- The Stereochemistry of Crown Ethers II -- 1H and 13C NMR Structural Study in Solution
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The 1H and 13C NMR spectra of a series of structurally related crown ethers have been recorded, and the signals assigned by means of 1H/13C heteronuclear correlated and 1H/1H homonuclear NOE difference spectroscopy.The NMR parameters so extracted have been used to discuss the preferred, rapidly interconverting solution conformations of the studied species.The high-field shifts in the 13C NMR spectra, within the so-called γ-fragments, and also the 1H/1H homonuclear NOEs, are particularly informative in this respect.The T1 relaxation times of the 13C atoms show segmental motions within the ether segments and suggest different intramolecular flexibilities for the studied compounds.KEY WORDS 1H/13C NMR 2D and 1D NOE difference spectroscopy Crown ethers Conformations in solution Intramolecular flexibility.
- Kleinpeter, E.,Gaebler, M.,Schroth, W.,Mattinen, J.,Pihlaja, K.
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- Microwave-assisted synthesis of dibenzo-crown ethers
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Microwave-assisted organic synthesis (MAOS) for dibenzo-substituted crown ethers is presented. Two routes were developed: (1) one-pot MAOS for symmetric dibenzo-crown ethers (DBC) and (2) a two-step MAOS via diphenol intermediates for both symmetric and asymmetric DBCs. MAOS were carried out in open or closed vessels, with or without temperature control at various microwave settings using different bases and reactants. Open vessel MAOS was limited by the volatility of reactants hence was less preferred than the closed vessel MAOS. DBC formation was highly affected by the cation size of the base, which acted as a template ion during DBCs ring closure. Closed vessel MAOS without temperature control was found most appropriate for DBC synthesis. Symmetric DBCs were conveniently obtained via one-pot MAOS whereas asymmetric DBCs were obtained from two-step MAOS via diphenol intermediates. The method was found expedient as it afforded satisfactory yields at considerably shorter reaction time than those in conventional methods.
- Torrejos, Rey Eliseo C.,Nisola, Grace M.,Beltran, Arnel B.,Park, Myoung Jun,Patil, Basavaraj R.,Lee, Seong-Poong,Seo, Jeong Gil,Chung, Wook-Jin
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p. 109 - 115
(2014/03/21)
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- Preparation of benzo- and polybenzocrown ethers by macrocyclization reactions
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Macrocyclization of crown ethers and their methods of preparation were explored. We present a robust, scalable method for the preparation of these macrocycles. Additionally, the effect of changing the structures of the precursors was explored to determine whether the 'cut' of bisphenol and dimesylate influenced the course of the reaction as measured by the yield. Further, using catechol derivatives, the method was used for the preparation of monobenzocrown ethers. Interestingly, for the preparation of monobenzocrown ethers, [2+2] adducts were discovered to be significantly contaminating the products. Dimesylates were chosen as the leaving group due to their ease or preparation and the ability to use the unpurified products with no apparent impact on the macrocyclization.
- Hanes, Robert E.,Lee, Jong Chan,Ivy, Sheryl N.,Palka, Anna,Bartsch, Richard A.
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p. 238 - 248
(2012/11/07)
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- An Improved Method for Preparing Dibenzo Crown Ethers
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In synthesis of symmetrical and unsymmetrical dibenzo crown ethers, the use of dimethyl sulfoxide as a solvent in cyclization allows an increase in the yields, a significant reduction in the reaction time, and simplification of refining.
- Kotlyar,Gorodnyuk,Grigorash,Chuprin
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p. 1135 - 1138
(2007/10/03)
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- Dibenzo-15-crown-5 ether and its sodium thiocyanate complex. X-ray crystallographic and NMR studies in the solid phase and in solution
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For the title systems, single crystal X-ray data indicate the presence of totally asymmetric structures.These features are reflected inn the multiplicity of resonances in the solid phase 13C NMR spectra.Some of the 13C chemical shifts trends are analyzed in terms of torsional angle influences.In solution, resonance assignment have been made with the aid of 2D methods.Stereochemical inferences are drawn from NOESY spectra and 1H-1H couplings.Low temperature solution experiments indicate that conformational interconversions in both the free ligand and the NaNCS complex are rapid on the NMR timescale down to 163 K.Key words: crown ethers, NMR, stereochemistry.
- Buchanan, G. W.,Mathias, S.,Lear, Y.,Bensimon, C.
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p. 404 - 414
(2007/10/02)
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