Aliphatic side chains branching

Fluorosilicones consist of PDMS backbones with some degree of fluoro-aliphatic side chains. The fluorinated group can be trifluoropropyl, nonafluorohexylmethyl, or fluorinated ether side group [78,28,79]. These polymers differ not only in substituent group, but also in the amount of fluoro-substitution relative to PDMS, the overall molecular weight and crosslink density, and the amount of branching. In most commercially available cases, these polymers are addition cure systems and the reactions are those discussed previously for silicone networks. 

Isoleucine lie NH2 CH, / 3 H—C CH 1 COOH CH2 CHj 6.0 Branched aliphatic side chain — has two asymmetric carbon atoms... 

Compounds of type 117, containing a tricycyc nucleus with a six-or seven-mem-bered central ring A, a three atom aliphatic side chain (saturated or unsaturated, linear or branched) bound to the central ring A, and a basic functional group (ter-... 

The authors studied the polymerization of formaldehyde with amines including tertiary amines at —78°C in various solvents (Table 1), and determined the conversion after 15 min reaction time. Tertiary amines are highly reactive initiators for formaldehyde polymerizations even at the level of 10 mole T per mole 1 of formaldehyde. The reactivity of the amine is related to its pXg value but also to the branching of the aliphatic side chains of the substituents on the nitrogen atom. Branched amines, especially when the branching is on the a-carbon atom as in the case of a tertiary butyl group, are less effective initiators than tertiary amines with n-alkyl chains. The pX a of the amine is not the essential feature for an efficient tertiary amine initiator, because pyridine was almost as effective as tri-n-butylamine but quinoline, with a similar pK g as pyridine, is almost inactive (Table 1). 

Carboxypeptidase A catalyzes the hydrolysis of the C-terminal amino acids in peptides and proteins it shows a preference for aromatic and branched aliphatic side chains at the C-terminal end. The enzyme also acts as an esterase. The X-ray structure of the native form of carboxypeptidase A, was solved by Rees and Lipscomb to 1.5 A... 

Fessner et al.[256] developed an efficient method for the synthesis of L-fucose analogs modified at the nonpolar terminus by means of L-fucose isomerase and l-fuculose 1-phosphate aldolase from E. coli. Various L-fucose analogs bearing linear or branched aliphatic side chains were prepared in about 30% overall yield with hydroxyaldehyde precursors and dihydroxyacetone phosphate as the starting materials (Fig. 17-32). 

The aliphatic side chains of valine, isoleucine, leucine and methionine have no reactive groups, only methylene, methyl (Me) and SMe groups. They are important, however, because they do not interact well with water, preferring each other. In addition, if they are branched as are isoleucine and valine, they stiffen the main chain and decrease its flexibility [30]. Phenylalanine is nonpolar like benzene or toluene. It has a methylene group which lengthens the side chain and prevents steric hindrance with the main chain. These are hydrophobic residues found in the center of proteins and provide a core around which the functional parts of the protein are assembled. The... 

The last section shows the effect of substituents (Rj, R2) on the aliphatic amide moiety. As for the aliphatic side chain, it was found that the alpha-branched alkyl side chain was essential for stabilizing the diamide structure. In the case of non-branched alkyl, the diamide derivatives tend to decompose to the corresponding phfhalimides. A variety of substituents were examined to improve the activity. As shown in Table I, the introduction of a heteroatom or a functional group increased the insecticidal activity especially a sulfur atom within the alkyl side chain markedly increased the activity. This sulfonylalkylamine is also novel as an amine residue in pesticide chemistry. In summary, flubendiamide has unique substituents as essential parts of the structure in three adjacent positions on the benzene ring, which characterizes the chemical structare of flubendiamide as totally novel. 

In the discussion of side reactions we have dealt, so far, only with electronic effects. It would be a mistake, however, to underestimate the importance of geometric factors and, in particular, the influence of bulky side chains. Branching of the aliphatic chain in valine and in isoleucine is at the jS-carbon atom, near to the reactive carbonyl group in activated intermediates. Hence, acylation reactions in which these amino acids are involved, do not proceed at a rate observed with other amino acids, for instance with leucine, in which branching is at the y-carbon atom. Steric hindrance becomes even more pronounced when the activating group is similarly bulky as in pentachlorophenyl esters ... 

The aliphatic side chains in alanine and leucine have no major influence but branching at the ) -carbon atom in valine and isoleucine can enhance racemization because the combination of electron release and steric hindrance results in reduced coupling rates. The ensuing increase in the life-time of the reactive intermediate provides an extended opportunity for proton abstraction by base. It is obvious from these examples that the effect of individual side chains, the influence of various methods of coupling and the conditions of the peptide bond forming reaction (solvents, concentration, temperature, additives) must be studied in well designed experiments. Several model systems have been proposed for this purpose. 

FIGURE 8.2 A Topliss Tree for aliphatic side chain substitutions. The Topliss schemes were constructed by consideration of hydrophobic and electronic factors and are designed such that the optimum substituent maybe found as efficiently as possible. It is assumed that the methyl substituted compound has been made, tested, and compared to the unsubstituted compound. There are three possibilities the analogs will have less (L), equal (E), or more (M) activity and this determines which branch of the tree should be followed next. 

CM is also very useful if the objective is to modify branched polymers or to introduce some convenient functional groups. This was the case of two original studies conducted by the Meier s research team. The first study [26] described the reaction of a poly(2-oxazoline) containing terminal double bonds in the side chains with various functional acrylates in the presence of a HG-II. The second (more recent) study [27] involved the synthesis of a polymalonate bearing C9 aliphatic side chains with terminal double bonds used for grafting-onto reactions by ruthenium-catalysed CM. 

At the beginning of the reaction of phenolic groups with epoxide, the concentration of aliphatic hydroxyl groups is zero. As the reaction progresses, the concentration of secondary alcoholic groups increases and the possibility of side chain branching increases. 

The peroxisomal /3-oxidation pathway also fulfils an important biosynthetic role. In the hepatic synthesis of bile acids from cholesterol, the aliphatic side chain, which resembles an a-methyl-branched-chain fatty acid, must be shortened. A single cycle of peroxisomal /3-oxidation will remove a three-carbon... 

The ansamycin antibiotics represent a new class of natural products, combining an aromatic nucleus with a branching aliphatic side chain, and considerable interest has centered on their formation in nature. Their common structural features suggest immediately a similar biosynthetic pathway. Moreover, the streptovaricins, rifamycins, tolypo-micin B and halomicin B all have the same number of carbon atoms in the ansa ring (C17), while geldanamycin and the maytansinoids also have the same number of carbon atoms (C15). Thus far, biosynthetic... 

The aliphatic amino acids (class 1) include glycine, alanine, valine, leucine, and isoleucine. These amino acids do not contain heteroatoms (N, 0, or S) in their side chains and do not contain a ring system. Their side chains are markedly apolar. Together with threonine (see below), valine, leucine, and isoleucine form the group of branched-chain amino acids. The sulfurcontaining amino acids cysteine and methionine (class 11), are also apolar. However, in the case of cysteine, this only applies to the undissociated state. Due to its ability to form disulfide bonds, cysteine plays an important role in the stabilization of proteins (see p. 72). Two cysteine residues linked by a disulfide bridge are referred to as cystine (not shown). 

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