Furan-terminated cationic cyclizations allylic alcohols, enones, and N-acyl iminium ions as initiators have been explored as routes to highly functionalized spiro[4,5]decanes, spiro[5,5]undecanes, spiro[4,6]undecanes, and spiro[5,6]dodecanes. 相似文献
The N7/O6 equatorial binding interactions of the antitumor active complex Rh(2)(OAc)(4)(H(2)O)(2) (OAc(-) = CH(3)CO(2)(-)) with the DNA fragment d(GpG) have been unambiguously determined by NMR spectroscopy. Previous X-ray crystallographic determinations of the head-to-head (HH) and head-to-tail (HT) adducts of dirhodium tetraacetate with 9-ethylguanine (9-EtGH) revealed unprecedented bridging N7/O6 guanine nucleobases that span the Rh-Rh bond. The absence of N7 protonation at low pH and the notable increase in the acidity of N1-H (pK(a) approximately 5.7 as compared to 8.5 for N7 only bound platinum adducts), suggested by the pH dependence titrations of the purine H8 (1)H NMR resonances for Rh(2)(OAc)(2)(9-EtG)(2) and Rh(2)(OAc)(2-)[d(GpG)],are consistent with bidentate N7/O6 binding of the guanine nucleobases. The pK(a) values estimated for N1-H (de)protonation, from the pH dependence studies of the C6 and C2 (13)C NMR resonances for the Rh(2)(OAc)(2)(9-EtG)(2) isomers, concur with those derived from the H8 (1)H NMR resonance titrations. Comparison of the (13)C NMR resonances of C6 and C2 for the dirhodium adducts Rh(2)(OAc)(2)(9-EtG)(2) and Rh(2)(OAc)(2)[d(GpG)] with the corresponding resonances of the unbound ligands [at pH 7.0 for 9-EtGH and pH 8.0 for d(GpG)], shows substantial downfield shifts of Deltadelta approximately 11.0 and 6.0 ppm for C6 and C2, respectively; the latter shifts reflect the effect of O6 binding to the dirhodium centers and the ensuing enhancement in the acidity of N1-H. Intense H8/H8 ROE cross-peaks in the 2D ROESY NMR spectrum of Rh(2)(OAc)(2)[d(GpG)] indicate head-to-head arrangement of the guanine bases. The Rh(2)(OAc)(2)[d(GpG)] adduct exhibits two major right-handed conformers, HH1 R and HH2 R, with HH1 R being three times more abundant than the unusual HH2 R. Complete characterization of both adducts revealed repuckering of the 5'-G sugar rings to C3'-endo (N-type), retention of C2'-endo (S-type) conformation for the 3'-G sugar rings, and anti orientation with respect to the glycosyl bonds. The structural features obtained for Rh(2)(OAc)(2))[d(GpG)] by means of NMR spectroscopy are very similar to those for cis-[Pt(NH(3))(2))[d(GpG)]] and corroborate molecular modeling studies. 相似文献
N-2-(4-picolyl)-N′-2-chlorophenylthiourea, 4PicTu2Cl, monoclinic, P21/c, a=10.068(5), b=11.715(2),
β=96.88(4)°,
and Z=4; N-2-(6-picolyl)-N′-2-chlorophenylthiourea, 6PicTu2Cl, triclinic, P-1, a=7.4250(8), b=7.5690(16), c=12.664(3) Å, =105.706(17), β=103.181(13), γ=90.063(13)°, V=665.6(2) Å3 and Z=2 and N-2-(6-picolyl)-N′-2-bromophenylthiourea, 6PicTu2Br, triclinic, P-1, a=7.512(4), b=7.535(6), c=12.575(4) Å, a=103.14(3), β=105.67(3), γ=90.28(4)°, V=665.7(2) Å3 and Z=2. The intramolecular hydrogen bonding between N′H and the pyridine nitrogen and intermolecular hydrogen bonding involving the thione sulfur and the NH hydrogen, as well as the planarity of the molecules, are affected by the position of the methyl substituent on the pyridine ring. The enthalpies of fusion and melting points of these thioureas are also affected. 1H NMR studies in CDCl3 show the NH′ hydrogen resonance considerably downfield from other resonances in their spectra. 相似文献
Two simple and fast C18 and HILIC liquid chromatography–electrospray mass spectrometry methods for the determination of hyaluronic acid (HA) in a mucoadhesive chitosan-based formulation were developed and validated. The performances of both methods were compared in terms of validation parameters and matrix effect. A simple sample preparation method based on sulphuric acid-based degradation was optimized for the detection of HA fragments (i.e. m/z 380 2-mer, m/z 759 4-mer, m/z 1,138 8-mer and m/z 1,518 16-mer). By operating under selected ion-monitoring mode, excellent selectivity towards chitosan fragments was obtained. For validation, good linearity, detection limits (<4 μg mL−1) and precision (RSD % < 16 %) were generally obtained on matrix with both columns. However, HILIC column exhibited improved performances in terms of HA fragment separation and selectivity. By analyzing on the C18 column the chitosan-based formulation and sample extracts from pig mucosa treated with the formulation, matrix effects exhibited a dependence of signal suppression degree (ranging from 37 to 83 %) as a function of the HA fragment dimension. The HILIC column afforded instead a significantly reduced suppression degree (ranging from 1 to 16 %) and a better separation. These findings demonstrated the improved performances of the HILIC column with respect to conventional C18 mechanism for the analysis of HA fragments in complex matrices.
NH3–plasma treated multi-walled carbon nanotubes (pn-MWCNTs) with cation traps for the detection of ultratrace quantities of Zn(II), Cd(II), Cu(II), and Hg(II) using square wave anodic stripping voltammetry (SWASV) is described. The pn-MWCNTs use their adsorption performance to enhance the sensitivity. It is found that under optimized conditions Zn(II), Cd(II), Cu(II) and Hg(II) were individually detected at potentials of −1.16, −0.78, −0.268 and 0.108 V, respectively. The detection limit (3σ method) of 0.314, 0.0272, 0.2263, and 0.1439 nM toward Zn(II), Cd(II), Cu(II), and Hg(II) is achievable, respectively. No interference could be seen during the simultaneous detection of Zn(II), Cd(II), Cu(II), and Hg(II). The pn-MWCNTs exhibit excellent selectivity owing to the different ability of adsorption. A study of the ability of pn-MWCNTs in practical application is carried out using a sample of water collected from Dongpu Reservoir in Hefei City, Anhui, China. It is found that the results were favorable when compared against inductively coupled plasma atomic emission spectrometry (ICP-AES) analysis. 相似文献