Aromaticity to explain unique properties of benzene derivatives has long played an important role in chemistry. To expand the aromatic world, replacement of skeletal carbon atoms in the aromatic compounds by silicon and germanium atoms was performed and aromaticity of the resulting silicon- and germanium-bearing compounds has been established. Based on the background, we decided to undertake investigation on whether or not introduction of tin and lead atoms into the aromatic skeletons would retain the aromaticity. We herein summarize our recent project on the synthesis of tin- and lead-bearing aromatic compounds, dilithiostannoles and a dilithioplumbole. Application of the dianionic aromatic compounds as ligands for transition-metal complexes is also demonstrated.
In recent years, we have disclosed that O-propargylic oximes, which have a propargyl group on the oxime oxygen atom, undergo skeletal rearrangement reactions by the action of π-Lewis acidic metal catalysts, leading to a wide variety of nitrogenous heterocyclic compounds in an efficient manner with high functional group compatibility. For example, the copper-catalyzed reaction of O-propargylic oximes proceeded via 2,3-rearrangement involving C-O bond cleavage. The resulting N-allenylnitrone intermediate subsequently underwent intramolecular reactions, such as 4π-electrocyclization, 6π-electrocyclization, and nitrone-amide rearrangement, according to the functional group on the oxime carbon atom. The key intermediate also underwent cascade reactions via intermolecular transformations with various external reagents, such as electron-deficient olefins, tosylcyanates, and azodicarboxylates, affording, oxazepines, dihydropyrimidines, and triazines, respectively. The rhodium-catalyzed reactions of O-propargylic oximes having a cyclopropyl or cyclobutyl group at the oxime moiety afforded the corresponding medium ring-sized nitrogenous heteromonocycles in good yields via 2,3-rearrangement followed by oxidative cyclization of the N-allenylnitrone with rhodium(I) catalyst. In contrast, gold-catalyzed rearrangement reaction of the formaldoximes proceeded via C=N bond cleavage, affording 4-methylenated 2-isoxazolines in an efficient manner. The mechanistic studies suggest that the reaction proceeds via cyclization — intermolecular methylene transfer sequence. Moreover, gold-catalyzed reactions of O-propargylic oximes, which have an electron-withdrawing p-nitrophenyl group at the oxime moiety, proceeded through N-O bond cleavage, producing 2H-1,3-oxazines. The use of Brønsted base as cocatalyst was effective for the rearrangement reaction via N-O bond cleavage.
Mercuric trifluoromethanesulfonate [Hg(OTf)2] with high reactivity toward carbon-carbon multiple bonds has been widely used as a well-known mercury reagent for organic synthesis. On the basis of the catalytic mechanism of Hg(OTf)2 in the hydration of alkynes, we have developed new catalytic systems using the salt-metathesis reaction of Hg(OAc)2/Sc(OTf)3, PhHgOAc/HOTf, m-carbaboranylmercury chloride (m-CBHgCl)/AgOTf, which can achieve C-C bond-forming cyclizations, heterocycle synthesis and cyclization initiated by 1,3-dienes at high catalytic turnovers under mild conditions. To further enhance the usability as well as synthetic utility of the catalytic systems, the solid-supported mercuric salts (Si-PhHgOTf and Si-CBHgOTf) were developed and found to act as powerful catalyst for most mercury salt-catalyzed reactions. These heterogeneous catalysts were recovered without serious mercury leakage by simple filtration.
Over the past few decades, a number of modified oligonucleotides have been synthesized and evaluated their biophysical properties. Modified oligonucleotides having high enzymatic stability, high duplex-forming ability toward single-stranded RNA (ssRNA), high sequence specificity, and good pharmacokinetic profiles, are highly desirable for therapeutic applications. We previously demonstrated that oligonucleotides modified with 2′-O,4′-C-methylene bridged nucleic acid/locked nucleic acid (2′,4′-BNA/LNA) have extremely high duplex-forming ability toward ssRNA. In recent years, we have been focusing on developing further strengthened bridged nucleic acids, e.g. 2′,4′-BNA/LNA analogues showing high enzymatic stability, high sequence specificity, high efficacy, and low toxicity. We herein report several promising bridged nucleic acids developed recently for therapeutic applications, especially in antisense and splice-switching oligonucleotides.
Cyclic polyarylene is a series of compounds, where the cyclic structure is constructed by a number of phenylene and/or heteroarylene moieties. Cyclic polyarylenes consisting of more than three arylene moieties are non-planar, and introduction of substituent(s) induces axial chirality. We comprehensively studied catalytic and enantioselective synthesis of various cyclic polyphenylenes by consecutive inter- and intramolecular cycloadditions of triynes, where 1,6-diyne and alkyne moieties were tethered by phenylene moiety. We further used this strategy for the construction of cyclic heteroarylenes using thiophene-tethered triynes.
Next, we focused on the enantioselective construction of sulfur-containing benzo-fused medium ring systems. For examples, intermolecular cycloaddition of 1,8-diynes tethered by sulfur and two ortho-phenylene moieties with an alkyne gave axially chiral poly-substituted tribenzothiepins possessing a seven-membered ring core. We succeeded in the enantioselective synthesis of seven to eleven-membered benzo-fused compounds by intramolecular reaction of triynes consisting of sulfur-tethered 1,6-diyne and 1, n-diyne moieties (n=8—11).
Enantioselective 1,3-dipolar cycloaddition reactions of cyclic carbonyl ylides have received a great deal of attention due to their synthetic utilities for asymmetric syntheses of medium-sized polycyclic ethers containing epoxy-bridged moieties such as 8-oxabicyclo[3.2.1]octane derivatives, which are recognized as common structural units in naturally occurring biologically important compounds. Rh(II)-catalyzed intramolecular carbenoid-carbonyl cyclization reactions of diazo carbonyl compounds are one of the most efficient methodologies for generation of cyclic carbonyl ylides. Although some catalytic asymmetric methods using chiral Rh catalysts have been developed, related works on other asymmetric catalysts including a dual catalytic system have rarely been demonstrated. We have developed a new catalytic system, an achiral Rh complex/chiral Lewis acid system, for the asymmetric 1,3-dipolar cycloaddition reactions of cyclic carbonyl ylides with benzyloxyacetaldehyde derivatives, α-keto esters, and 3-alkenoyl-2-oxazolidinones. Our methodology can be applied not only to normal electron-demand cycloadditions but also to inverse electron-demand cycloadditions using electron-rich dipolarophiles such as vinyl ethers and N-methylindoles. This article describes the range of the diazocarbonyl compounds as the cyclic carbonyl ylide precursors and their synthetic applications to chiral indolizidine alkaloids, including (+)-Tashiromine. Asymmetric 1,3-dipolar cycloadditions between diazo imine-derived cyclic azomethine ylides and a 2-acryloyl-3-pyrazolidinone using the dual catalytic system, which afford optically active 8-azabicyclo[3.2.1]octane derivatives, are also reported.
Cyanostar was developed as a pseudo-C5h-symmetric macrocyclic anion receptor which possesses an electropositive central cavity surrounded by five weak hydrogen-bonding donors. Cyanostar strongly binds large non-coordinating anions, such as BF4-, ClO4-, and PF6-, as 2 : 1 sandwich complexes. Addition of cyanostar to LiPF6 solution, a representative electrolyte in lithium ion battery, facilitated salt dissociation to generate free Li+ and enhanced conductivity and Li+ transference number. This finding opens new opportunity for the development of novel battery electrolytes.