The total synthesis of biselide E, a marine polyketide isolated from the Okinawan ascidian, has been accomplished. The highlight of this approach is the use of the β-elimination reaction of the chloroacetoxy group for the construction of an unstable six-membered α,β,γ,δ-unsaturated lactone portion at the late stage of the total synthesis.

The antitumor and actin-depolymerizing marine macrolide aplyronine A (ApA) synergistically binds to tubulin in association with actin, and prevents spindle formation and mitosis. While the crystal structure of the actin ApA complex was solved in 2006, its interaction with the tubulin heterodimer has not been clarified. To investigate the binding modes of ApA as a unique protein–protein interaction (PPI)-inducer between these two cytoskeletal proteins, we prepared its photoaffinity acetylene and fluorescent derivatives with the aid of molecular modeling studies for probe design. Among these three derivatives, the ApA–PPA–TAMRA probe specifically photoreacted with both actin and tubulin in vitro. However, the photolabeling yield of tubulin was quite low (up to ∼1%), and one of the major side-reactions was the addition of a water molecule to the carbene species generated from an aryldiazirine moiety on the hydrophilic surface of actin.Download high-res image (88KB)Download full-size image

Second-generation total synthesis of aplyronine A, a potent antitumor marine macrolide, was achieved using Ni/Cr-mediated coupling reactions as key steps. The overall yield of the second-generation synthetic pathway of aplyronine A was 1.4%, obtained in 38 steps based on the longest linear sequence. Compared to our first-generation synthetic pathway of aplyronine A, the second-generation synthesis greatly improved both the yield and number of steps. In particular, we improved the stereoselectivity in the construction of the C13 stereogenic center and the C14–C15 (E)-trisubstituted double bond using the asymmetric Ni/Cr-mediated coupling reaction. Furthermore, we established efficient reaction conditions for the asymmetric Ni/Cr-mediated coupling reaction between the C21–C28 segment and C29–C34 segment. Thus, this coupling reaction proceeded with an equimolar ratio of each segment.

•Aplysiasecosterol A, a 9,11-secosterol compound, was isolated from Aplysia kurodai.•This compound has a unique tricyclic γ-diketone skeleton including a hemiacetal.•The tricyclic core was constructed by intramolecular radical cyclization.Aplysiasecosterol A, a 9,11-secosterol compound, has a unique tricyclic γ-diketone skeleton including a hemiacetal. The novel tricyclic core of aplysiasecosterol A is constructed by using intramolecular radical cyclization as a key step.Download high-res image (106KB)Download full-size image

The antitumor macrolide aplyronine A induces protein–protein interaction (PPI) between actin and tubulin to exert highly potent biological activities. The interactions and binding kinetics of these molecules were analyzed by the surface plasmon resonance with biotinylated aplyronines or tubulin as ligands. Strong binding was observed for tubulin and actin with immobilized aplyronine A. These PPIs were almost completely inhibited by one equivalent of either aplyronine A or C, or mycalolide B. In contrast, a non-competitive actin-depolymerizing agent, latrunculin A, highly accelerated their association. Significant binding was also observed for immobilized tubulin with an actin–aplyronine A complex, and the dissociation constant KD was 1.84 μM. Our method could be used for the quantitative analysis of the PPIs between two polymerizing proteins stabilized with small agents.

We have discovered O6-benzyl glaziovianin A, which showed stronger inhibition of microtubule polymerization (IC50 = 2.1 μM) than known α,β-tubulin inhibitors, such as colchicine and glaziovianin A. Also, we performed competition binding experiments of O6-benzyl glaziovianin A and revealed that O6-benzyl glaziovianin A binds to the colchicine binding site with high affinity. It is interesting that glaziovianin A derivatives change their mode of action in benzylation at the O6 (α,β-tubulin inhibitor) or O7 (γ-tubulin-specific inhibitor) position.

Two new 9,11-secosteroids with a cis-fused 3β,5β-dihydroxy-1,4-quinone structure, aplysiasecosterols B and C, were isolated from the sea hare Aplysia kurodai. Their structures were determined by 1D- and 2D-NMR spectroscopic analysis, molecular modeling studies, and a modified Mosher’s method. Aplysiasecosterol B might be the biosynthetic precursor of aplysiasecosterol A, another 9,11-secosteroid with a tricyclic γ-diketone structure from A. kurodai, via two α-ketol rearrangements and intramolecular acetalization.

Covering: up to 2014 Various marine natural products that target cytoskeletal proteins have been discovered. A few of these compounds have recently been shown to induce or inhibit protein–protein interactions. Lobophorolide, an actin filament-disrupting macrolide, binds to actin with a unique 2:2 stoichiometry in which two lobophorolide molecules cooperate to stabilize an actin dimer. Adociasulfates, merotriterpenoid derivatives, inhibit microtubule-stimulated ATPase activity of a motor protein kinesin by blocking both the binding of microtubules and the processive motion of kinesin along microtubules. The antitumor macrolide aplyronine A synergistically binds to tubulin in association with actin, and prevents spindle formation and mitosis. In this highlight, we address recent chemical biology studies on these mechanistically-attractive marine natural products. These findings may be useful for the design and development of new pharmacological tools and therapeutic agents.

Yuzurimine-type alkaloids make up a sub-family of Daphniphyllum alkaloids structurally featuring a [6-7-5-5] tetracarbocyclic core framework. In this manuscript, we describe our construction of the [6-7-5-5] tetracarbocyclic core of yuzurimine-type alkaloids by using a unique in situ intramolecular Wittig reaction and Sm-mediated cyclization as key steps.

Halichonine B is a sesquiterpene alkaloid isolated from the marine sponge Halichondria okadai Kadota. Halichonine B has exhibited cytotoxicity against mammalian cancer cells and induced apoptosis in the human leukemia cell line HL60. Here we established a practical route for the synthesis of halichonine B and its analogues, and we evaluated their biological activities. It was revealed that the secondary amino groups in the side chain portion are important for the strong cytotoxicity of halichonine B and that the N11-prenyl group is unimportant. Halichonine B and its analogues were also observed to induce apoptosis in HL60 cells.

Aplyronine A (ApA) is a marine natural product that shows potent antitumor activity. While both ApA and ApC, a derivative of ApA that lacks a trimethylserine ester moiety, inhibit actin polymerization in vitro to the same extent, only ApA shows potent cytotoxicity. Therefore, the molecular targets and mechanisms of action of ApA in cells have remained unclear. We report that ApA inhibits tubulin polymerization in a hitherto unprecedented way. ApA forms a 1:1:1 heterotrimeric complex with actin and tubulin, in association with actin synergistically binding to tubulin, and inhibits tubulin polymerization. Tubulin-targeting agents have been widely used in cancer chemotherapy, but there are no previous descriptions of microtubule inhibitors that also bind to actin and affect microtubule assembly. ApA inhibits spindle formation and mitosis in HeLa S3 cells at 100 pM, a much lower concentration than is needed for the disassembly of the actin cytoskeleton. The results of the present study indicate that ApA represents a rare type of natural product, which binds to two different cytoplasmic proteins to exert highly potent biological activities.

•A new cyclic heptapeptide, stylissatin A, was isolated from the marine sponge.•The structure of stylissatin A is cyclo-[Tyr1–Ile2–Phe3–Pro4–Ile5–Pro6–Phe7].•Stylissatin A inhibited NO production in LPS-stimulated macrophage RAW264.7 cells.A new cyclic heptapeptide, stylissatin A (1), was isolated from the Papua New Guinean marine sponge Stylissa massa. Through the use of 1D and 2D NMR spectroscopic analysis, Marfey’s method, and MS/MS analysis, its structure was determined to be cyclo-[Tyr1–Ile2–Phe3–Pro4–Ile5–Pro6–Phe7]. Stylissatin A inhibited nitric oxide production in LPS-stimulated murine macrophage RAW264.7 cells with an IC50 value of 87 μM.

The antitumor and apoptogenic macrolide aplyronine A (ApA) is a potent actin-depolymerizing agent. We developed an ApA acetylene analog that bears the aryldiazirine group at the C34 terminus, which formed a covalent bond with actin. With the use of the photoaffinity biotin derivatives of aplyronines A and C, Arp2 and Arp3 (actin-related proteins) were specifically purified as binding proteins along with actin from tumor cell lysate. However, Arp2 and Arp3 did not covalently bind to aplyronine photoaffinity derivatives. Thus, actin-related proteins might indirectly bind to ApA as the ternary adducts of the actin/ApA complex or through the oligomeric actin.

Synthesis of the macrolactone part of biselides A (1) and B (2), marine cytotoxic polyketides, was achieved by using regioselective allylic oxidation as a key step.

Biselides A and B are cytotoxic marine polyketides. We have achieved synthesis of the C-1–C-15 segment of biselides A and B by using Stille coupling and regioselective oxidative cleavage as key steps. Furthermore, we constructed the α,β-unsaturated lactone part of biselide E by using a similar strategy.

13-Oxyingenol and its derivatives have high levels of anti-HIV activity. A fully substituted tetracyclic skeleton of 13-oxyingenol is constructed by using spiro-cyclization and ring-closing olefin metathesis as key steps.

An efficient synthesis of the C1−C19 segment of aplyronine A is described. Stereoselective construction of the C14−C15 (E)-trisubstituted double bond and the C13 stereocenter was achieved by using an asymmetric Nozaki−Hiyama−Kishi coupling.

Various derivatives of glaziovianin A, an antitumor isoflavone, were synthesized, and the cytotoxicity of each against HeLa S3 cells was investigated. Compared to glaziovianin A, the O7-allyl derivative was found to be more cytotoxic against HeLa S3 cells and a more potent M-phase inhibitor.

The artificial analogs of aurilide (1), a potent cytotoxic cyclodepsipeptide of marine origin, were synthesized, and the structure–activity relationships were investigated.The artificial analogs of aurilide (1), a potent cytotoxic cyclodepsipeptide of marine origin, were synthesized, and the structure–activity relationships were investigated.

Three saponins, isotheasaponins B1–B3, were isolated from the leaves of the tea plant Camellia sinensis var. sinensis, and their structures were determined to be theasapogenol B [β-d-galactopyranosyl(1 → 2)][β-d-xylopyranosyl(1 → 2)-α-l-arabinopyranosyl(1 → 3)]-β-d-gulcopyranosiduronic acid with two acyl groups by spectroscopic analysis.Three saponins, isotheasaponins B1–B3, were isolated from the leaves of the tea plant Camelia sinensis var. sinensis, and their structures were determined by spectroscopic analysis.

A hybrid compound consisting of aplyronine A and mycalolide B was synthesized, and its biological activities were evaluated. The hybrid compound was found to have somewhat more potent actin-depolymerizing activity than aplyronine A. In contrast, the hybrid compound possessed about 1000-fold less cytotoxicity than aplyronine A. These results indicated that there is no direct correlation between actin-depolymerizing activity and cytotoxicity.

Yuzurimine-type alkaloids make up a sub-family of Daphniphyllum alkaloids structurally featuring a [6-7-5-5] tetracarbocyclic core framework. In this manuscript, we describe our construction of the [6-7-5-5] tetracarbocyclic core of yuzurimine-type alkaloids by using a unique in situ intramolecular Wittig reaction and Sm-mediated cyclization as key steps.

Halichonine B is a sesquiterpene alkaloid isolated from the marine sponge Halichondria okadai Kadota. Halichonine B has exhibited cytotoxicity against mammalian cancer cells and induced apoptosis in the human leukemia cell line HL60. Here we established a practical route for the synthesis of halichonine B and its analogues, and we evaluated their biological activities. It was revealed that the secondary amino groups in the side chain portion are important for the strong cytotoxicity of halichonine B and that the N11-prenyl group is unimportant. Halichonine B and its analogues were also observed to induce apoptosis in HL60 cells.

We have established an efficient synthetic methodology for the 13-oxyingenol natural derivative (13-oxyingenol-13-dodecanoate-20-hexanoate), featuring a ring-closing olefin metathesis reaction for the “direct” construction of a highly strained inside–outside framework and a Mislow–Evans-type [2,3]-sigmatropic rearrangement for the stereoselective introduction of the hydroxy group at C5. We also synthesized artificial analogs of 13-oxyingenol and ingenol by using our synthetic strategy. In vitro activation assays of protein kinase C (PKC) α and δ revealed that the dodecanoyl group at O13 on 13-oxyingenol analogs had a significant role in PKCδ activation. The PKCα- or PKCδ-activating 13-oxyingenol and ingenol analogs induced both distinct morphological changes and increases of CD11b expression in HL-60 cells, which would be typical signs of HL-60 cell differentiation to macrophage-like cells, as expected by previous reports. Intriguingly, however, similar differentiation phenotypes were observed with the use of 13-oxyingenol natural derivatives and 13-oxyingenol-13-dodecanoate showing a remarkably less potent PKCα or PKCδ activation ability, which the PKC inhibitor Gö6983 diminished. This indicated the involvement of other PKC isozymes or related kinase activities. 13-Oxyingenol analogs, which induced HL-60 cell differentiation, also induced HL-60 cell death, similar to the action of a phorbol ester, a strong PKC activator.

Second-generation total synthesis of aplyronine A, a potent antitumor marine macrolide, was achieved using Ni/Cr-mediated coupling reactions as key steps. The overall yield of the second-generation synthetic pathway of aplyronine A was 1.4%, obtained in 38 steps based on the longest linear sequence. Compared to our first-generation synthetic pathway of aplyronine A, the second-generation synthesis greatly improved both the yield and number of steps. In particular, we improved the stereoselectivity in the construction of the C13 stereogenic center and the C14–C15 (E)-trisubstituted double bond using the asymmetric Ni/Cr-mediated coupling reaction. Furthermore, we established efficient reaction conditions for the asymmetric Ni/Cr-mediated coupling reaction between the C21–C28 segment and C29–C34 segment. Thus, this coupling reaction proceeded with an equimolar ratio of each segment.