ABSTRACT The synthesis of the glycosyl donors 2,3-di-O-acetyl- and 2,3-di-O-chloroacetyl- 4,6-O-ethylidene-ß-D-glucopyranose (10β) and (11β) and their use for the glycosidation of 4'-O-benzyloxycarbonyl- and 4'-O-chloroacetyl-4'-O-demethyl-4-epi-podophyllotoxins (12) and (13) is described. Starting from benzyl β-D-glucopyranoside (6), benzyl 2,3-di-O-acetyl- and 2,3-di-O-chloroacetyl-4,6-O-ethylidene-ß-D-glucopyranoside (8) and (9) were prepared. Hydrogenolysis of the benzyl group in 8 or 9 afforded the β-hydroxy glucopyranose donors 10β and 11β. Condensation of 10β or 11β with 4'-O-Z-epi-podophyllotoxin 12 in the presence of BF3-etherate gave selectively the 4-O-(2,3-di-O-acetyl- or -2,3-di-O-chloroacetyl-4,6-O-ethylidene-β-D-glucopyranosyl)-epi-podophyllotoxins 14β and 15β, respectively. The β-glycoside 16 was prepared in the same manner starting from 11β and 4'-O-chloroacetyl-epi-podophyllotoxin 13. By deblocking (Dowex 1X8, 3:2 methanol-chloroform) of the chloroacetyl groups in 15β and the following hydrogenolysis of the benzyloxycarbonyl group in 17 etoposide 1 was obtained. The deacylation of 16 afforded 1 in a one step procedure. 相似文献
A concise and efficient total synthesis of (-)-umbelactone 1, an occurring γ-hydroxymethyl-α, β-butenolide from Memycelon umbelatum Burro, is described. The synthesis features the use of a ring closing metathesis strategy. 相似文献
This paper reports a number of recent developments in the intercalation chemistry of Al(OH)(3). From Rietveld refinement and solid-state NMR, it has been possible to develop a structural model for the recently reported [M(II)Al(4)(OH)(12)](NO(3))(2)·yH(2)O family of layered double hydroxides (LDHs). The M(2+) cations occupy half of the octahedral holes in the Al(OH)(3) layers, and it is thought that there is complete ordering of the metal ions while the interlayer nitrate anions are highly disordered. Filling the remainder of the octahedral holes in the layers proved impossible. While the intercalation of Li salts into Al(OH)(3) is facile, it was found that the intercalation of M(II) salts is much more capricious. Only with Co, Ni, Cu, and Zn nitrates and Zn sulfate were phase-pure LDHs produced. In other cases, there is either no reaction or a phase believed to be an LDH forms concomitantly with impurity phases. Reacting Al(OH)(3) with mixtures of M(II) salts can lead to the production of three-metal M(II)-M(II)'-Al LDHs, but it is necessary to control precisely the starting ratios of the two M(II) salts in the reaction gel because Al(OH)(3) displays selective intercalation of M nitrate (Li > Ni > Co ≈ Zn). The three-metal M(II)-M(II)'-Al LDHs exhibit facile ion exchange intercalation, which has been investigated in the first energy dispersive X-ray diffraction study of a chemical reaction system performed on Beamline I12 of the Diamond Light Source. 相似文献
A novel and versatile approach for the mixing of ZnO nanofillers into a host polymer matrix, poly(ethyl methacrylate) (PEMA), is reported. Firstly, ZnO nanoparticles are deposited onto the surface of polystyrene (PS) colloidal particles in a “raspberry‐like” fashion and subsequently obtained PS/ZnO composite particles are mixed into the PEMA matrix in the range of 0.5 to 5 wt.‐%. Microscopic analyses reveal a homogenous distribution of PS/ZnO domains into the PEMA matrix even at 5 wt.‐% loading level. Thermogravimetric analysis and differential scanning calorimetry results indicate an improvement in thermal stability of PEMA matrix after mixing with PS/ZnO filler particles. A significant enhancement in mechanical properties of PEMA matrix in the presence of PS/ZnO particles has been evidenced by dynamic mechanical analysis and three point bending measurements.
Lewis-base adducts of dichlorodioxomolybdenum(VI) and dimethyldioxomolybdenum(VI) react in an equilibrium reaction with excess t-butylhydroperoxide (TBHP) under the formation of a seven-coordinated molybdenum(VI) complexes displaying a η1-alkylperoxo-ligand. HCl/CH4 elimination or the protonation of the Lewis-base ligand is not observed, the TBHP hydrogen atom is instead transferred to one of the terminal oxo ligands under the formation of a molybdenum bound OH moiety. The peroxo species is assumed to be the active catalyst in olefin epoxidation. 相似文献
The title intermediate () is produced on photolysis of hexakis(2,6-diethyl-phenyl)cyclotrigermane () or bis(2,6-diethylphenyl)bis(trimethylsilyl)germane () as evidenced by trapping experiments, and thermally dimerizes to tetrakis(2,6-diethyl-phenyl)digermene (). Diarylgermylenes such as do not form stable triethylamine adducts (e.g. ) as has been previously reported. 相似文献
Some new class of amino isoxazolidine derivatives have been synthesized from α‐amino nitrones in water with good to excellent yields. The new isoxazolidines are further functionalized into new scaffolds of peptides with good synthetic potentiality. Simple reaction methodology, greener approaches, atom efficiency, noninvolvement of catalysts, faster reaction conditions, excellent yields, and a new approach in peptide synthesis are the attractions in these syntheses. 相似文献
Methyl(vinyl)dichlorosilane reacts with DMSO in the presence of hexamethyldisiloxane to give the corresponding linear oligosiloxanes
of the general formula Me3Si(OSiMeVin)nOSiMe3 (n=1–6) as well as MeSi(OSiMe3)3 and Me3Si(MeOSiVin)mOSi(OSiMe3)(Me)OSiMe3 (m=1–2). The same reaction in the presence of chlorotrimethylsilane results in oligomers of the general formula Me3Si(OSiMeVin)nCl (n=1–3). A possible scheme of their formation is discussed.
Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 8, pp. 1614–1616, August, 1998. 相似文献
The synthesis of the two new phosphoramidites 5 and 8 bearing a carbostyril (=quinolin‐2(1H)‐one) chromophore used as donor entity in our recently developed new FRET (fluorescence‐resonance‐energy transfer) system is described (Schemes 1 and 2) The high stability of the chromophore to basic conditions enables the incorporation of the phosphoramidites directly into DNA during solid‐phase synthesis (Schemes 3 and 4). Since this is also possible for the (bathophenanthroline)ruthenium(II) complex used as acceptor (Scheme 4, Steps d and e), the whole labelling procedure to insert the FRET system into synthetic DNA is straightforward and represents a major improvement to our previous strategy. 相似文献