We have achieved a total synthesis of telomestatin and determined it absolute configuration to be (R). During the synthetic study for its enantiomer, serious epimerization in the macrolactamization was observed. The problem was overcome under the reaction conditions utilizing DPPA/HOBt/DMAPO and the (S)-isomer was synthesized. Measurement of Tm values obtained from the CD melting curves indicated that (S)-isomer stabilizes the antiparallel G-quadruplex structure of telomea more strongly than telomestatin. The (S)-isomer exhibited more potent inhibitory activity against telomerase than telomestatin. We also synthesized telomestatin analogues that are heptaoxazole macrocycles including different numbers of methyloxazole moieties or containing a bromooxazole. The bromooxazole-containing heptaoxazole analogue underwent Suzuki-Miyaura coupling leading to aryl-substituted oxazole-containing heptaoxazole analogues. Although the substituents on the oxazole moieties in the heptaoxazole macrocyclic analogues did not affect telomerase inhibitory activity, one of amino-linked S-alkylated analogues exhibited potent inhibitory activity as telomestatin.
Transition metal-catalyzed cycloisomerization of carbon-carbon multiple bonds has emerged as powerful method for construction of highly substituted all-carbon or heteroatom-containing organic flameworks, which are important and core structures in organic and natural product synthesis. However, most of the reported methods required the use of noble and precious metal catalysts. Herein, we report iron- and bismuth-catalyzed introduction of carbon- and heteroatom nucleophiles into carbon-carbon multiple bonds such as alkene, alkynes, and enynes by means of their unique Lewis acid character: σ,π-chelation effect. In addition, divalent iron complex is found to be an effective promoter for a generation of aryl radicals from aryl boronic acids. By using these methods, direct arylation of benzoquinones and heteroarenes are also reported.
Carbon-carbon single bonds constitute the main frameworks of organic molecules. They are non-polar, thermodynamically stable, and kinetically inert in general. Nonetheless, it would streamline synthetic pathways if carbon-carbon single bonds are subjected to site-selective cleavage and directly utilized for construction and/or functionalization of organic skeletons. We herein describe the synthetic methods we have developed based upon carbon-carbon bond activation during the past four years. Rhodium(I) complexes cleave the carbon-carbon bond of strained tert-alcohols by β-carbon elimination to generate organorhodium(I) intermediates, and thus trigger reconstruction of their frameworks into totally different ones with excellent atom economy. These reactions are combined with Norrish-Yang type photo-reactions to make thermodynamically disfavored transformations possible. Ring expansion of orthocyclophanes to metacyclophanes and enantioselective synthesis of 3-hydroxyindolines from α-(N-aryl)amino ketones present prototypical examples of organic synthesis driven by photo-energy. Also described is a restructuring reaction of N-arenesulfonylazetidinols into benzosultams.
Basic concept and applications of AIM dual functional analysis (AIM-DFA) are discussed. Total electron energy densities (Hb(rc)) are plotted versus Hb(rc)-Vb(rc)/2 [=(ħ2/8m)∇2ρb(rc)] at bond critical points (BCPs) of interactions in question, where Vb(rc) are potential energy densities at BCPs. In our treatment, data of perturbed structures around fully optimized ones are employed for the plot, in addition to those of the fully optimized structures. Data of fully optimized structures correspond to the static nature, whereas those of perturbed structures with fully optimized ones represent the dynamic behavior of interactions, which we have proposed. Methods to generate the perturbed structures are also proposed. The treatment enables us to evaluate, classify, and understand well the weak to strong interactions in a unified form. Applications of AIM-DFA are explained, after confirmation of very good coincidence between calculated and experimental results, supported by the high resolution X-ray determination of ρ(r).
The utilization of lanthanoid metal salts and organolanthanoid compounds as synthestic reagents or catalysts in organic synthesis has been steadily interesting due to their unique chemical properties. Low-valent, trivalent, and tetravalent lanthanoid compounds have been widely used in organic reactions, however, the direct use of lanthanoid metal is still rare due to their low solubility in organic solvents or their instability under aerobic conditions. In this paper, we show the various organic reactions by the direct use of lanthanum metal as a reagent. Various unstable carbon species, such as alkyl radicals, o-quinodimethane, carbene, benzyne and alkylidene carbene, were easily generated by the reaction of organohalogen compounds with lanthanum metal. Carbonyl compounds were also reduced by lanthanum metal and the generated reducing species were widely used on the new carbon-carbon bond formations. The synthesis of organochalcogenides was successfully achieved by the reaction of dichalcogenides, alkyl halides, and lanthanum metal.
Toward the preparation of heterocycles using 1,3-dipoles, the approaches have been generally limited to concerted 1,3-dipolar cycloaddition. A strategy consisted of a stepwise addition to 1,3-dipoles followed by cyclizaion could be an alternative pathway to synthesize of heterocycles. We have been interested in nucleophilic addition reactions to nitrones. In pursuing them, synthesis of heterocycles based on a new strategy of stepwise nucleophilic addition-cyclization and related transformations consisted of [1,3]-sigmatropic rearrangement was developed. A novel [5+1] cycloaddition reaction of C,N-cyclic-N’-acyl azomethine imines as 1,5-dipoles, 1,3-dipoles with extended conjugation, with isocyanides was explored. A [3+1+1] cycloaddition of azomethine ylides generated from aziridines in situ with isocyanides was also realized by the use of Lewis-acid catalyst. Furthermore, unprecedented ring enlargement reaction of the azomethine imines with sulfonium ylide was developed.
In 2009, Buchwald group demonstrated the first catalytic nucleophilic fluorination to aryl fluorides in the presence of Pd-catalyst based on tBuBrettPhos with CsF as a fluoride source. Based on the significant report, the palladium-catalyzed nucleophilic fluorination system has been modified to improve the reactivity. Additionally, some groups reported the Cu-mediated and -catalyzed nucleophilic fluorination, wherein the reactions are considered to involve the formation of Cu(III) intermediates.
Carbene ligands such as N-heterocyclic carbenes (NHC) and cyclic alkyl(amino)carbenes (cAAC) have been utilized for the synthesis of low-coordinated silicon compounds. The reduction of NHC- and cAAC-coordinated silicon tetrachlorides 2 and 7 gives unique silicon compounds 3-5 and 8-12, respectively. These studies should be important from the viewpoint of the isolation of the reactive small molecules such as Si2 and SiCl2. The properties of the central silicon fragments of NHC derivatives 3-5 and cAAC derivatives 8-12 are different from each other reflecting the stronger π-accepting character of cAAC compared with that of NHC.