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91.
Facile cleavage C‐terminal to ornithine residues in gas phase peptides has been observed and termed the ornithine effect. Peptides containing internal or C‐terminal ornithine residues, which are formed from deguanidination of arginine in solution, were fragmented to produce either a y‐ion or water loss, respectively, and the complementary b‐ion. The fragmentation patterns of several peptides containing arginine were compared to those of the ornithine analogues. Conversion of arginine to ornithine results in a decrease of the gas phase proton affinity of the residue, thereby increasing the mobility of the ionizing proton. This alteration allows the nucleophilic amine to facilitate a neighboring group reaction to induce a cleavage of the adjacent amide bond. The selective cleavage at the ornithine residue is proposed to result from the highly favorable generation of a six‐membered lactam ring. The ornithine effect was compared with the well‐known proline and aspartic acid effects in peptide fragmentation using angiotensin II, DRVYIHPF and the ornithine analogue, DOVYIHPF. Under conditions favorable to either the aspartic acid (i.e. singly protonated peptide) or proline effect (i.e. doubly protonated peptide), the ornithine effect was consistently observed to be the more favorable fragmentation pathway. The highly selective nature of the ornithine effect opens up the possibility for conversion of arginine to ornithine residues to induce selective cleavages in polypeptide ions. Such an approach may complement strategies that seek to generate non‐selective cleavages of the related peptides. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
92.
Surfactant (SDS)‐mediated cleavage of imines was achieved with acetic anhydride to the corresponding carbonyls (aldehydes and ketones) and acetanilides in water at 25–30°C with very good to excellent yields, thus contributing significantly to the green chemistry concept. 相似文献
93.
Sankareswaran Srimurugan Paulsamy Suresh Thirukkallam Kanthadai Varadarajan 《合成通讯》2013,43(15):2483-2490
A facile method for the synthesis of (2R,3R)‐1,4‐dimethoxy‐1,1,4,4‐tetrasubstituted‐2,3‐butanediols involving oxidative cleavage of benzylidene acetal as a key step is described. These sterically hindered diols unusually formed cyclic sulfites as the major product under methanesulfonylation reaction conditions. 相似文献
94.
95.
Bogdan Boduszek Tomasz Olszewski Waldemar Goldeman Magdalena Konieczna 《Phosphorus, sulfur, and silicon and the related elements》2013,188(4):787-795
The synthesis of a series of new pyridine aminomethylphosphinic acids is described. These compounds were obtained in the reaction of the corresponding pyridine aldehydes with primary amines and with ethyl phenylphosphinate, or methylphosphinate, in the presence of bromotrimethylsilane. In aqueous, strong acid solutions, pyridine aminophosphinic acids were split, forming the phenyl-, or methylphosphonic, acid and the corresponding secondary pyridyl-alkylamines. The kinetics of some observed cleavages were measured, and a mechanism of the cleavage has been proposed. 相似文献
96.
In this paper, the mechanism of the full catalytic cycle for binuclear Cu(I)-catalyzed sulfonyl azide-alkyne cycloaddition reaction for the synthesis of triazolopyrimidines was rationalized by density functional theoretical (DFT) calculations. The computed reaction route consists of: (a) formation of dicopper intermediates, including C−H activation of terminal alkyne, 3+2 ring cycloaddition and ring-reducing reaction and transmetalation, (b) interrupted CuAAC reaction, including di-copper catalyzed ring-opening of 2H-azirines and C−C bond formation to generate the copper-triazoles and -ketenimines, (c) two-step C−N cross-coupling and following (d) multi-step hydrogen transfer by the hydrogen bonding chain of water to promote the C−N formation and another C−N cleavage through the removal of p-tolyl sulfonamides. Our DFT results indicate that the multi-step hydrogen transfer process is the rate-determining step along the potential energy surface profile. The explicit water model was used for systematic determination of barrier for C−C cross-coupling, C−N bond formation and cleavage, and p-tolylsulfonamide removal. A critical insight in the interrupted CuAAC reaction was proposed. Further prediction interprets H2O hydrogen bond chain plays an important role in C−N bond formation and cleavage, and the removal of p-tolylsulfonamide. This may have fundamental guidance on the design of 1, 5-herterocyclic functionalized triazolopyrimidines via interrupted CuAAC rearrangement reaction, as well as hydrogen bond chain of water. 相似文献
97.
98.
Lorenzo Di Terlizzi Prof. Stefano Protti Prof. Davide Ravelli Prof. Maurizio Fagnoni 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(26):e202200313
With the aim of generating new, thermally inaccessible diradicals, potentially able to induce a double-strand DNA cleavage, the photochemistry of a set of chloroaryl-substituted carboxylic acids in polar media was investigated. The photoheterolytic cleavage of the Ar−Cl bond occurred in each case to form the corresponding triplet phenyl cations. Under basic conditions, the photorelease of the chloride anion was accompanied by an intramolecular electron-transfer from the carboxylate group to the aromatic radical cationic site to give a diradical species. This latter intermediate could then undergo CO2 loss in a structure-dependent fashion, according to the stability of the resulting diradical, or abstract a hydrogen atom from the medium. In aqueous environment at physiological pH (pH=7.3), both a phenyl cation and a diradical chemistry was observed. The mechanistic scenario and the role of the various intermediates (aryl cations and diradicals) involved in the process was supported by computational analysis. 相似文献
99.
Wei-Jing Gao Chiu Marco Lam Bao-Guo Sun R. Daniel Little Cheng-Chu Zeng 《Tetrahedron》2017,73(17):2447-2454
The electrochemically oxidative cleavage of lignin β-O-4 model compounds mediated by iodide ion has been studied. The results indicate that electrolytic conditions play a predominant role in determining the distribution of cleavage products. The preparative-scale electrolysis proceeds in a simple undivided cell, employing a catalytic amount of NaI as the redox mediator and supporting electrolyte in methanol. Under these conditions, the CβO bond is selectively cleaved with 2,2-dimethoxy-2-arylacetaldehyde being the main product. In some cases, the reaction gives a good yield of cleavaged products. The results further demonstrate that the indirect electrolysis mediated by halide is a versatile approach for chemical transformation. 相似文献
100.
A metal-free protocol for the selective cleavage of unstrained C–C single bonds was developed. Under the catalysis of KI and in the presence of NaHCO3, the readily available α-chloro-β-hydroxy ketones underwent bond breaking and sulfonylation smoothly to afford β-ketosulfones with high efficiency and broad substrate scope. Mechanism investigations, both experimental and theoretical, showed that a retro-aldol cleavage/nucleophilic substitution sequence might be involved. 相似文献