been identified, 2,3 approximating 6,000!As we have seen in this brief review, the frequency of the discovery of new naturally occurring organohalogen compounds -100-200 per year -has sustained for the year 2017.Marine and terrestrial organisms alike continue to amaze with their inexorable output of novel halogen-containing compounds.Given the fact that of the 500,000 estimated marine organisms 111 -which are the source of most organohalogens -only a small percentage have been investigated for their chemical content, it is certain that myriad new natural organohalogens are awaiting discovery.Of the estimated 1.5 million species of fungi, secondary metabolites have been characterized from only 5,000 species. 112The future is bright for the collector of naturally occurring organohalogens!This increased activity in natural products research, of all types, may be attributed to modern collection methods (e.g., SCUBA and remote submersibles for the collection of previously inaccessible marine organisms), selective bioassay for identifying biologically active compounds, powerful multidimensional NMR and mass spectral techniques for characterizing sub-milligram amounts of compounds, and new separation and purification techniques (e.g., counter-current chromatography and HPLC).All of this ensures that even the most structurally intricate natural products can be identified.Finally, knowledge and appreciation of folk medicine and ethobotany will continue to guide chemists to new biologically active natural products, including organohalogen compounds.
But what is the raison d'être for the existence of natural organohalogens?This is the big question!Some organohalogens function as pheromones and hormones, as antifeedants and antifoulants, as natural pesticides, as recyclers of halogen, and as structural proteins.Many organohalogen compounds display enormous biological activity that may lead to clinical drugs of benefit to mankind.

Figure 3 .
Figure 3. Structures of new Laurencia sp.halogenated metabolites from Japanese costal waters.

Figure 11 .
Figure 11.Structures of new Indonesian sponge metabolites.

Figure 19 .
Figure 19.Structures of new chlorinated metabolites from marine fungi.

Figure 21 .
Figure 21.Structures of new chlorinated metabolites from marine fungi.

Figure 22 .
Figure 22.Structures of new brominated metabolites from marine bacteria.

Figure 24 .
Figure 24.Structures of new chlorinated metabolites from cyanobacteria.

Figure 25 .
Figure 25.Structures of new chlorinated metabolites from Moorea sp.cyanobacteria.

Figure 28 .
Figure 28.Structures of new halogenated metabolites from marine animals.

Figure 29 .
Figure 29.Structures of new chlorinated metabolites from terrestrial plants.

Figure 30 .
Figure 30.Structures of new chlorinated metabolites from terrestrial plants.

Figure 31 .
Figure 31.Structures of new chlorinated phenolic glycosides from terrestrial plants.

Figure 34 .
Figure 34.Structures of new chlorinated alkaloids from terrestrial plants.

Figure 35 .
Figure 35.Structures of new chlorinated metabolites from terrestrial fungi.

Figure 36 .
Figure 36.Structures of new halogenated metabolites from terrestrial Penicillium sp.fungi.

Figure 37 .
Figure 37. Structures of new chlorinated metabolites from terrestrial fungi.

Figure 38 .
Figure 38.Structures of new chlorinated metabolites from terrestrial Penicilium fungi.

Figure 39 .
Figure 39.Structures of new chlorinated metabolites from terrestrial fungi.

Figure 40 .
Figure 40.Structures of new chlorinated metabolites from terrestrial fungi.

Figure 41 .
Figure 41.Structures of new chlorinated metabolites from terrestrial bacteria.

Figure 42 .
Figure 42.Structures of new chlorinated cyclic peptides from terrestrial bacteria.

Figure 43 .
Figure 43.Structures of new chlorinated cyclic peptides from terrestrial bacteria.