Eight-Membered Rings With One Oxygen Atom

marine polyether ladders, points out some mechanistic hypotheses for the contiguous rings formation that questioned the well-established structure of maitotoxin. 33 Meanwhile, based on NMR studies, the conformation of maitotoxin and yessotoxin (described later), mode of action and their interactions in biomembranes, have been briefly reviewed by Murata et al. 34


Introduction
This chapter appears as a complement of the material and information previously published in Comprehensive Heterocyclic Chemistry II (CHEC-II) 1 and CHEC-III, 2 providing a privileged point of entry to the eight-membered ring oxygen heterocycle systems chemistry.
Throughout the last decade, the numerous reviews on the synthesis of naturally occurring derivatives have highlighted the importance of this heterocycle moiety.Reviews describing the synthesis of brevetoxins (PbTxs) and ciguatoxins (CTXs), 3 on the most recent developments on the synthesis of maitotoxin (MTX) 4 and CTX, 5 on the synthesis of brevetoxin A (PbTx-1) on the 1990s 6 and on the isolation and synthesis of heliannuols, namely A, G, H, K, and L, 7 have appeared.Nicolaou reviewed their work on the total synthesis of several natural compounds, including the synthesis of PbTx-1, which subunits B and G were obtained through lactonization and dithioketal cyclization reactions, respectively, and of brevetoxin B (PbTx-2), which subunit H was synthesized via hydroxyl dithioketal cyclization reaction. 8The synthesis of subunit H of PbTx-2 proposed by Nicolaou was also reviewed by T. Nakata. 9he importance of specific reagents, namely SmI 2 -promoted cyclizations applied to the synthesis of some cyclic ethers of PbTx-2 10,11 and oxiranyl anions as building blocks in the construction of CDEFGHIJ-ring system of yessotoxin (YTX) and adriatoxin 12 have been disclosed.
H. Oguri studied the absolute configuration of CTX using an approach which combined partial structure synthesis, microscale chemical transformations, and the CD exciton chirality method.In addition, the partial structures of CTXs were synthesized as haptens for the preparation of anti-ciguatoxin antibodies. 13eviews dedicated to toxic naturally occurring seafood contaminants have appeared, including the biosynthesis of PbTxs and YTX, 14 biosynthesis of PbTxs and CTXs, 15 monitoring and toxicity assessment of PbTxs in molluscan shellfish, 16 inflammatory and immune effects of PbTxs and CTXs. 17 critical overview on the chemistry, toxicity, distribution and fate of CTX derivatives has appeared. 18Friedman and a large group of collaborators of different scientific areas published in 2017 an update review on the effects of exposure to CTXs on human health, diagnosis, human pathophysiology of Ciguatera Fish Poisoning (CFP), treatment, detection of CTXs in fish, epidemiology of the illness, global dimensions, prevention, future directions, and recommendations not only for clinicians but also for patients. 19In the same year, reviews on the mechanism of action of CTXs and PbTxs, both activators of the same target, voltage-gated Na + channels, 20 14.02.3Experimental structural methods

Natural products
Many marine natural products, including marine toxins, possess complex and unusual chemical structures containing at least one 8-O-membered ring.Brevetoxins (PbTxs) are one of most popular examples of toxins, produced by marine dinoflagellates belonging to the genus Karenia.These neurotoxins are associated to the harmful algal blooms and responsible for the deaths of several marine organisms or for their accumulation in the marine food chain with consequent intoxication in human consumers, known as neurotoxic shellfish poisoning (NSP).In the previous chapter of CHC-III dedicated to 8-O-membered rings, the identification and characterization of numerous brevetoxins has been assigned, due to the intense and challenging marine discovery programs back in the 1980s but also to the development of the analytical techniques for their structural assignment. 26n 2007, Twiner et al. extracted from K. brevis of Texas Gulf coast using a C18 solid-phase extraction disc (SPEC) methodology, a series of brevetoxins and made their quantification by four independent methods: receptor binding assay (RBA), radioimmunoassay (RIA), neuroblastoma (N2A) cytotoxicity assay, and liquid chromatography/mass spectrometry (LC/MS).In addition, LC/MS analysis also provided the identification of several individual congeners (PbTx-1 (also known as brevetoxin A) 2, PbTx-2 (also known as brevetoxin B) 3, PbTx-3 4, PbTx-7 5, and PbTx-9 6) and of their hydrolyzed products (resulting from the lactone A ring opening). 27udies on the conformational analysis of a model structure for the trans-fused FGH ether rings in PbTx-1 2, involving several classical force field-solvent combinations provided ensembles containing 30-60 structures with the central G-ring, the ring that plays an important role in the ion channel active binding site, adopting the crown, twist-crown, boat-chair, and boat conformations.Moreover, quantum mechanical calculations showed that twist-crown and boat-chair conformers were typically lowest in energy, in agreement with other reports describing the need for the 8-membered G-ring of PbTx-1 2 to adopt the boat-chair conformation in order to bind to the ion channel active site.Other calculations including solvation indicated that solvent does not play a major role in the conformational preferences. 28bTx-2 3 was isolated for the first time from New Zealand and Delaware strains of K. papilionacea, being produced 25 times more in the former strain than in the latter.The toxin production also increased in response to hypoosmotic stress, as previously reported in K. brevis.The structural assignment of PbTx-2 3 was carried out by mass spectrometry (MS) and nuclear magnetic resonance (NMR) spectroscopy.Indeed, a strong overlap was observed in 1 H NMR spectra of the New Zealand extract with the PbTx-2 3 standard, namely in the signals corresponding to the aldehyde proton, the exomethylene protons and to the protons on the unsaturated bonds in the A and H-rings. 29wo novel marine ladder-frame polyethers named brevisulcatic acid-  O 13 ) and their structural elucidation was obtained after detailed analysis of low-temperature NMR spectra and high-energy MALDI TOF MS/MS spectra.The b-hydroxy-g-methylene valeric acid side chain and the ether ring skeleton of both brevisulcatic acids are common to both acids, but BSX-4 8, has a g-lactone as the 5-membered A-ring while BSX-1 7 is the seco acid analog.BSX-4 8 has structural and bioactivity similarities to PbTx-1 2. 30 Ciguatoxins (CTXs) are synthesized by microalgae of the dinoflagellate genus Gambierdiscus and composed by a ladder shaped structure made by 13 or 14 ether rings.Soliño and Costa suggested a classification of this class of compounds according to the differences in the number and substitution of the rings as well as their regional occurrence: Pacific Ocean, Caribbean Sea and Indian Ocean. 18Pacific CTXs possess 13 rings with main differences in E ring, lack of the side-chain substituent and can be separated into CTX-I (CTX1B type) and CTX-II (CTX3C type).Caribbean and Indian CTXs have a structure similar to CTX-II with an extra fused ring.Humans of isolated island communities which depends mainly on fishing for their food supply can eat fish containing these naturally occurring CTXs and develop Ciguatera Fish Poisoning (CFP) illness.It is characterized by gastrointestinal, neurologic, and cardiovascular symptoms. 18sing high-performance liquid chromatography tandem mass spectrometry (LC-MS/MS), the identification and quantification of 13 CTXs (CTX-1B 9, 52-epi-54-deoxyCTX-1B 10, 54-deoxyCTX-1B 11, CTX-4A 12, CTX-4B 13, M-seco-CTX-4A/B 14, CTX-3C 15, 40-epi-CTX-3C 16, 51-hydroxyCTX-3C 17, 2-hydroxyCTX-3C 18, 2,3-dihydroxyCTX-3C 19, M-seco-CTX-3C 20 and M-seco-CTX-3C methyl acetal 21) in fish flesh collected at different locations of Japan were accomplished.Regional differences were also possible to be made: ciguatoxin-1B-type were found in snappers and groupers from Okinawa, ciguatoxin-3C-type were found in a spotted knifejaw, Oplegnathus punctatus, from Miyazaki and both types of toxins were found in a red snapper, Lutjanus bohar, from Minamitorishima (Marcus) Island.In addition, 10 CTXs (compounds 10-17, 20, 21) were identified in the dinoflagellate Gambierdiscus toxicus collected in French Polynesia.This technique revealed high sensitivity, specificity, and rapidity in monitoring CTXs in these type of specimens. 31Using quantitative LC-MS/MS, it was possible to estimate the production of CTX (18.2 pg per cell) in G. polynesiensis CAWD212, isolated from samples of seawater of Rarotonga, Cook Islands. 32itotoxin (MTX-1, 22) is the largest non-polymeric natural compound known so far.It is a polycyclic ether isolated from the marine dinoflagellate G. toxicus, and similarly to CTXs, that are responsible for CFP illness.A critical review on the biosynthesis of Eight-Membered Rings With One Oxygen Atom marine polyether ladders, points out some mechanistic hypotheses for the contiguous rings formation that questioned the wellestablished structure of maitotoxin. 33Meanwhile, based on NMR studies, the conformation of maitotoxin and yessotoxin (described later), mode of action and their interactions in biomembranes, have been briefly reviewed by Murata et al. 34 Using quantitative LC-MS/MS, it was possible to estimate the production of MTX (8.3 pg per cell) in G. australes CAWD149 isolated from samples of seawater of Rarotonga, Cook Islands.Moreover, the production of MTX-3 23, a MTX analog, was also monitored in three species of Gambierdiscus: G. pacificus CAWD213, G. australes CAWD149 and 216. 32LC-MS analysis of G. cf.yasumotoi CAWD210 isolated from the Bay of Islands, Northland, New Zealand revealed the production of MTX-3 23 but not of MTX-1 22 or CTXs by this species. 35Years later, the isolation of two Gambierdiscus species, G. australes and a previously unknown Gambierdiscus species named as CAWD242, collected from North Meyer Island, Kermadec Islands, was accomplished.The production of MTX-1 22 in 10 of the isolates from G. australes ranged from detectable to 36.6 pg per cell while the presence of MTX-3 23 was detected in all isolates of G. australes.In addition, for the Gambierdiscus species CAWD242 was detected the production of MTX-3 23 but not of MTX-1 22. 36 The structural identification of MTX-3 23 (initially reported as 44-methylgambierone) was assigned, by comparison of its NMR and MS data with those of gambierone 24, for the first time in 2019 by Boente-Juncal et al. from G. belizeanus collected in the Caribbean Sea 37 and by Murray et al. from G. australes collected at Raoul Island (Rangitahua/Kermadec Islands). 38 new MTX analog named as MTX-4, isolated from two Canary Islands G. excentricus strains VGO791 and VGO792, was reported by Pisapia et al. in 2017, which HRMS/MS spectra analysis allowed their identification (accurate mono-isotopic mass of 3292.4860Da, as free acid form).In addition, the authors screened the identification and quantification of four MTX analogs (MTX-1 22, putative MTX-2, MTX-3 23 and MTX-4) in the extracts of two strains of Fukuyoa ruetzleri and 42 Gambierdiscus strains using LC-LRMS (low resolution mass spectrometry)/MS analysis.To note that MTX-4 was detected and quantified (13.0-72.8pg MTX equiv.cell −1 ) in all seven strains of G. excentricus analyzed, independently of their origin (Brazil, Canary Islands, and Caribbean) but not detected in any other species under study.Unfortunately, the structural elucidation of MTX-4 was not provided by the authors. 39ery recently, MTX and two MTX-like compounds from Caribbean strains of Gambierdiscus spp.were isolated by liquid chromatography: one from G. belezianus, isolate CCMP 399 from St. Barthelemy Island, Greater Antilles, Caribbean Sea; and the second from both Gambierdiscus ribotype-2, isolate CCMP 1655 from Martinique, Lesser Antilles, Caribbean Sea, and Gambierdiscus sp., isolate SIU 350 from Virgin Gorda, British Virgin Islands, Caribbean Sea.Besides MTX, HRMS analysis suggested that the first unknown compound is a desulfo-MTX congener and the second one is a didehydro-demethyl-desulfo-MTX congener.Furthermore, the authors used the Advanced Chemistry Development program, "ACD/C +H NMR Predictors and Data Base," to predict the 13 C chemical shifts of some carbons. 40n 2013, Watanabe et al. identified a novel epoxy polyether compound named gambieroxide 25, isolated from G. toxicus (GTP2 strain) collected at Papeete, Tahiti, French Polynesia.The assignment of this structure was based on the analysis of NMR and MS spectroscopic data and comprises 12 contiguous trans-fused ether rings, a sulfate ester group, an epoxide and two olefins in side chains. 41ssotoxin (YTX, 26) is a disulfated polyether compound with a characteristic ladder-shape formed by 11 adjacent ether rings of different sizes, with two sulfate ethers in one terminus of the chain and an acyclic unsaturated side chain constituted by nine carbons in the opposite side.This compound was firstly isolated in 1987 from the scallop Pecten yessoensis and since then, several metabolites have been found in shellfish from throughout the world.Various YTX analogs have also been isolated from three dinoflagellate algae Protoceratium reticulatum, Lingulodinium polyedrum, and Gonyaulax spinifera.
In 2006 a new YTX analog, 45,46,47-trinorhomoyessotoxin 27, was isolated from cultures of P. reticulatum collected at Yamada Bay, Iwate in Japan. 42YTX and a novel yessotoxin analog (furanoyessotoxin 28) containing a furan ring in the side chain have been isolated (by filtration followed by solid-phase extraction) and purified from a large-scale culture (226 L) of P. reticulatum strain CAWD129 from the western coast of South Island, New Zealand. 43All these structures have been established by 1D and 2D NMR experiments and MS studies.
From the GG1AM strain of P. reticulatum collected from phytoplankton cultures at the Centro Oceanográfico de Vigo, YTX, glycoyessotoxin (G-YTXA 29) and a novel polyether named adriatoxin-B 30 were isolated.The latest compound is a C 13 terminal truncated YTX analogous metabolite and was characterized using a combination of NMR spectroscopy experiments and conformational analysis. 44Very recently, Rajotte et al. identified YTX and the analog (24R)-24-hydroxyyessotoxin 31 from a Namibian isolate of Gonyaulax spinifera.The configuration at C-24 of 31 was based on the correlations observed in the ROESY NMR experiment.Thus, the signal of H-24 correlated with the signals of H-22a, H-25a and H-26a.In addition, on the a-face, the signal of H-28a correlated with the signals of H-22a and H-26a; on the b-face, the signal of H-25b correlated with the signals of 23-Meb and H-27b. 45e analysis of ethyl acetate extract obtained from the marine organism Laurencia obtusa, collected in Corsica, France, allowed the identification of the known microcladallene A 32. 46 Chromatographic fractionations of the extract were made and spectroscopic analysis (NMR and MS experiments) supported the assignment of this compound.
The genus Physalis belongs to the Solanaceae family and includes about 120 species widely distributed around tropical and subtropical regions of the world.Some of these species are used as food as well in folk medicine for the treatment of several illnesses such as malaria, hepatitis, hyperglycemia, rheumatism, asthma, among others.There was a series of seco-steroid compounds named physalins that have been isolated from this plant family, since the firstly isolation of physalin A 44 in 1969. 55In this article, we will only present the characterization of novel physalins reported after 2006, since the other derivatives have been characterized by NMR and MS techniques in the previous CHC chapters.Nevertheless, many of them have been isolated from different species and geographic regions for their biological evaluation, namely physalins B 45, D 46, E 47, F 48, G 49, H 50, I 51, J 52 and U 53 from Physalis angulata L. plant collected in Brazil, [56][57][58][59][60][61][62][63] Republic of China [64][65][66][67][68] or Republic of Congo. 69For physalins B and F, crystallization of the dichloromethane extract from acetone allowed the structural determination of both compounds by X-ray crystallography. 69From the whole Physalis angulata L. plant collected in Tainan Hsien, Taiwan, Republic of China a new physalin, physalin W 54, was isolated and fully characterized by a combination of 1D and 2D NMR and MS studies.The relative configuration assignments were based on NOESY correlations and coupling constants data. 64A novel physalin, 25-hydroxylisophysalin B 55, were isolated from whole plant of Physalis angulata collected in Hainan Province, China.The structure of such compound was based on the analysis of their spectroscopic data. 65e known 6a-chloro-5b-hydroxy-5,6-dihydrophysalin B 56 and the new 5a-ethoxy-6b-hydroxy-5,6-dihydrophysalin B 57 were obtained from the whole Physalis angulata L. plant collected in Guangdong, Republic of China, and their structures assigned through MS, IR, NMR spectroscopy analyses and X-ray crystallography. 68The structural elucidation of novel physalin X 58 was accomplished by spectral analysis of NMR and MS data and the configuration was based on NOESY experiments which showed correlations of 5-OH with H-9, 6-OH with 19-CH 3 , 13-OH with H-9/21-CH 3 , and of H-22 with 28-CH 3 , giving a-orientations for 5-OH and 13-OH and b-orientations for 6-OH and H-22.The absolute configuration was deduced as 5R,6R,8R,9S,10R,13S,14R,16S, 17R,20S,22R,24S from the X-ray structure of physalin P, the skeletal structure of physalins, and their biosynthetic background. 70imilarly, to Physalis angulata L. plant, a number of known physalins have been isolated mainly from the calyces of P. alkekengi var.Franchetii for pharmacological purposes.Examples are physalins A 44, B 45, D 46, E 47, F 48, G 49, J 52, L 59, O 60 and X 58, [71][72][73][74][75][76][77][78][79] which structures were elucidated on the basis of NMR and MS studies and confirmed by comparison with a reference sample and spectroscopic data already published.More details on the ethnomedical, phytochemical and pharmacological properties of Physalis alkekengi L. var.Franchetii (Mast.)Makino can be seen in the review of Li et al. published in 2018. 80 study reported the quantification of physalin D in methanolic extracts of fruit (mature and immature) and calyx of P. alkekengi collected from wild plants growing in the Mures district, Romania.The quantification was determined by RP-HPLC-UV method, being their content in a relation of immature calyx > mature calyx > immature fruits > mature fruits. 81Qiu et al. isolated from P. alkekengi var.Franchetii calyces hydroalcoholic extract two new physalins, physalin Y 61 and Z 62, along with the already known 3b-hydroxy-2,3-dihydro-4,7-didehydrophysalin B 63. 71 From the same type of extracts, Li et al. isolated three new physalins which structures were elucidated by UV, MS, 1D and 2D NMR spectroscopy as 5a,6b-dihydroxy-25,27-dihydro-7-deoxyphysalin A 64, 5a,6b-dihydroxyphysalin R 65 and 3b-hydroxy-2-hydrophysalin A 66. 82 Two years later, from the fruits of P. alkekengi L. var.Franchetii (Mast.)Makino collected in Qiqihar City, China, was identified a novel physalin, 5a,6a-dihydroxyphysalin D. This compound is an epimer of physalin D, and the authors named it as physalin D 1 67.Their structural assignment was based on 1D and 2D NMR, IR and MS spectroscopic data.Thus, the main differences in 13 C NMR spectra of physalin D 1 67 and D 46 are the changes of the chemical shift of C-4, C-19 and C-11 due to the space field effect of the 6-a (equatorial) hydroxy group in physalin D 1 instead of the 6-b (axial) hydroxy group in physalin D. The equatorial orientation of 6-OH in physalin D 1 67 was further confirmed by NOESY correlations of H-6 with 19-Me and of H-6 with H-28. 78ore recently, seven new physalins (7b-methoxylisophysalin B 68, 7b-methoxylphysalin C 69, physalin V 70, physalin VII 71, isophysalin I 72, 7b-ethoxylisophysalin C 73 and 3b-ethoxyl-2,3-dihydro-4,7-didehydrophysalin B 74) were also isolated from a Chinese P. alkekengi var.Franchetii calyces hydroalcoholic extract and identified by NMR and MS studies, along with 18 already known physalins, elucidated by comparison of the literature spectral data. 83,84A novel and efficient HPTLC (High Performance Thin-Layer Chromatography)-MS/(MS) methodology was developed for the first time for the analysis of physalins in crude plant extracts of different parts of P. alkekengi L. collected in Slovenia and harvested at different stages of maturity.This method proved to be more selective, sensitive and with higher resolution in the separation of physalin L standard and its impurity (2,3,25,27tetrahydrophysalin A). 85 More examples of physalins were collected from other plants of the genera Physalis and Withania.Thus, physalin F was isolated form P. minima L. whole plant collected in Arau-Perlis, Malaysia, 86 while from that collected in Guangdong, Republic of China, 87 five derivatives was isolated using a ultra-performance liquid chromatographic (UHPLC) method: isophysalin B 75, physalin G 49, physalin D 46, physalin I 51 and 5a,6b-dihydroxyphysalin R 65. Physalins A 44, B 45, D 46, F 48 and H 50 were also isolated from the extracts of the aerial parts of P. divericata D. Don 88,89 while from the leaves of Witheringia solanacea were isolated physalins B, D, and F. 90 All structures were elucidated based on ultraviolet spectrophotometry, NMR and MS studies and confirmed by comparison with spectroscopic data already published.

Reactivity of substituents attached to ring carbon atoms
Several hydrogenation reactions of benzoxocines 76-78 over Pd/C have been reported to prepare benzoxocanes 79-81.][93] Asymmetric total synthesis of trilobacin 87 was achieved through a multistep strategy starting from glycolate oxazolidinone 83 and undecanal.Initially, it involves a 13-step sequence for the formation of the a,a 0 -cis-oxocene 85 that includes a ring-closing metathesis promoted by 1st generation Grubbs catalyst [bis(tricyclohexylphosphine)benzylidene ruthenium (IV) dichloride] (Grubbs I, 84).Then, additional 5-steps including an organoselenium-mediated oxonium ion formation/SiO 2 -promoted fragmentation to form erythro-bis(2,2 0 )-tetrahydrofuran moiety 86 are required for the total synthesis of 87, in a 14% overall yield (Scheme 2). 94n order to construct the PET probe 91 with improved tumor-to-background ratios in neurotensin receptor (NTR) imaging, a diastereomeric mixture of trans-5-oxocene 88 reacted with triethylene glycol di(p-toluenesulfonate), followed by treatment with 18 F-TBAF to provide 18 F-labeled trans-5-oxocene 89, which after mixing a few seconds with tetrazine-peptide conjugate 90, afforded the desired probe 91 (Scheme 3). 95The computational guided synthesis of 90 was previously reported in seven steps from the commercially available glycidol and involves Sakurai allylation, olefin metathesis and flow enabled photoisomerization as key steps.Wallace et al. also demonstrated the potential use of trans-5-oxocene in studies where hydrophilicity and bioconjugation are required.Thus they showed the rapid and quantitative inverse electron-demand Diels-Alder (IED-DA) reaction of trans-5-oxocene with a water-soluble tetrazine derivative and with a green fluorescence protein encoded with an unnatural tetrazine containing amino acid. 96tereoselective synthesis of (+)-cis-lauthisan 95 is accomplished in a three-step approach starting from eight-membered lactone 92 (the synthesis of 92 is described in Section 14.02.6.3).It involves saturation of the oxocene ring and conversion to thionolactone 93 by using Lawesson reagent, treatment with ethyl lithium followed by iodomethane to form intermediate 94 and finally reductive removal of sulfur in the presence of tributyltin hydride and a catalytic amount of 2,2 0 -azobis(2-methylpropionitrile) (AIBN) in refluxing toluene (Scheme 4). 97he synthesis of oxocene Holmes intermediate 102, which is involved in the formal synthesis of natural (+)-laurencin 103 can be achieved through a multistep transformation of bridged cyclic ether 96 (the synthesis of 96 is described in Section 14.02.6.2).It starts by the removal of the sulfur bridge via Raney nickel reduction in the presence of acetate buffer (pH ¼ 4.6); decarboxylation of the oxocane formed 97 via microwave heating in aqueous DMF to afford ketone 98; reducing with L-selectride to form a 2:1 epimeric mixture of alcohols 99; formation of xanthate 100 by reaction with NaH, CS 2 and MeI; elimination reaction carried out by microwave heating in O-dichlorobenzene giving a 9:1 mixture of oxocene regioisomers (C5]C6 and C6]C7) 101; and finally acetate cleavage to provide Holmes intermediate 102 (Scheme 5). 98.02.5 Ring syntheses classified by number of ring atoms in each component

Hydroxydithioketal cyclization
The hydroxydithioketal cyclization approach was applied in the construction of the oxocene ring in some natural fused polycyclic ethers isolated from marine sources.Examples are the formation of the F-ring in the total synthesis of PbTx-1 2 (Scheme 6) 99,100 and of the B 0 -ring in the convergent synthesis of the ent-ZA 0 B 0 C 0 D 0 -ring system of MTX-1 22 (Scheme 7). 101,102heme 3 Scheme 4

Scheme 8 Scheme 9
Two-directional double RCM was used to synthesize bis-oxocine-fused naphthalene 133 starting from 2,6-bis(prop-2en-1-yloxy)-1,7-bis(but-3-en-1-yloxy)naphthalene 132 in the presence of Grubbs I catalyst 84 in dichloromethane at room temperature under nitrogen atmosphere (Scheme 12). 140CM was also used to prepared eight-membered lactones.Protected ester 134 undergo RCM in the presence of Grubbs I catalyst 84 at room temperature overnight, to give the unsaturated lactone 135 in quantitative yield.Meanwhile, RCM of unprotected derivative 136 was only possible with Grubbs II catalyst 118 leading to unsaturated lactone 137.However, the best conditions for the cyclization process of triene 138 was using Grubbs II 118 as catalyst and Ti(O i Pr) 4 as cocatalyst in refluxing dichloromethane producing unsaturated lactone 139 (Scheme 13). 141

Haloetherification of unsaturated alcohols
A single study reported iodine-promoted 8-endo-oxacyclizations of several dienyl alcohols 140 carried out in the presence of sodium bicarbonate in THF to provide iodinated oxocenes 141 in moderate yields (Scheme 14). 142

Lactonization
The synthesis of the lactone core of natural (−)-octalactin A 113 was accomplished by two different approaches starting from a-diazoketone 177: (1) treatment with silver benzoate and 4-(dimethylamino)pyridine (DMAP) in dilute tetrahydrofuran to give lactone 178 in 26% yield; (2) photolytic Wolff rearrangement in aqueous tetrahydrofuran provided seco-acid 179 which suffered lactonization, after treatment with benzoic anhydride (Scheme 22). 154n efficient lactone formation reaction was developed through the use of 2-methyl-6-nitrobenzoic anhydride (MNBA) as a coupling reagent and nucleophilic catalysts, namely acyl-transfer catalysts such as DMAP, 4-pyrrolidinylpyridine and 4-(dimethylamino)pyridine N-oxide (DMAPO). 155Octalactin B 113 was synthesized from the commercially available methyl 3-butenoate and isobutyraldehyde, being the lactonization reaction one of the key-steps.Thus, the eight-membered ring 181 was obtained from the reaction of seco-acid 180 with MNBA and DMAP in toluene at room temperature (Scheme 23). 156This reaction conditions were also applied for the synthesis of the lactone core of astakolactin 182. 157Meanwhile, 3 years later, the same group revised the structure of astakolactin to a pyran derivative 182a. 158maguchi lactonization was the key step in the stereoselective synthesis of the eight-membered lactone (+)-cephalosporolide D 185.It involved the reaction of acid 183 in the presence of N,N-diisopropylethylamine (DIPEA), 2,4,6-trichlorobenzoyl chloride and DMAP in benzene for the construction of the macrolactone 184 and subsequent removal of the protecting group in acid conditions (Scheme 24). 159 series of eight-membered lactones 188 were accomplished via [6 + 2] cyclization reaction of (1,6)-amphoteric systems, 2-(oxetan-3-yl)arylaldehydes 186, with siloxy alkynes 187 mediated by triflimide (HNTf 2 ) in dichloromethane at room temperature (Scheme 25). 160Oxidative lactonization of 1,7-diol 189 carried out in the presence of a catalytic amount of TEMPO and PhI(OAc) 2 afforded eight-membered lactone 190.This strategy was applied in a concise total synthesis of (AE)-isolaurepan 191 (Scheme 26). 161

Scheme 52
Eight-membered lactones 325 were synthesized in moderate yields from the reaction of b-silyl ketone enolates 323 with MeLi in THF, subsequent treatment with epoxides and BF 3• OEt 2 in THF to yield the desired hemiketals 324 and finally, addition of PhI(OAc) 2 and iodine in dichloromethane (Scheme 58).One of these lactones was applied in the stereoselective total synthesis of (+)-cis-lauthisan 95, previously described in Section 14.02.4. 97

Syntheses of particular classes of compounds and critical comparison of the various routes available
As described along this chapter, a huge number of publications have emerged during the last decade focusing the eight-Omembered heterocyclic chemistry.Among the impressive number of methodologies developed, we can mention those dedicated to the synthesis of several marine natural products with unusual and complex structures such as PbTxs, MTX, CTXs and YTXs as well as some natural derivatives extracted from terrestrial origin, namely, heliannuols, pysalins, lauthisan and related derivatives.In fact, seven different routes have been established concerning the total synthesis of natural oxocane derivative lauthisan.
RCM was the most popular approach to construct the eight-membered ring and used as the key reaction in the synthesis of many types of natural compounds, or their precursors.Several reactions promoted by palladium catalysts also deserved special attention in the synthesis of several natural eight-membered oxacycles.Regio-and diastereoselectivity were achieved in the three-carbon ringenlargement of tetrahydrofuran derivatives assisted by metals such as tin and antimony.

Important compounds and applications
The pharmacological importance of eight-O-membered heterocycles among the scientific community is demonstrated by the large number of reviews and original papers related to this topic, published every year.
Extensive studies have been devoted to the toxicology of dinoflagellate toxins such as PbTxs, MTX, CTXs and YTXs, with environmental and human impact.Their molecular mechanisms of action were compiled and described in several reviews (see Section 14.02.1), which can be complemented with the information provided in CHEC III.Here, it is our intention to give a brief presentation on the main therapeutic applications of natural eight-O-membered heterocyclic compounds, providing some of the most relevant results.It is noteworthy to mention that no synthetic derivative was evaluated in any biological assay.
Ansellone C 34 showed moderate antitumor activity against A549 (non-small cell lung cancer) and K562 (human leukemia) cell lines with LC 50 values of 3.9 and 4.5 mM, respectively, when compared with the positive control doxorubicin (LC 50 of 1.1 and 1.5 mM, respectively).No antibacterial activity was observed against Staphylococcus aureus and Proteus hauseri. 49ntistaphylococcal activity against a panel of multidrug and methicillin-resistant S. aureus was tested for eight-membered cyclic ethers 36-41.Derivatives 36 and 40 did not show any activity at 128 mg/mL, although the remaining compounds exhibited significant antistaphylococcal activity, with minimum inhibitory concentrations (MICs) in the range of 8-256 mg/mL.Derivative 37 was the most active one (MIC 8-16 mg/mL) probably due to the presence of the two acetyl groups that improves its cellular bioavailability by being more lipophilic. 51he antioxidant activity of methyl 3,4,5,8-tetrahydro-2H-oxocine-5-carboxylate 42 were evaluated and compared to a-tocopherol and the synthetic antioxidants butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA).Similar 1,1diphenyl-2-picrylhydrazyl (DPPH) and 2,2 0 -azino-bis-3-ethylbenzothiozoline-6-sulfonic acid (ABTS) radical scavenging activities were observed for compound 42, BHA and BHT but slightly higher than a-tocopherol.For the anti-inflammatory assays, compound 42 exhibited higher inhibitory activity against cyclooxygenase-1 (COX-1, IC 50 0.92 mg/mL) than cyclooxygenase-2 (COX-2, IC 50 1.05 mg/mL) isoform.No significant differences were recorded for the in vivo 5-lipoxidase (5-LOX) inhibitory activity of derivative 42 (IC 50 0.95 mg/mL) and the control ibuprofen (IC 50 0.93 mg/mL). 52ome studies have been carried out on the isolation of sunflower constituents with biological activity, with special emphasis on phytotoxicity, an indication for the possible allelopathic activity of sunflower.Heliannuol A 43 have been tested in the etiolated wheat coleoptile bioassay and showed high inhibitory activity on coleoptile elongation at 10 −3 M, with a value of 85% (very close to the value of the commercial herbicide logran, 93%), and of 50% at 3 Â 10 −3 M concentration, but the activity decreased significantly with dilution.Derivative 43 was also selected for the evaluation of phytotoxicity on tomato seed growth, in terms of percentage inhibition of germination and shoot and root length, revealing strong inhibitory effects, with activities similar to that of the herbicide logran. 53,54 great number of seco-steroids named physalins are widely used in folk medicine in many tropical countries around the world.Its known pharmacological applications are antioxidant, anti-inflammatory, antimicrobial, antitumor, sedative, among others.Herein, we will describe the therapeutic potential of the eight-O-membered physalins evaluated during the last decade.

Scheme 58
Immunomodulatory activities of physalins B 45, F 48 and G 49 in lymphocyte proliferation, cytokine production and in transplantation were studied.The addition of physalins B 45, F 48 and G 49 to concanavalin A-activated splenocyte cultures induced a concentration-dependent inhibition of proliferation.Physalin B 45 also inhibited IL-2 production by concanavalin A-activated spleen cells.Rejection of allogeneic heterotopic heart transplant was delayed in mice treated with physalins B 45, F 48 or G 49. 56 The immunosuppressive effect of physalin H 50 on T cells both in vitro and in vivo were also evaluated.The results point out that the immunosuppressive activity might be attributed to the suppression of T cell activation and proliferation, the modulation of Th1/Th2 cytokine balance and the induction of HO-1 in T cells. 66he activity of physalin F 48 was investigated in mice models of collagen-induced arthritis and allergic airway inflammation.Oral treatment caused a marked decrease in paw edema and joint inflammation while no effects were observed in mice with allergic airway inflammation.In addition, a different inhibitory mechanism is proposed for physalin F 48 when compared with that of glucocorticoids. 58opic anti-inflammatory potential of physalin E 47 was demonstrated in acute and chronic models of dermatitis induced in mouse ear. 62he antimalarial activity of pysalins B 45, D 46, F 48 and G 49 was evaluated using in vitro assays against Plasmodium falciparum.Although these physalins exhibited antiplasmodial activity, lower cytotoxicity was recorded when compared to the standard antimalarial drug, mefloquine. 59he inhibitory effects of some physalins on nitric oxide (NO) production induced by lipopolysaccharide (LPS) in macrophages were tested.Physalin X 58 exhibited a significant inhibitory effect against NO production with an IC 50 value of 68.50 AE 1.51 mM (positive control hydrocortisone, IC 50 58.79AE 3.32 mM). 70Another study indicated that physalin F 48 possessed the strongest effect, with an IC 50 of 0.33 AE 0.17 mM.Pysalins A 44, B 45, and O 60 also showed strong inhibition with IC 50 ranging from 0.84 to 17.11 mM.Moderate activities were observed for physalins Y 61 and Z 62 and 3b-hydroxy-2,3-dihydro-4,7-didehydrophysalin B 63, which were close to hydrocortisone (IC 50 64.34AE 7.49 mM). 71Other papers reported the anti-inflammatory potential of physalins A 44, G 49, L 59, O 60 74 and of physalins B 45 and F 48. 90 The antileishmanial activity of physalins B 45, D 46, F 48 in vitro and in vivo in models of cutaneous leishmaniasis was evaluated.For the in vitro assays, physalins B 45 and F 48 were able to reduce the percentage of leishmania-infected macrophages and the intracellular parasite number at concentrations non-cytotoxic to macrophages.For the in vivo assays, physalin F 48 reduced the lesion size, the parasite load and histopathological alterations in the infected mice. 63n interesting study was undertaken to evaluate a new pharmacological property of physalins B 45, D 46, F 48 and G 49 in inflammatory and centrally mediated pain tests in mice.The results suggested that physalins present antinociceptive properties associated with central but not anti-inflammatory events. 60everal in vitro and in vivo antitumor activity studies were performed for physalins.Thus, physalin B 45 and D 46 displayed considerable cytotoxicity against several cancer cell lines [B-16 (murine skin), HCT-8 (human colon), PC3 (human prostate), MDA-MB-435, MDA-MB-231 and MCF-7 (human breast), K562, CEM and HL-60 (human leukaemia)], showing IC 50 values of 0.58-15.18mg/mL for physalin B 45, and 0.28-2.43mg/mL for physalin D 46.The antitumor activity of both compounds was confirmed in vivo using mice bearing sarcoma 180 tumor cells. 61Physalins B 45, D 46, F 48, J 52, U 53 and W 54 were tested against a panel of human cancer cell lines [A549, ZR751 (estrogen receptor positive breast cancer), A431 (EGFR overexpressing skin cancer), LNCAP (AR-dependent prostate cancer), HCT-8, PC-3, KB (nasopharyngeal carcinoma), KB-VIN (vincristine-resistant KB subline)].The results showed that physalins B 45, D 46, F 48 displayed strong cytotoxicity against most of the cell lines with EC 50 values lower than 3 mg/mL.However, physalin B 45 showed moderate activity against LNCAP and A549, and physalin D 46 against KB-VIN tumor cell lines.In addition, physalin W 54 was only slightly active against 1A9 and physalin J 52 only against A431 cell lines.Physalin U 53 showed modest activity against all human tumor cell line panel. 64Other paper reported that physalin B 45 showed the highest cytotoxic activities against HeLa (human cervical), SMMC-7721 (human hepatoma) and HL-60 tumor cell lines while physalins A 44, D 46, D 1 67, L 58 and X 58 show moderate cytotoxic activities. 78Physalins B 45, F 48 and J 52 were evaluated for cytotoxicity against human HL-60, SMMC-7721 (hepatoma cell line), A-549, MCF-7 and SW-480 (colon cancer cell line) and displayed potent cytotoxicity (IC 50 <5 mM) while physalin V 70 exhibited a modest cytotoxicity. 83High cytotoxicity was observed for physalin H 50 against HXT-116 (human colorectal carcinoma) and NCI-H460 (human non-small cell lung cancer) cells but weak activity for physalins A 44, B 45, D 46 and F 48. 88 Physalins A 44 and B 45 significantly inhibited the growth of two androgen-independent cell lines CWR22Rv1 and C42B, induced apoptosis and decreased androgen receptor expression. 73Other studies referred that physalin A 44 also induced cytotoxic effects in human melanoma A375-S2 cells 75 and human fibrosarcoma HT1080 cells 76 and the selective cytotoxicity of physalin B 45 for human melanoma A375 cells 67 and HXT-116 cells, 89 of physalin F 48 for human breast T-47D carcinoma 86 and of 7bethoxylisophysalin C 73 for PC-3 cancer cell lines. 84he phase II enzyme quinone reductase plays an important role in anticancer, detoxification pathways and antioxidant defense.From the tested compounds, physalin H 50 exhibited strong quinone reductase inhibitory activity while physalin G 49, 5bhydroxy-6a-chloro-5,6-dihydrophysalin B 56 and 5a-ethoxy-6b-hydroxy-5,6-dihydrophysalin B 57 showed weak inhibitory activity. 68The inhibitory effects on quinone reductase activity by physalin A 44 79 and isophysalin B 75 87 were also reported.
Physalin D 46 showed low antioxidant potential in the DPPH radical and thiobarbituric acid (TBA) assays.Low antibacterial and antifungal activity was also recorded for physalin D 46. 72 Further antibacterial assays indicated that physalins B 45 and J 52 showed high antibacterial activities against Bacillus subtilis and Escherichia coli. 83 Scheme 10