Of aromatic side chains

Titov claims that the free radical mechanism applies for nitration of aliphatic hydrocarbons, of aromatic side chains, of olefins, and of aromatic ring carbons, if irf the latter case the nitrating agent is ca 60—70% nitric acid that is free of nitrous acid, or even more dil acid if oxides of nitrogen are present... 

See Shoppee (1972 359), citing C. K. Ingold and E. Rothstein, "The Nature of the Alternating Effect in Carbon Chains. Pt. XXV. The Mechanism of Aromatic Side-Chain Substitution," JCS 131 (1928) ... 

C. R. Coan, L. M. Hinman, and D. A. Deranleau, Charge-transfer studies of the availability of aromatic side chains of proteins in guanidine hydrochloride, Biochemistry 14, 4421 4427... 

Etard reagents (chromyl chloride and some derivatives) suffer from the problem that occasionally they can exhibit a lack of selectivity and low yields. They are useful in the selective oxidation of aromatic side-chains to a carbonyl group, aldehyde or ketone but in many instances, the formation of the initial complex is slow and yields are low because of difficulties in the work-up which lead to undesired over-reaction. Attempts have been made to solve the problems by the use of sonication [134]. A simple preparation of the liquid reagent was proposed and the Etard reaction itself together with the hydrolytic step were conducted under sonication, with some success (Scheme 3.25). 

Modification of Chemical Structure of Drug The use of a Hammett linear free-energy relationship to investigate the effects of substituents on the rates of aromatic side-chain reactions such as hydrolysis of esters has been alluded to earlier vis-a-vis attainment of optimum stability [9,10]. Degradation of erythromycin under acidic pH conditions is inhibited by substituting a methoxy group for the C-6 hydroxyl as found for the acid stability of clathromycin, which is 340 times greater than that of erythromycin [70]. 

J. and Bernal, 1. (2004) A critical look on the nature of the intra-molecular iriterligarid Jt,Jt-stackrng interaction in mixed-ligand copper(II) complexes of aromatic side-chain amino acidates and a,a -diimines. Cryst Eng Comm, 6, 627. 

Oxidative cleavage of olefins, terminal alkynes, or aromatic rings 9-11 Oxidation of aromatic side chains 9-21 Oxidation of amines 9-22 Oxidation of primary alcohols or ethers... 

The reactivity of aromatic side chains to undergo dealkylation is in line with the stability of the corresponding carbocations. This indicates the possible involvement of carbocations in dealkylation, which was proved to be the case. The intermediacy of the rm-butyl cation in superacid solution was shown by direct spectroscopic observation.228,229 Additional proof was provided by trapping the ferf-butyl cation with carbon monoxide during dealkylation 230... 

Reaction 1 has been postulated both in oxidations of alkanes in the vapor phase (29) and in the anti-Markovnikov addition of hydrogen bromide to olefins in the liquid phase (14). Reaction 2 involves the established mechanism for free-radical bromination of aromatic side chains (2). Reaction 4 as part of the propagation step, established in earlier work without bromine radicals (26), was not invoked by Ravens, because of the absence of [RCH3] in the rate equation. Equations 4 to 6, in which Reaction 6 was rate-determining, were replaced by Ravens by the reaction of peroxy radical with Co2+ ... 

Figure 7-17 The structure of insulin. (A) The amino acid sequence of the A and B chains linked by disulfide bridges. (B) Sketch showing the backbone structure of the insulin molecule as revealed by X-ray analysis. The A and B chains have been labeled. Positions and orientations of aromatic side chains are also shown. (C) View of the paired N-terminal ends of the B chains in the insulin dimer. View is approximately down the pseudo-twofold axis toward the center of the hexamer. (D) Schematic drawing showing packing of six insulin molecules in the zinc-stabilized hexamer.

Methoxy-2,2,6,6-tetramethyl-1 -oxopiperidinium chloride, 183 Pyridinium chlorochromate-Benzotria-zole, 262 of amines to nitro compounds Dimethyldioxirane, 120 other nitrogen compounds /-Butyl hydroperoxide-Dichlorotris-(triphenylphosphine)ruthe-nium(II), 54 Di-/-butylimoxyl, 94 Potassium ruthenate, 259 Ruthenium tetroxide, 268 of aromatic side chains Trimethylsilyl chlorochromate, 327 of diols... 

Potassium nitrosodisulfonate, 258 Trimethylsilyl chlorochromate, 327 By hydrolysis of acetals or thioacetals Amberlyst ion-exchange resin, 152 Methylthiomethyl p-tolyl sulfone, 192 By isomerization of allylic alcohols N-Lithioethylenediamine, 157 By oxidation of aromatic side chains Trimethylsilyl chlorochromate, 327 From oxidative cleavage of alkenes [Bis(salicylidene-7-iminopropyl)-methylamine]cobalt(II)... 

Second, what type of residues are best at the a and d sites This is also a difficult question to answer direcdy because, as will be addressed in more detail over the next few sections, the nature of residues at these sites influences coiled-coil stability, oligomer state, partner selection, and helix-helix orientation (Table I). However, in general terms, natural coiled-coil sequences tend to use the aliphatic hydrophobic residues (Ala, lie, Leu, Met, and Val) at these positions, rather than the aromatic hydrophobic side chains (Phe, Trp and Tyr) (Parry, 1982 Woolfson and Alber, 1995). The reason for this is probably a combination of bulk and steric constraints presented by the aromatic residues. However, a thorough understanding of the possible exclusion of aromatic side chains from coiled-coil... 

A good yield of p-nitrobenzoic acid2 is obtained from p-nitrotoluene if a lead peroxide anode be employed in a mixture of sulphuric and acetic acids. Investigations upon the oxidation of aromatic side-chains have been conducted by Smith3 and also by Law and Perkin.4... 

To conclude, in a manner complementary to the interactions of oxygens and sulfurs with the hydrogens of aromatic rings, amino group hydrogens can make enthalpically favorable, weak, polar interactions with the tt-electrons of aromatic rings. These interactions are of sufficient importance to cause a marked anisotropy in the distribution of N—H and NH3 groups in the vicinity of aromatic side chains. 

Spectral Properties of Aromatic Side Chains. The CD of proteins arises from signals from aromatic side chains (Phe, Tyr, and Trp) as well as disulfides. As the CD of each of these groups is dependent on the local environment, this leads to the view that perturbation of the near UV indicates disruption of the localized folding pattern. The CD and absorption bands display maxima near 280-290 nm for Trp, near 275-280 nm, and around 250-260 nm for Phe [7, 8],... 

Contributions of Aromatic Side Chains to the Far UV CD of Proteins. Numerous theoretical studies of the effects of aromatic groups on both the far and near UV CD spectra of proteins have been conducted by Hooker and co-workers [143-154], While the calculations on larger proteins were limited in scope, they do provide the only comprehensive attempt to include these chromophores into CD calculations (see below). Other researchers have attempted coupled-oscillator calculations on proteins such as insulin [155, 156], to assess the effects of tertiary structure on near UV CD spectra. More recent work by Woody and co-workers expanded the matrix method to include more elaborate descriptions of... 

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