Synthesis and structures of deoxyribonucleoside analogues for triazole-linked DNA (DNA)

Deoxyribonucleoside analogues bearing acetylene group at the pseudo-5′-position and azido group at the pseudo-3′-position have been synthesized by transglycosylation reaction of deoxythymidine analogue with adenine, cytosine, and guanine nucleobases as nucleophiles. The structures of analogues were studied in crystalline state by X-ray crystallography as well as in solution phase by NMR spectroscopy and showed the puckering conformations similar to the natural congeners.     

Palladium complex catalyzed reaction of diphenylketene with terminal acetylenes giving disubstituted acetylenes

The reaction of diphenylketene with terminal acetylenes catalyzed by tetrakis-(triphenylphosphine)palladium gave disubstituted acetylenes in high yields.     

Diverse reactions of nitroxide-radical adducts of silylene, germylene, and stannylene

The results of the thermolysis of 1:2 adducts of stable group-14 element divalent compounds [R₂M:, R₂=1,1,4,4-tetrakis(trimethylsilyl)butane-1,4-diyl; 1b, M=Ge; 1c, M=Sn] to TEMPO radical are discussed in detail. Whereas the thermal reactions of the 1:2 adducts [R₂M(OR^′)₂, R^′=2,2,6,6-tetramethylpiperidin-N-yl; 3b, M=Ge; 3c, M=Sn] are understood to proceed by the initial homolysis of an M-O bond to give the corresponding aminoxy-substituted group-14 element radicals [R₂(R^′O)M; 2b, M=Ge; 2c, M=Sn] and TEMPO, the subsequent reactions of 2b and 2c were remarkably different to each other; 2b favors the N-O bond fission (path b) to give the corresponding germanone, while 2c prefers the M-O bond fission (path a) to give stannylene (1c). In combining with our previous results for aminoxysilyl radical (2a) [R₂(R^′O)Si], the origin of the remarkable differences in the reactivity among group-14 element radicals 2a-2c is discussed on the basis of the theoretical calculations for model reactions.Improved syntheses of the precursor dichlorogermane and dichlorostannane of germylene (1b) and stannylene (1c), respectively, are described in Section 3.     

Synthesis of (22S, 23S)-homobrassinolide and brassinolide from stigmasterol

(22S, 23S)-Homobrassinolide (2α, 3α, 22S, 23S-tetrahydroxy-24S-etyl-B-homo-7-oxa-5α-cholestan-6-one) and brassinolide (2α, 3α 22R, 23R-tetrahydroxy-24S-methyl-B-homo-7-oxa-5α-cholestan-6-one) were synthesized from stigmasterol and shown to promote plant growth.     

Anionic WS-TEMPO-mediatory electrooxidation of alcohols in water: halide-free oxidation directed towards a totally closed system

Electrooxidation of alcohols in water with water-soluble N-oxyl derivatives bearing a sulfonic acid group (anionic WS-TEMPOs) as a mediator proceeded to afford the corresponding ketones and aldehydes in moderate to good yields. The aqueous solution containing WS-TEMPOs could be readily recovered and reused for the electrooxidation of alcohols, thereby providing a totally closed system.     

A stable fused bicyclic disilene as a model for silicon surface

We synthesized the first fused bicyclic disilene 1 representing topologically a partial structure of the Si(001) surface up to the third layer. In the solid state, the five-membered rings adopt the envelope conformation, and the Si=Si double bond in 1 exists in the slightly cis-bent form (bent angle theta is 3.6 degrees ) compared to that of the highly cis-bent dimer on the Si(001) surface. Highly symmetric 1H NMR spectral pattern of 1 remains even at -80 degrees C, indicating the facile ring flipping of the bicyclic skeleton in solution. While syn-adduct was obtained in the reaction of 1 with water, anti-addition of chlorine atoms across the Si=Si double bond in 1 was observed in the reaction with carbon tetrachloride. The structural characteristics of the 9,10-phenanthrenequinone adduct 7 are in good accord with those of the proposed structure of the 9,10-phenanthrenequinone molecule adsorbed on the Si(001) surface.     

Comparative chemistry of isolable divalent compounds of silicon, germanium, and tin

Recent studies of the synthesis, structures, spectroscopic properties, and reactions of a series of isolable metallylenes (R2E:, E = Si (1), Ge (2), and Sn (3); R2 = 1,1,4,4-tetrakis(trimethylsilyl)butane-1,4-diyl) are summarized. Because these group-14 metallylenes bear the same helmet-like ligand, a straightforward discussion of the element-dependence of the intrinsic properties of the group-14 element divalent compounds is possible. All these metallylenes were monomeric both in solution and in the solid state, indicating the effective steric protection by the ligand against dimerization. A small sigma-pi conjugation between C-Si(substituent) sigma orbitals and the vacant npz orbitals of divalent atoms in R2E: exists and the extent decreases in the order E = Si > Ge > Sn, as evidenced by UV-vis and NMR spectroscopies and X-ray crystallography. However, the extent of the sigma-pi conjugation in metallylenes 1-3 was much smaller than the electron-donating effects of neighboring nitrogen atoms in known stable cyclic diamino-substituted metallylenes, and hence metallylenes 1-3 are regarded as the least electronically perturbed. Comparative studies of the unique reactions among these metallylenes are also discussed.     

Synthesis of aminocyclohexitol via carbon-carbon bond-forming radical cyclization of oxime ether

The stannyl radical mediated-cyclization of oxime ether, derived from D-glucose, gave the aminocyclohexitol derivative. Stereoselective C-C bond forming cyclization proceeded via favorable conformers minimizing A(1,3)-strain between the oxime ether group and alpha-substituents.     

Stereoselective synthesis of beta-hydroxyphenylalanines using imino 1,2-Wittig rearrangement of hydroximates

The imino 1,2-Wittig rearrangement of hydroximates containing a furan ring provides a novel method for the synthesis of beta-hydroxy-alpha-amino acids. Upon treatment with LDA, hydroximates smoothly underwent the rearrangement to give Z-2-hydroxyoxime ethers in good yield, which were converted into both cis- and trans-oxazolidinones with high stereoselectivity. The cis- and trans-oxazolidinones were stereoselectively converted into erythro- and threo-beta-hydroxyphenylalanines, respectively, via the oxidative cleavage of a furan ring, ring-opening of oxazolidinone, and deprotection.