Tandem Macrolactone Synthesis: Total Synthesis of (-)-Exiguolide by a Macrocyclization/Transannular Pyran Cyclization Strategy.

Tetrahydropyran-containing macrolactones were synthesized by integrating Meyer-Schuster rearrangement, macrocyclic ring-closing metathesis, and transannular oxa-Michael addition under gold and ruthenium catalysis. Single-step access to a variety of 14- to 20-membered macrolactones containing a tetrahydropyran ring was possible from readily available linear precursors in good yields and with moderate to excellent diastereoselectivity. A 13-step synthesis of (-)-exiguolide, an anticancer marine macrolide, showcased the feasibility of our tandem reaction sequence for macrolactone synthesis and also demonstrated the power of transannular reactions for rapid assembly of the tetrahydropyran rings of the target natural product.

Cobalt-Catalyzed Hartung-Mukaiyama Cyclization of γ-Hydroxy Olefins: Stereocontrolled Synthesis of the Tetrahydrofuran Moiety of Amphidinolide N.

Cobalt-catalyzed Mukaiyama-type cyclization of γ-hydroxy olefins is known as an atom- and step-economical means for stereoselective synthesis of 2,5-trans-substituted tetrahydrofuran derivatives. In this study, we investigated the synthesis of a series of 2,5-substituted tetrahydrofuran derivatives by means of a cobalt-catalyzed Hartung-Mukaiyama cyclization. The stereochemical consequence of the reaction was found to be largely dependent on the substitution pattern and relative configuration of γ-hydroxy olefins. 2,5-cis-Substituted tetrahydrofuran derivatives could be obtained diastereoselectively from appropriately substituted γ-hydroxy olefins. Additionally, relatively bulky olefin substituents and unprotected hydroxy groups at non-interfering positions (e.g., α and δ) were well tolerated in the reaction. Finally, the synthetic versatility of the Hartung-Mukaiyama cyclization was demonstrated through a stereocontrolled synthesis of the tetrahydrofuran moiety of amphidinolide N, a potent cytotoxic macrolide of marine origin. This study expands the capacity of Mukaiyama-type cyclization in that it can be used in convergent assembly of complex tetrahydrofuran motifs from internal olefins.

Stereoselective Synthesis of the Southern Hemisphere Acyclic Domain of Neaumycin B.

A stereocontrolled synthetic entry to the southern hemisphere C3-C17 acyclic domain of neaumycin B, a highly potent cytotoxic macrolide natural product, has been developed. The present synthesis is based on (i) a tandem olefin cross-metathesis/hemiacetalization/intramolecular oxa-Michael addition, (ii) a regioselective reductive acetal opening for differential protection of the C14 hydroxy group, (iii) a Horner-Wadsworth-Emmons reaction for the stereoselective formation of the C8-C9 olefin, and (iv) a Corey-Bakshi-Shibata asymmetric reduction to create the C7 stereogenic center.

Ruthenium-Catalyzed Intramolecular Double Hydrofunctionalization of Alkynes. Synthesis of Spirocyclic Hemiaminal Ethers and Their Lewis Acid-Mediated Cleavage/Nucleophilic Addition.

[RuCl2(p-cymene)]2/AgNO3-catalyzed intramolecular double hydrofunctionalization of internal alkynes having nitrogen and oxygen nucleophilic groups at appropriate positions provided a series of spirocyclic hemiaminal ether derivatives in good to excellent yields. The product spiro-hemiaminal ethers underwent Lewis acid-mediated chemoselective cleavage, and in situ-generated iminium/oxocarbenium ions could be trapped with nucleophiles to afford a range of nitrogen and oxygen heterocycles.

Synthesis-Driven Stereochemical Assignment of Marine Polycyclic Ether Natural Products.

Marine polycyclic ether natural products have gained significant interest from the chemical community due to their impressively huge molecular architecture and diverse biological functions. The structure assignment of this class of extraordinarily complex natural products has mainly relied on NMR spectroscopic analysis. However, NMR spectroscopic analysis has its own limitations, including configurational assignment of stereogenic centers within conformationally flexible systems. Chemical shift deviation analysis of synthetic model compounds is a reliable means to assign the relative configuration of "difficult" stereogenic centers. The complete configurational assignment must be ultimately established through total synthesis. The aim of this review is to summarize the indispensable role of organic synthesis in stereochemical assignment of marine polycyclic ethers.

Total Synthesis, Stereochemical Revision, and Biological Assessment of Iriomoteolide-2a.

Iriomoteolide-2a is a marine macrolide metabolite isolated from a cultured broth of the benthic dinoflagellate Amphidinium sp. HYA024 strain. This naturally occurring substance was reported to show remarkable cytotoxic activity against human cancer cell lines HeLa and DG-75 and in vivo antitumor activity against murine leukemia P388 cell line. Herein, the total synthesis, stereochemical revision, and biological assessment of iriomoteolide-2a are reported in detail. Total synthesis of the proposed structure 1 of iriomoteolide-2a featured a late-stage convergent assembly of three components by a Suzuki-Miyaura coupling, an esterification, and a ring-closing metathesis. However, the NMR data of synthetic 1 were not identical to those of the natural product. Careful analysis of the NMR data of the authentic material and synthesis/NMR analysis of appropriately designed model compounds led to consideration of four possible stereoisomers 2-5 as candidates for the correct structure. Accordingly, total syntheses of 2-5 were achieved by taking advantage of the convergent strategy, and comparison of the NMR spectra of synthetic 2-5 with those of the natural product led to the conclusion that 5 shows the correct relative configuration of iriomoteolide-2a. The absolute configuration of this natural product was finally established through chiral HPLC analysis of synthetic 5/ent-5 with the authentic sample. The antiproliferative activity of the synthetic compounds was assessed against HeLa and A549 cells to show that, in contrast to expectation, synthetic 5 and ent-5 were only marginally active in these cell lines. This work clearly underscores the vital role of total synthesis in the establishment of the structure and biological activity of natural products.

Fluorescence-labeled neopeltolide derivatives for subcellular localization imaging.

Design, synthesis and functional analysis of fluorescent derivatives of neopeltolide, an antiproliferative marine macrolide, are reported herein. Live cell imaging using the fluorescent derivatives showed rapid cellular uptake and localization within the endoplasmic reticulum as well as the mitochondria.

Total Synthesis of (-)-Enigmazole†A.

Total synthesis of (-)-enigmazole A, a marine macrolide natural product with cytotoxic activity, has been accomplished. The tetrahydropyran moiety was constructed by means of a domino olefin cross-metathesis/intramolecular oxa-Michael addition of a δ-hydroxy olefin. After coupling of advanced intermediates, the macrocycle was formed through gold-catalyzed rearrangement of a propargylic benzoate, followed by ring-closing metathesis of the resultant α,ß-unsaturated ketone.

Total Synthesis and Stereochemical Revision of Iriomoteolide-2a.

Total syntheses of the proposed and correct structures of iriomoteolide-2a, a cytotoxic marine macrolide natural product with an unusual 23-membered macrolactone skeleton, have been accomplished for the first time. The synthesis of the correct structure involves an asymmetric epoxidation/diepoxide cyclization cascade for the construction of the bis(tetrahydrofuran) moiety, a Suzuki-Miyaura coupling for the fragment assembly, and a ring-closing metathesis for the closure of the macrocyclic backbone. In addition, the original stereochemical assignment of iriomoteolide-2a was revised.

A Synthetic Analogue of Neopeltolide, 8,9-Dehydroneopeltolide, Is a Potent Anti-Austerity Agent against Starved Tumor Cells.

Neopeltolide, an antiproliferative marine macrolide, is known to specifically inhibit complex III of the mitochondrial electron transport chain (mETC). However, details of the biological mode-of-action(s) remain largely unknown. This work demonstrates potent cytotoxic activity of synthetic neopeltolide analogue, 8,9-dehydroneopeltolide (8,9-DNP), against starved human pancreatic adenocarcinoma PANC-1 cells and human non-small cell lung adenocarcinoma A549 cells. 8,9-DNP induced rapid dissipation of the mitochondrial membrane potential and depletion of intracellular ATP level in nutrient-deprived medium. Meanwhile, in spite of mTOR inhibition under starvation conditions, impairment of cytoprotective autophagy was observed as the lipidation of LC3-I to form LC3-II and the degradation of p62 were suppressed. Consequently, cells were severely deprived of energy sources and underwent necrotic cell death. The autophagic flux inhibited by 8,9-DNP could be restored by glucose, and this eventually rescued cells from necrotic death. Thus, 8,9-DNP is a potent anti-austerity agent that impairs mitochondrial ATP synthesis and cytoprotective autophagy in starved tumor cells.

Total Synthesis and Complete Stereostructure of a Marine Macrolide Glycoside, (-)-Lyngbyaloside†B.

We have described in detail the total synthesis of both the proposed and correct structures of (-)-lyngbyaloside B, which facilitated the elucidation of the complete stereostructure of this natural product. Our study began with the total synthesis of 13-demethyllyngbyaloside B, in which an esterification/ring-closing metathesis (RCM) strategy was successfully used for the efficient construction of the macrocycle. We also established reliable methods for the introduction of the conjugated diene side chain and the l-rhamnose residue onto the macrocyclic framework. However, the esterification/RCM strategy proved ineffective for the parent natural product because of the difficulties in acylating the sterically encumbered C-13 tertiary alcohol; macrolactionization of a seco-acid was also extensively investigated under various conditions without success. We finally completed the total synthesis of the proposed structure of (-)-lyngbyaloside B by means of a macrolactonization that involves an acyl ketene as the reactive species. However, the NMR spectroscopic data of our synthetic material did not match those of the authentic material, which indicated that the proposed structure must be re-examined. Inspection of the NMR spectroscopic data of the natural product and molecular mechanics calculations led us to postulate that the configuration of the C-10, C-11, and C-13 stereogenic centers had been incorrectly assigned in the proposed structure. Finally, our revised structure of (-)-lyngbyaloside B was unambiguously verified through total synthesis.

Complete Stereochemical Assignment of Campechic Acids A and B.

Campechic acids A and B are anti-invasive polyketide antibiotics isolated from Streptomyces sp. CHI93 strain. Herein we describe stereoselective synthesis of the C-16-C-30 fragment of campechic acids A and B via a biosynthesis-inspired epoxide-opening cascade and its NMR spectroscopic comparison with the authentic degradation product, resulting in configurational assignment of the C-21, C-24, C-25, and C-28 stereogenic centers and reassignment of the C-18 stereogenic center.

Progress toward the Total Synthesis of Goniodomin A: Stereocontrolled, Convergent Synthesis of the C12-C36 Fragment.

Goniodomin A is a marine polyether macrolide natural product isolated from the dinoflagellate Alexandrium hiranoi. In this paper, we report stereocontrolled, convergent synthesis of a fully functionalized C12-C36 fragment of goniodomin A. The synthesis of the C12-C25 vinylstannane involved a Wittig reaction and a reductive cycloetherification for the construction of the dihydropyran ring. The C26-C36 thioester was synthesized via a Nozaki-Hiyama-Kishi reaction of an aldehyde and an iodoalkyne, the former of which was easily prepared from (R)-malic acid as a chiral source by taking advantage of substrate-controlled diastereoselective reactions. Finally, a palladium-catalyzed coupling of the C12-C25 vinylstannane and the C26-C36 thioester completed the synthesis of the target compound.

Diastereoselective Ring-Closing Metathesis as a Means to Construct Medium-Sized Cyclic Ethers: Application to the Synthesis of a Photoactivatable Gambierol Derivative.

This paper describes a concise synthesis of six- to eight-membered α,α'-substituted cyclic ethers by exploiting diastereoselective ring-closing metathesis (RCM) of 1,4-pentadien-3-yl ether derivatives. The RCM precursors could be efficiently prepared via a vinylation of the corresponding α-acetoxy ether derivatives using divinylzinc. Diastereoselective RCM of 1,4-pentadien-3-yl ether derivatives afforded a series of six- to eight-membered α,α'-substituted cyclic ethers with moderate to good diastereoselectivity. The stereochemical consequence of the diastereoselective RCM appeared to be dependent on the structure of the ring being forged. The diastereoselectivity of six- and seven-membered cyclic ethers appeared to be largely under kinetic control irrespective of the catalyst reactivity, whereas that of an eight-membered cyclic ether could be controlled by the catalyst reactivity. Finally, the diastereoselective RCM chemistry was applied to the synthesis of a biotin-tagged photoactivatable derivative of gambierol.

Effect of carbon chain length in acyl coenzyme A on the efficiency of enzymatic transformation of okadaic acid to 7-O-acyl okadaic acid.

Okadaic acid (OA), a product of dinoflagellate Prorocentrum spp., is transformed into 7-O-acyl OA in various bivalve species. The structural transformation proceeds enzymatically in vitro in the presence of the microsomal fraction from the digestive gland of bivalves. We have been using LC-MS/MS to identify OA-transforming enzymes by detecting 7-O-acyl OA, also known as dinophysistoxin 3 (DTX3). However, an alternative assay for DTX3 is required because the OA-transforming enzyme is a membrane protein, and surfactants for solubilizing membrane proteins decrease the sensitivity of LC-MS/MS. The present study examined saturated fatty acyl CoAs with a carbon chain length of 10 (decanoyl), 12 (dodecanoyl), 14 (tetradecanoyl), 16 (hexadecanoyl) and 18 (octadecanoyl) as the substrate for the in vitro acylation reaction. Saturated fatty acyl CoAs with a carbon chain length of 14, 16 and 18 exhibited higher yields than those with a carbon chain length of 10 or 12. Acyl CoAs with carbon chain lengths from 14 to 18 and containing either a diene unit, an alkyne unit, or an azide unit in the carbon chain were synthesized and shown to provide the corresponding DTX3 with a yield comparable to that of hexadecanoyl CoA. The three functional units can be conjugated with fluorescent reagents and are applicable to the development of a novel assay for DTX3.

Contemporary Strategies for the Synthesis of Tetrahydropyran Derivatives: Application to Total Synthesis of Neopeltolide, a Marine Macrolide Natural Product.

Tetrahydropyrans are structural motifs that are abundantly present in a range of biologically important marine natural products. As such, significant efforts have been paid to the development of efficient and versatile methods for the synthesis of tetrahydropyran derivatives. Neopeltolide, a potent antiproliferative marine natural product, has been an attractive target compound for synthetic chemists because of its complex structure comprised of a 14-membered macrolactone embedded with a tetrahydropyran ring, and twenty total and formal syntheses of this natural product have been reported so far. This review summarizes the total and formal syntheses of neopeltolide and its analogues, highlighting the synthetic strategies exploited for constructing the tetrahydropyran ring.

Evaluation of gambierol and its analogs for their inhibition of human Kv1.2 and cytotoxicity.

Gambierol and its heptacyclic and tetracyclic analogs were tested for inhibitory activity against the human voltage-gated potassium channel Kv1.2 (hKv1.2), which was stably expressed in Chinese hamster ovary (CHO) cells. Gambierol, the heptacyclic analog, and the tetracyclic analog inhibited the potassium current evoked by a step pulse from -80mV to 40mV. The IC50 values for the three compounds were 0.75±0.15nM, 7.6±1.2nM, and 28±4.0nM (the mean±SEM, n=3), respectively. The cytotoxic activity was examined in order to assess a relationship between cytotoxicity and inhibition of the hKv1.2. The IC50 values for gambierol, the heptacyclic analog, and the tetracyclic analog in the wild-type CHO cells were 95±7.1µM, 6.5±0.8µM (the mean±SEM, n=3), and >100µM (n=3), respectively, whereas those in the CHO cells stably expressing hKv1.2 were 78±5.8µM, 6.0±1.0µM (the mean±SEM, n=3), and >100µM (n=3). These results suggested that cytotoxicity is not triggered by inhibition of the human Kv1.2. The electrophysiological recording at the resting potential in the presence of gambierol, the heptacyclic analog, and the tetracyclic analog revealed the dose-dependent leak current, which was largest when the heptacyclic analog was administered to the cells. We thus propose that the leak current induced by these compounds might cause a fatal effect on the cultured cells.

Total synthesis, stereochemical reassignment, and biological evaluation of (-)-lyngbyaloside B.

(-)-Lyngbyaloside B is a 14-membered macrolide glycoside isolated from the marine cyanobacterium Lyngbya sp. as a cytotoxic substance by Moore and co-workers. The first total synthesis of (-)-lyngbyaloside B and the reassignment of its stereostructure is described. The synthesis features an Abiko-Masamune aldol reaction, a vinylogous Mukaiyama aldol reaction, and a macrocyclization involving an acyl ketene intermediate for the construction of the macrocyclic backbone, which contains an acylated tertiary alcohol. The antiproliferative activity of selected compounds against a small panel of human cancer cell lines is also reported.

Total synthesis and structure revision of didemnaketal B.

Didemnaketal B, a structurally complex spiroacetal that exhibits potent HIV-1 protease inhibitory activity, was originally discovered by Faulkner and his colleagues from the ascidian Didemnum sp. collected at Palau. Its absolute configuration was proposed on the basis of degradation/derivatization experiments of the authentic sample. However, our total synthesis of the proposed structure of didemnaketal B questioned the stereochemical assignment made by Faulkner et al. Here we describe in detail our first total synthesis of the proposed structure 2 of didemnaketal B, which features 1) a convergent synthesis of the C7-C21 spiroacetal domain by means of a strategy exploiting Suzuki-Miyaura coupling, 2) an Evans syn-aldol reaction and a vinylogous Mukaiyama aldol reaction for the assembly of the C1-C7 acyclic domain, and 3) a Nozaki-Hiyama-Kishi reaction for the construction of the C21-C28 side chain domain. The NMR spectroscopic discrepancies observed between synthetic 2 and the authentic sample as well as careful inspection of the Faulkner's stereochemical assignment led us to postulate that the absolute configuration of the C10-C20 domain of 2 has been erroneously assigned. Accordingly, the total synthesis of the revised structure 65 was achieved to show that the NMR spectroscopic properties of synthetic 65 were in good agreement with those of the authentic sample. Furthermore, application of the phenylglycine methyl ester (PGME) method to the C7-C21 spiroacetal domain enabled us to establish the absolute configuration of didemnaketal B.

Total synthesis and complete structural assignment of gambieric acid A, a large polycyclic ether marine natural product.

More than thirty years after the discovery of polycyclic ether marine natural products, they continue to receive intense attention from the chemical, biological, and pharmacological communities because of their potent biological activities and highly complex molecular architectures. Gambieric acids are intriguing polycyclic ethers that exhibit potent antifungal activity with minimal toxicity against mammals. Despite the recent advances in the synthesis of this class of natural products, gambieric acids remain unconquered due to their daunting structural complexity, which poses a formidable synthetic challenge to organic chemists. This paper reviews our long-term studies on the total synthesis, complete configurational reassignment, and structure-activity relationships of gambieric acid A over the last decade.