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64.
Stephen G. Davies Ian A. Hunter Rebecca L. Nicholson Paul.M. Roberts Edward D. Savory Andrew D. Smith 《Tetrahedron》2004,60(35):7553-7577
α-Dibenzylamino- and α-benzyloxy- derivatives of N-acetyl-(S)-4-benzyl-5,5-dimethyloxazolidin-2-one readily undergo highly stereoselective boron mediated syn-aldol reactions with a range of aromatic and aliphatic aldehydes, generating the syn-aldol products in good to excellent yields as single diastereoisomers after purification. In the α-dibenzylamino series, deprotection of the functionalised aldol fragments to the corresponding α-amino-β-hydroxy methyl ester or α-amino-β-hydroxyaldehyde proved problematic, with a range of N- and O-protecting groups giving mixtures of products arising from endocyclic and exocyclic cleavage pathways. However, in the α-benzyloxy series, O-silyl protection of the aldol products, and subsequent DIBAL reduction gives stereoselectively the corresponding N-1′-hydroxyalkyloxazolidin-2-ones, which undergo base promoted fragmentation to the desired highly functionalised and differentially protected α,β-dihydroxyaldehydes in good yields and without loss of stereochemical integrity. 相似文献
65.
Kerry J. Adams Thomas D. McGrath Georgina M. Rosair Andrew S. Weller Alan J. Welch 《Journal of organometallic chemistry》1998,550(1-2)
Analysis of the structures of 8,8-(PPh3)2-8,7-nido-RhSB9H10 and 9,9-(PPh3)2-9,7,8-nido-RhC2B8H11 by RMS misfit calculations has confirmed that these rhodaheteroboranes possess nido 11-vertex cluster geometries in apparent contravention of Wade's rules. However, examination of the molecular structures of both species shows that the {RhP2} planes are inclined by ca. 66° with respect to the metal-bonded SB3 or CB3 faces, and that two weak ortho-CHRh agostic interactions occupy the vacant co-ordination position thereby created. As a consequence of these agostic bonds the Rh atom, and hence the overall cluster, is provided with an additional electron pair, meaning that their nido structures are now fully consistent with Wade's rules. The chelated diphosphine compound 8,8-(dppe)-8,7-nido-RhSB9H10 is similar to the PPh3 compound in showing the same agostic bonding. Attempts to prepare a bis-P(OMe)3 analogue result in ligand scavenging and the formation of 8,8,8-{P(OMe)3}3-8,7-nido-RhSB9H10. Similarly, reaction between Cs[6-arachno-SB9H12] and RhCl(dmpe)CO does not result in CO loss but in formation of 8,8-(dmpe)-8-(CO)-8,7-nido-RhSB9H10, shown to exist as a mixture of two of three possible rotamers. Deprotonation of 8,8-(PPh3)2-8,7-nido-RhSB9H10 and 8,8-(dppe)-8,7-nido-RhSB9H10 with MeLi yields the anions [1,1-(PPh3)2-1,2-closo-RhSB9H9]− and [1,1-dppe-1,2-closo-RhSB9H9]−, respectively, with octadecahedral cage structures. It is argued that anion formation causes the agostic bonding to be `switched-off' and results in the cluster adopting the closo architecture predicted by Wade's rules. This structural change is fully reversible on reprotonation, and if reprotonation of [1,1-(dppe)-1,2-closo-RhSB9H9]− is carried out in MeCN, the product 8,8-(dppe)-8-(MeCN)-8,7-nido-RhSB9H10 forms. Interestingly, 8,8-(dppe)-8-(MeCN)-8,7-nido-RhSB9H10 reconverts to 8,8-(dppe)-8,7-nido-RhSB9H10 on standing in CDCl3, suggesting that the agostic bonding is sufficiently strong to displace co-ordinated MeCN. All new compounds are fully characterised by multinuclear NMR spectroscopy and, in many cases, by single crystal X-ray diffraction. 相似文献
66.
Thomas Huber Andrew E. Torda Wilfred F. van Gunsteren 《Journal of computer-aided molecular design》1994,8(6):695-708
Summary The concept of memory has been introduced into a molecular dynamics algorithm. This was done so as to persuade a molecular system to visit new areas of conformational space rather than be confined to a small number of low-energy regions. The method is demonstrated on a simple model system and the 11-residue cyclic peptide cyclosporin A. For comparison, calculations were also performed using simulated temperature annealing and a potential energy annealing scheme. Although the method can only be applied to systems with a small number of degrees of freedom, it offers the chance to generate a multitude of different low-energy structures, where other methods only give a single one or few. This is clearly important in problems such as drug design, where one is interested in the conformational spread of a system. 相似文献
67.
Stulz E Scott SM Ng YF Bond AD Teat SJ Darling SL Feeder N Sanders JK 《Inorganic chemistry》2003,42(20):6564-6574
The synthesis of linear multiporphyrin arrays with mono- and bisphosphine-substituted porphyrins as ligand donors and ruthenium(II) or rhodium(III) porphyrins as ligand acceptors is described. With appropriate amounts of the building blocks mixed, linear dimeric and trimeric arrays have been synthesized and analyzed by (1)H NMR and (31)P NMR spectroscopy. The Ru/Rh acceptor porphyrins can be located either at the periphery or in the center of the array. Likewise, the monophosphine porphyrins can be positioned at the periphery, thus allowing a high degree of freedom in the overall composition of the arrays. This way, both donor and acceptor porphyrins can act as chain extenders or terminators. One of the trimeric complexes with two nickel and one ruthenium porphyrin has also been analyzed by X-ray crystallography. Attempts have also been made to synthesize higher order arrays by mixing appropriate amounts of the porphyrins; however, from the NMR data it cannot be concluded if monodisperse five, seven, or nine porphyrin arrays are present or if the solutions are composed of a statistical mixture of smaller and larger arrays. 相似文献
68.
Crystal Structure of AgCu3Cu(AsO4)3 and its Structural Relations to AgCo3H2(AsO4)3 and AgZn3H2 (AsO4)3 The compound AgCu3Cu(AsO4)3 was synthesized and investigated by X-rays. It crystallizes in the monoclinic space group C2/c with a = 1 212.7(2), b = 1 249.0(2), c = 727.8(1) pm, β = 117.94(1)°, Z = 4. The structure is closely related to the structures of AgCo3H2(AsO4)3 and AgZn3H2(AsO4)3. Only two hydrogen atoms are replaced by an additional copper atom forming a copper coordination square instead of two hydrogen bridges. The remaining copper atoms are sixfold coordinated with the generally observed Jahn-Teller distortion. Whereas in AgCo3H2(AsO4)3 and AgZn3H2(AsO4)3 silver has a (4+4) coordination, it is in this compound distinctly eightfold coordinated. 相似文献
69.
On P? P Containing Cyclic Compounds. II. Reaction of N,N'-dimethylthiourea with phenyl dichlorophosphane yields 1.4-dimethyl-2. 3-diphenyl-1.4.2.3-diazadiphospholidine-5-thione-2-sulfide ( II ). The compound is characterized by its nmr, vibration and mass spectra as well as the results of an X-ray structural analysis. Byproduct of the reaction is N-(N,N'-dimethylcarbamimidoyl)-N,N'-dimethyl thiourea hydrochloride ( III ), which crystallizes with pyridinium chloride. Molecular and crystal structure are reported. With methyl dichlorophosphane N,N'-dimethylthiourea forms 1.2.3.4-tetramethyl-1.4.2.3-diazadiphospholidine-5-thione-2-sulfide ( IV ), which is characterized by its nmr and vibration spectra. 相似文献
70.
Malkinson JP Anim MK Zloh M Searcey M Hampshire AJ Fox KR 《The Journal of organic chemistry》2005,70(19):7654-7661
[reaction: see text] In this article, the first solid-phase-based total synthesis of TANDEM, a synthetic analogue of triostin A, is described. In initial studies, the synthesis incorporated depsipeptide formation, introduction of chromophores, and disulfide bond formation on the solid phase, prior to a final solution-phase macrolactamization, to give the target molecule. Although pure TANDEM was obtained in an overall yield comparable to those for all syntheses to date, the yield of the final cyclization was low (11%). A more efficient approach involved removal from the solid phase prior to disulfide bond formation. The resulting linear peptide underwent macrolactamization under mild conditions and high dilution. Final disulfide bond formation was essentially quantitative and gave the target molecule, TANDEM, in an overall yield of 18%. The final compound was assessed for its ability to bind to 5'-TpA sequences on DNA by DNase I footprinting. This efficient synthesis sets the stage for a study of the structure-activity relationship of TANDEM and the natural product triostin A, with analogues containing "point mutations" at every site within the cyclic compounds. 相似文献