共查询到20条相似文献,搜索用时 0 毫秒
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Robert W. Lenz R. A. Awl W. R. Miller E. H. Pryde 《Journal of polymer science. Part A, Polymer chemistry》1970,8(2):429-444
The glycerol acetal of methyl azelaaldehydate is an ω-hydroxy ester that exists as a mixture of dioxolanyl and dioxanyl isomers, each having two geometric isomers. Each of the four isomers was isolated by chromatographic (gas–liquid and column) and fractionating (crystallization and distillation) methods. Structural assignments were made on the basis of nuclear magnetic resonance and infrared spectral data. Linear poly(ester-acetals) were prepared from each of the cis and trans forms of the dioxanyl isomers and from a mixture of the dioxolanyl geometric isomers. Physical properties of these polymers were correlated with their structures. When prepared with basic condensation catalysts, the polymers retained the geometric configuration and structural identity of the monomer. When prepared with lead acetate, structural rearrangement as well as polycondensation occurred, resulting in an enrichment of the dioxolanyl isomer and simultaneous increase in polymer crystallinity. The enrichment took place at elevated temperatures and also, unexpectedly, at room temperature upon long standing of the polymer. Isomer redistribution at room temperature appears to be antithermodynamic in the reaction sense, yielding a polymer of unusually high dioxolanyl unit content. The driving force for this isomerization may be crystallization of these units by a phenomenon termed a “crystallization-induced reaction.” 相似文献
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Simon J. Coles David B. Davies Michael B. Hursthouse Hanıfe İbişoğlu Adem Kılıç Robert A. Shaw 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(1):o51-o54
The reaction of spermidine with 2,2,4,4-tetrachloro-6,6-diphenylcyclotriphosphazene produces a mixture of products of which one of the fractions is a spiro-cis-ansa derivative, namely 12-chloro-14,14-diphenyl-2,6,11,13,15,16-hexaaza-1,12-diphosphatricyclo[10.3.1.01,6]hexadeca-12,14-diene, C19H26ClN6P3. Recrystallization of this fraction from different solvents results in the formation of two different crystalline forms. The rod morphology formed in dichloromethane–n-hexane (1:1) produces a triclinic structure with three molecules in the asymmetric unit. These three molecules adopt different conformations as a result of two NH groups flipping in an ansa-bridged ring system. The plate morphology crystals, grown in dichloromethane–n-hexane–ethyl acetate (1:1:1), produce a C-centred monoclinic structure that adopts a conformation that is essentially the same as one of the forms in the triclinic structure. 相似文献
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A.S.R. Anjaneyulu A.V. Raghava Reddy D.S.K. Reddy R.S. Ward D. Adhikesavalu T. Stanley Cameron 《Tetrahedron》1984,40(21):4245-4252
The isolation of resveratrol and a new dibenzoxepin derivative, pacharin, from the heartwood of Lank is reported. The structure of pacharin has been established as l,7-dlhydroxy-3-methoxy-2-methyl-dibenzo(2,3-6,7) oxepin (Va) by a study of its chemical and spectroscopic properties, including X-ray analysis. 相似文献
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Heller M Sukopp M Tsomaia N John M Mierke DF Reif B Kessler H 《Journal of the American Chemical Society》2006,128(42):13806-13814
The conformation of the cyclic pentapeptide cyclo(-D-Pro-Ala(4)-) in solution and in the solid state was reinvestigated using modern NMR techniques. To allow unequivocal characterization of hydrogen bonds, relaxation behavior, and intramolecular distances, differently labeled isotopomers were synthesized. The NMR results, supported by extensive MD simulations, demonstrate unambiguously that the preferred conformation previously described by us, but recently questioned, is indeed correct. The validation of the conformational preferences of this cyclic peptide is important given that this system is a template for several bioactive compounds and for controlled "spatial screening" for the search of bioactive conformations. 相似文献
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The protected hydroxyethylene dipeptide isostere of Phe-Arg and the tripeptide derivative 1 were synthesized as components of potential peptidase inhibitors. Key to the success of these syntheses is selective rhodium-catalyzed hydroboration in the presence of a readily reduced lactone. A convenient one-pot conversion of azides to diprotected guanidines was developed on the basis of the Staudinger reaction. 相似文献
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[structure: see text] The protected hydroxyethylene dipeptide isostere of Gln-Arg and the tripeptide derivative 1 were synthesized as components of potential peptidase inhibitors. 相似文献