Sarcophytonolides are cembranolide diterpenes isolated from the soft corals of genus Sarcophyton since 2005. Herein, unified total synthesis of sarcophytonolides C, E, F, G, H, and J and isosarcophytonolide D is reported. The key transformations in the synthetic route are fragment coupling, alkoxycarbonylallylation, macrolactonization, and transannular ring-closing metathesis. These total syntheses resulted in the absolute stereochemical confirmation of sarcophytonolide H, determination of sarcophytonolides C, E, F, and G, and revision of sarcophytonolide J and isosarcophytonolide D. We also evaluated the antifouling activity and toxicity of the synthetic sarcophytonolides H and J and their analogues as well as the cytotoxicity of the synthetic sarcophytonolides and their key synthetic intermediates.
Accumulated evidences suggest that soluble oligomers of amyloid β42 and 40 (Aβ42 and 40) would be responsible for the Alzheimer’s disease. We proposed putative toxic dimer and trimer structures of Aβ42 with a turn at positions 22 and 23 (toxic turn) based on the systematic proline replacement, electron spin resonance, and solid-state NMR. Based on these structures, we synthesized several dimer models of the toxic-conformation-restricted E22P-Aβ40 using an ʟ,ʟ-2,6-diaminopimeric acid (DAP) or an ʟ,ʟ-2,8-diaminoazelaic acid linker (DAZ) at position 30 or 38 (Fig. 4, 1～3). The trimer models of E22P-Aβ40 using 1,3,5-phenyltris-ʟ-alanine (PtA) as a linker at position 34, 36, or 38 were also synthesized (Fig. 9, 4～6). Ion mobility-mass spectrometry suggested that high-molecular weight oligomers (12～24-mer) were formed only in the dimer model with DAP linker at position 38 (3), and this model exhibited potent neurotoxicity against SH-SY5Y cells. Although only the trimer model with PtA at position 38 (6) existed as high-molecular weight oligomers (9～21-mer), its neurotoxicity was far less than that of the corresponding dimer model (3). These data indicate that the C-terminal hydrophobic core plays an important role for the formation of high molecular weight oligomers (～20-mer) with neurotoxicity, and such a propeller-type trimer model (6) could not give oligomers with potent neurotoxicity. Similar results were obtained using the corresponding dimer and trimer models of E22P-Aβ42 (E22P-V40DAP-Aβ42 dimer, E22P-V36PtA-Aβ42 trimer, and E22P-V40PtA-Aβ42 trimer), suggesting that the results obtained in the Aβ40 dimer and timer models could be extended to the models of more toxic Aβ42.
The synthesis and property of new three-dimensional curved heterocyclic π-electron molecules with embedded nitrogen atoms such as helix-, butterfly-, bowl-, and flake-shaped molecules were investigated. Particularly, novel heterocyclic molecules with an embedded hydrazine structure were focused. Hydrazine-embedded helix-shaped bicarbazole, bidimethylacridine, and biphenothiazine were synthesized by dimerization of the corresponding heterocycles. The hydrazine-embedded helical molecules were found to undergo electron transfer disproportionation by acid stimulus, forming their stable radical cations and reduced species through the acid-triggered N-N bond cleavage reaction. In the case of bidimethylacridine, the disproportionation reaction is highly reversible through neutralization with NEt3 through back electron transfer and N-N bond formation. Utilizing their reversible redox property, the hydrazine-embedded molecules were applied to cathode active materials of rechargeable lithium organic batteries with excellent performance. Ni-mediated coupling of dibromo-dimethylacridine, phenothiazine, or acridone was found to afford double-butterfly-shaped cyclodimers in high yields. Following N-N bond formation gave hydrazine-embedded butterfly-shaped or planar molecules. The double-butterfly-shaped cyclobiphenothiazine was converted to a cyclodimer of carbazole, cyclo-1,8-carbazolylene, through reductive desulfurization. N-N bond formation of cyclo-1,8-carbazolylene afforded hydrazine-embedded bowl-shaped bicarbazole. The bicarbazole was found to possess bowl and twist structures in the neutral state, a shallow bowl structure in the monocation state, and a planar structure in the dication state by X-ray analysis, 13C NMR, and DFT calculations. Hydrazine-embedded helix-, butterfly-, and bowl-shaped molecules expressed reversible oxidation at a wide range of potentials and red-shift of absorption and fluorescence. The bowl-shaped dimer, a flake-shaped cyclotrimer, a square-shaped cyclotetramer, and a hexagon-shaped cyclohexamer of carbazole were synthesized by Ni-mediated coupling of 1,8-dibromocarbazole, and were named [n]cyclo-1,8-carbazolylenes (n=3,4,6). A flake-shaped cyclo-4,6-dibenzofuranylene was also obtained from dibromodibenzofuran. B, P, and Si atoms were introduced to cyclo-1,8-carbazolylene, giving flake-shaped complexes.
Peptides are recognized as capable asymmetric catalysts, and have a great potential for the application to unique selective reactions. Most of the reported peptide catalysts consist of a few amino acid residues, and their structures are relatively simple. Our group designed peptide catalysts by combining multiple secondary structural units, and found that these peptides with turn and helical motifs were effective to promote various amine-catalyzed reactions enantioselectively. Such turn-helix-type peptides were applicable to the selective reactions that could not be performed with conventional low-molecular-weight catalysts; an enzyme-co-catalyzed reaction, regioselective reduction, diastereoselective cyclopropanation, kinetic resolution of a planar-chiral compound, and desymmetrization reaction to induce planar chirality were attained. We also explored novel peptide scaffolds by the screening of peptide libraries. Histidine-containing peptide catalysts with a substrate-binding ability and a highly active peptide catalyst in aqueous media were successfully developed.
[n]Radialenes consisting of an n-membered ring and n exocyclic double bonds are a class of cyclic π-conjugated molecules. They have much attention from the viewpoint of their unique molecular and electronic structures due to the cyclic cross-conjugation systems. Various derivatives substituted at the terminal position of the exomethylene carbons were synthesized so far. The derivatives and analogs of [n]radialene substituted with redox-active 1,3-dithiol-2-ylidenes (DTs) (n=4-6) are of interest as multistep redox systems that n DT sites work. Among them, DTradialene derivatives (9) indicate a unusual redox behavior. However, the detailed studies on the synthesis, structural and electronic properties of 9 are in the process of development for the functional materials. In this paper, we revealed the molecular and electronic structures, significant redox behavior, the reactivity of 9 and its oxidized species. Herein, we introduce the synthesis and redox properties of DT[n]radialenes: hydration reaction of (9)4+ and the molecular structure of its products (13, 14, 15), the possible mechanism ; the synthesis and properties of analogs bearing aromatic rings (20) and π-extended derivatives (22) of 9; the synthesis and redox properties of TTF-fused donor exhibiting a simultaneous four-or eight-electron transfer (24, 25, 27, 33, 34, 35, 36).
Synthesis and characterization of a dimer of an aluminyl anion complex have been achieved by using a xanthene-based bulky tridentate ligand. The fact that this species acts as a nucleophile provides the evidence of umpolung at the central aluminium atoms. In addition, this dimeric species and its monomeric analogs displayed a distinctive reactivity to cause the C-H and C-C oxidative addition of benzene, respectively. Furthermore, the reaction of the monomeric analog with a gold reagent affords an aluminum-gold heteronuclear complex, which exhibits nucleophilic reactivity with the insertion of CO2 at the gold center.