Modification chains

Substitution Reactions on Side Chains. Because the benzyl carbon is the most reactive site on the propanoid side chain, many substitution reactions occur at this position. Typically, substitution reactions occur by attack of a nucleophilic reagent on a benzyl carbon present in the form of a carbonium ion or a methine group in a quinonemethide stmeture. In a reversal of the ether cleavage reactions described, benzyl alcohols and ethers may be transformed to alkyl or aryl ethers by acid-catalyzed etherifications or transetherifications with alcohol or phenol. The conversion of a benzyl alcohol or ether to a sulfonic acid group is among the most important side chain modification reactions because it is essential to the solubilization of lignin in the sulfite pulping process (17). 

Although the activity of methoxylated monobactams could be improved by appropriate side-chain modifications, difficulty of synthesis and poor chemical stabihty focused attention on the nonmethoxylated analogues. Both high intrinsic activity and excellent P-lactamase stabiUty are exhibited by monobactams that combine C-3 arninothia2ole oxime side chains and 4-alkyl, 4-alkenyl, and 4-alkynyl groups (19). 

Side chain modification has been carried out on several aziridines for example, various aziridine methanols have been prepared from 2-methoxycarbonylaziridines (70JOC3424). Aziridine methanols (296) react with thionyl chloride in the presence of base to give both rearranged (299) and unrearranged (298) chlorides (70JOC3428). The rearrangement product is thought to be formed via the azabicyclobutane intermediate (297). 

This fact can serve as a model system for the study of PS-MA-Lewis acid systems. Results obtained allow one to present the general scheme for the side chain modification of PS in the presence of Lewis acid in the following way ... 

In view of these constraints, we recently suggested a different strategy for the improvement of the material properties of synthetic poly (amino acids) (12). Our approach is based on the replacement of the peptide bonds in the backbone of synthetic poly(amino acids) by a variety of "nonamide" Linkages. "Backbone modification," as opposed to "side chain modification," represents a fundamentally different approach that has not yet been explored in detail and that can potentially be used to prepare a whole family of structurally new polymers. 

In addition to the two major processes, cross-linking and chain modification (or cyclization), chain scission doubtless occurs also to varying degrees during conventional vulcanizations. Processes of this nature are not difficult to envisage in the presence of free radicals. The radical intermediate (II) may, for example, undergo /3-fission as follows ... 

Many studies on side-chain modifications in PF were initially based on the idea of excimer formation, resulting in the green emission during LED operation or in solid-state PL on annealing PF films. This resulted in several proposed strategies for the design of fluorene side-chain homopolymers, where bulky substituents at position 9 of the fluorene moiety should sterically prevent (hinder) interchain interaction and thus improve the stability of blue emission. 

Imaging Processes Based on Side-Chain Modification of Polymers... 

The radiation induced side-chain modification of polymers containing pendant phenyl ester groups has been the object of several studies as some undergo a photo-Fries rearrangement (JJ while others such as... 

FRECHETETAL. Imaging Based on Polymer Side-Chain Modification... 

Table 3.9. THE ACTIVITY OF SIDE-CHAIN MODIFICATIONS OF GUANETHIDINE...

The influence of chain length and side-chain modifications of ACTH-derived peptides on active avoidance behaviour in rats will be discussed. H-Met(02)-Glu-His--Phe-D-Lys-Phe-OH (Org 2766) emerged from these studies as an orally active peptide with an increased potency and selectivity of action. Physico-chemical data (from the literature) on the reference peptide ACTH--(4-10) did not point to a preferred conformation in solution, whereas in the crystalline state an antiparallel 3-pleated sheet structure was found. At the receptor site we suggested an a-helical conformation in which the Phe and Met residues are close together. Additional support for this suggestion came from the behavioural activity of [des-Tyr", Met ]enkephalin and of cyclo--(-Phe-Met-cAhx-), eAhx merely serving as a spacer. 

Figure 1.2 A schematic representation of the versatility of reversible, supramolecular side-chain modification and selected examples of interactions that can be employed.

Ilhan F, Gray M, Rotello VM. Reversible side chain modification through noncovalent interactions. Plug and play polymers. Macromolecules 2001 34 2597-2601. 

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