- Stabilization of Azapeptides by Namide···H-NamideHydrogen Bonds
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An unusual Namide···H-Namide hydrogen bond (HB) was previously proposed to stabilize the azapeptide β-Turns. Herein we provide experimental evidence for the Namide···H-Namide HB and show that this HB endows a stabilization of 1-3 kcal·mol-1 and enforces the trans-cis-Trans (t-c-T) and cis-cis-Trans (c-c-T) amide bond conformations in azapeptides and N-methyl-Azapeptides, respectively. Our results indicate that these Namide···H-Namide HBs can have stabilizing contributions even in short azapeptides that cannot fold to form β-Turns.
- Baruah, Kalpita,Sahariah, Biswajit,Sakpal, Sushil S.,Deka, Jugal Kishore Rai,Bar, Arun Kumar,Bagchi, Sayan,Sarma, Bani Kanta
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supporting information
p. 4949 - 4954
(2021/06/28)
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- N-(Acetamido)thiourea based simple neutral hydrogen-bonding receptors for anions
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N-(Acetamido)-N′-phenylthioureas (4-6) were found to be efficient anion receptors with higher anion affinity than their N-benzamido-N′- phenylthiourea counterparts (1 and 2). The N′-phenylthiourea moiety in 4-6 was shown to be the chromophore with an absorption maximum at ca. 270 nm. It was found that, in the presence of anions, the absorption at ca. 270 nm of 4-6 (except 5f) in acetonitrile (MeCN) was blue shifted and enhanced while a red-shifted shoulder appeared at ca. 295 nm, together with an isosbestic point at ca. 240 nm. The 1:1 anion binding constants of 4-6, for example at 10 6-107 M-1 order of magnitude for AcO - in MeCN, were found to be higher than those of 1 and 2, although the acidity of the thioureido -NH protons in 4-6 is lower than that in 1 and 2. 1H NMR data indicates that the N-N single bond in 4-6 is twisted but less than that in 1 and 2. A conformation change at the N-N single bond of 4-6 was suggested to occur upon anion binding which leads to a planar hydrogen-bonding network in the anion binding complex in which a charge transfer takes place with the N-acyl moiety being the electron acceptor. Variations in the CD signals of a proline derivative 6 bearing a chiral center in the N-amido moiety provide direct evidence for this conformation change upon its binding with anions in MeCN. The amplified effect of substituent X at the N′-phenyl ring of 5 on the anion binding constant supports the conclusion of anion-binding switched charge transfer in the anion binding complex. 1H NMR and absorption titrations for 5 indicated that the anion-receptor interaction was of a hydrogen-bonding nature until the N′-phenyl substituent X is as electron-withdrawing as m-CF3 (5e). With X being the more electron-withdrawing p-NO2 (5f), deprotonation of the thioureido -NH occurs in the presence of anion. Results reported here confirm that N-amidothioureas derived from both N-aliphatic and N-aromatic amides can in general be a family of efficient hydrogen-bonding receptors, with the aliphatic N-amido derivatives being more efficient. This provides a wider structural diversity for designing thiourea-based functional molecules such as anion receptors and organocatalysts. Preliminary experiments confirm that 6 could catalyse efficiently the reduction of nitrostyrene in CH2Cl2 and MeCN.
- Liu, Wen-Xia,Yang, Rui,Li, Ai-Fang,Li, Zhao,Gao, Yu-Feng,Luo, Xing-Xing,Ruan, Yi-Bin,Jiang, Yun-Bao
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experimental part
p. 4021 - 4028
(2009/12/06)
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- Superior reactivity of thiosemicarbazides in the synthesis of 2-amino-1,3,4-oxadiazoles
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(Chemical Equation Presented) A facile and general protocol for the preparation of 2-amino-1,3,4-oxadiazoles is reported. This method relies on a tosyl chloride/pyridine-mediated cyclization of a thiosemicarbazide, which is readily prepared by acylation o
- Dolman, Sarah J.,Gosselin, Francis,O'Shea, Paul D.,Davies, Ian W.
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p. 9548 - 9551
(2007/10/03)
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- The rapid preparation of 2-aminosulfonamide-1,3,4-oxadiazoles using polymer-supported reagents and microwave heating
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Herein, we report on the preparation of a library of 5-substituted-2-amino- 1,3,4-oxadiazoles and the corresponding thiadiazole analogues. Presented is a one-pot preparation of the 2-aminosulfonylated analogues through a three component coupling of an acylhydrazine, an isocyanate and sulfonyl chloride promoted by a polymer-supported phosphazine base under microwave dielectric heating. Also described is the optimization process and details pertaining to the elucidation of the reaction products.
- Baxendale, Ian R.,Ley, Steven V.,Martinelli, Marisa
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p. 5323 - 5349
(2007/10/03)
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- An efficient synthesis of 1,3,4-oxadiazoles from N,N′-diacylhydrazines using Ph3P·Br2, Ph3P·CCl4 or Ph3P·CBr4 adducts as condensing agents
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1,3,4-Oxadiazoles including 2-amino derivatives were effectively synthesized by treatment of N,N′-diacylhydrazines or N-acylsemicarbazides with Ph3P·Br2, Ph3P·CCl4 or Ph3P·CBr4 adducts in the presence of Et3N in CH2Cl2 under mild conditions.
- Mazurkiewicz,Grymel
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- NEW METAL COMPLEXES OF SOME DIPOSITIVE METAL IONS WITH THIOSEMICARBAZIDE AND SEMICARBAZIDE DERIVATIVES
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The preparation and characterization of some dipositive metal ions with 1-acetyl-4-phenyl-3-thiosemicarbazide (ATSC) and 1-acetyl-4-phenyl-3-semicarbazide (ASC) are reported.The data of elemental analyses, molar conductivities, polarographic, magnetic mea
- El-Asmy, Ahmed Abdel-Hamid,Bekheit, Magdy Mohamed,Ibrahim, Kamal Mohamed,Mostafa, Mohsen Mahmoud
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p. 391 - 402
(2007/10/02)
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