Modifications chain elongation

Important for both the synthesis and modification of polymers is also the elongation of polymer chains. Bisnitrile oxides have been claimed as reagents for chain elongation of polyimides containing terminal groups with C=C, C=C, C=N, C=0, and C=N bonds (504). 

Attractive Compounds. The female-produced sex pheromone of the yellow mealworm beetle, Tenebrio molitor, is (R)-4-methyl- 1-nonanol [316] 163 (Scheme 18). Careful investigations on the biosynthesis of this compound [317] revealed that it is produced through a modification of normal fatty acid biosynthesis (Fig. 1, Fig. 2) propanoate serves as the starter, while formal chain elongation with acetate, propanoate, and acetate (accompanied by removal of the oxygens) produces 4-methylnonanoate which yields the pheromone alcohol after reduction. The structures and role of proteins that are present in the hemolymph or secreted by the tubular accessory glands of T. molitor, and that may carry lipophilic chemical messengers (like pheromones) are under investigation [318,319]. 

Because carbohydrates are so frequently used as substrates in kinetic studies of enzymes and metabolic pathways, we refer the reader to the following topics in Ro-byt s excellent account of chemical reactions used to modify carbohydrates formation of carbohydrate esters, pp. 77-81 sulfonic acid esters, pp. 81-83 ethers [methyl, p. 83 trityl, pp. 83-84 benzyl, pp. 84-85 trialkyl silyl, p. 85] acetals and ketals, pp. 85-92 modifications at C-1 [reduction of aldehydes and ketones, pp. 92-93 reduction of thioacetals, p. 93 oxidation, pp. 93-94 chain elongation, pp. 94-98 chain length reduction, pp. 98-99 substitution at the reducing carbon atom, pp. 99-103 formation of gycosides, pp. 103-105 formation of glycosidic linkages between monosaccharide residues, 105-108] modifications at C-2, pp. 108-113 modifications at C-3, pp. 113-120 modifications at C-4, pp. 121-124 modifications at C-5, pp. 125-128 modifications at C-6 in hexopy-ranoses, pp. 128-134. 

The biosynthesis of many hydroxylated natural products proceeds through regio- and enantioselective modification of polyketides, which are assembled through chain elongation via acetate or propionate units [2]. The enzymes responsible for the chain elongation and subsequent reduction, elimination, aromatiza-tion, and further modifications are classified as polyketide synthases [3]. These multifunctional enzymes have been used for whole-cell biotransformation toward unnatural metabolites that are within the scope of combinatorial biosynthesis... 

Through the modification of the known L-type VDCC inhibitors verapamil and its desmethoxy analog emopanil, Eli Lilly identified a novel series of amino acid containing phenylalkylamines which demonstrate submicromolar inhibition of neuronal non-L-type VDCC while showing markedly decreased activity on L-type channels. They initially investigated the modification of the phenethylamine moiety and found that chain elongation and incorporation of a second phenyl substituent (compound 1) markedly shifted activity away from L-towards non-L-type inhibition. 

Polyketides are made by the sequential activity of domains of large, multifunctional enzymes called polyketide synthases (PKSs) (Fig. 6a and b). Polyketides are formed by the condensation and modification of acyl units derived from acyl-CoA precursors. Domains are organized in modules and each module carries out the series of steps necessary for one cycle of polyketide chain elongation. A single protein can have more than one module, and several different proteins together can make up a PKS. The number of modules determines the size of the polyketide. A growing polyketide chain is tethered to the enzyme as a thiol ester and moves sequentially from the N- to the C-terminus of a module, lengthened by two carbon units per module. The first module in a PKS... 

Zero-length cross-linkers are reactive molecules that activate functional groups on surfaces without any chain elongation or incorporation in molecules that have to be attached. In this approach, all reactive surface modifications, for example aldehydes, epoxy-groups, and halogenated surfaces are also termed zero-length cross-linkers. 

Modular polyketide synthases synthesize nascent polyketide scaffolds in an assembly line-like process, with each module participating in a single round of chain elongation and modification. At a minimum, all modules posses a B-keto synthase catalytic domain, an acyl transferase domain, and an acyl carrier protein domain. In general, specialized modules exist at the N-terminus of specific... 

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