New pathways

In 1891, Miolati (75) confirmed the cyclic formula of Liebermann and Lange by preparing the compound by three new pathways (1) reaction of CS2 on thiohydantoin, (2) condensation of ammonium dithiocarbamate with chloroacetic ester, (3) reaction of HjS on thiocyanoacetic acid. 

An alternative approach (78,79) is based on a set of possible reaction schemes that are used to generate potential new pathways. Under both approaches, the problem, in part, is how to evaluate the utiUty of a particular scheme. A computer-assisted approach to predicting potentially useful reactions has been developed (80). The union of existing capabiUties in modeling chemical stmctures with selecting reaction pathways has not yet taken place. 

The reactions of enamines with positively activated olefins have been extended to arylations with />-quinones (350,362-369) and quinone sulfoni-mides (365-368,370). Thus a new pathway for the facile formation of benzofurans and indoles became available. 

As mentioned previously, pyrylium salts are dehydration products of the conjugate acids of as-2-ene-l,5-diones, and therefore, most syntheses of pyrylium salts are, in fact, syntheses of 1,5-enediones. Dilthey has devised his new pathways to pyrylium salts by analogy... 

Natural product synthesis poses the challenge to consider and develop new pathways of structural transformation. Natural products as targets for synthetic research possess a special fertility in this regard, because the structural channels of biosynthesis are not necessarily the conduits of organic synthesis. A. Eschenmoser19... 

An alternative strategy for producing new derivatives by directed biosynthesis is to produce mutants in which particular pathways may be blocked or a new pathway created. Again, we will use a specific example to illustrate this approach. 

The new pathway, too, is a chain reaction Note that the first term of Eq. (8-31) does not give a meaningful transition state composition. Since the scheme in Eqs. (8-20M8-23) seems valid for the Cu2+-free reaction, we can seek to modify it to accommodate the new result. This approach is surely more logical than inventing an entirely new sequence. To arrive at the needed modification, we simply replace Eq. (8-23) by a new termination step, Eq. (8-30). With that, and the steady-state approximation, the rate law is... 

A catalyst speeds up a reaction by providing an alternative pathway—a different reaction mechanism—between reactants and products. This new pathway has a lower activation energy than the original pathway (Fig. 13.34). At the same temperature, a greater fraction of reactant molecules can cross the lower barrier of the catalyzed path and turn into products than when no catalyst is present. Although the reaction takes place more quickly, a catalyst has no effect on the equilibrium composition. Both forward and reverse reactions are accelerated on the catalyzed path, leaving the equilibrium constant unchanged. 

A reaction rate increases by a factor of 1000. in the presence of a catalyst at 25°C. The activation energy of the original pathway is 98 kj-mol. What is the activation energy of the new pathway, all other factors being equal In practice, the new pathway also has a different pre-exponential factor. 

First activated Effect of complex catalyst (new pathway lowers E )... 

A new pathway of CO2 fixation proposed (to become the Wood-Ljungdahl... 

To accommodate this new finding and the previous results, we considered a new pathway (Scheme 3), in which acetate or its derivative condenses with arginine followed by decarboxylation. Such Claisen-type condensation on alpha-amino acid has some precedent in biochemical systems (6). To prove this hypothesis, we synthesized [2- C, 2-arginine and ornithine and fed to A, flos-aquae (5). 

Kretzer A, JR Andreesen (1991) A new pathway for isonicotinate degradation by Mycobacterium sp. INAL... 

Small FJ, JK Tilley, SA Ensign (1995) Characterization of a new pathway for epichlorohydrin degradation by whole cells of Xanthobacter strain Py2. Appl Environ Microbiol 61 1507-1513. 

Gescher J, A Zaar, M Mohamed, H Schagger, G Fuchs (2002) Genes coding a new pathway of aerobic benzoate metabolism in Azoarcus evansii. J Bacterial 184 6301-6315. 

Groseclose EE, DW Ribbons (1981) Metabolism of resorcinolic compounds by bacteria a new pathway for resorcinol catabolism in Azotobacter vinelandii. J Bacteriol 146 460-466. 

Oltmanns RH, R Muller, MK Otto, F Lingens (1989) Evidence for a new pathway in the bacterial degradation of 4-fluorobenzoate. Appl Environ Microbiol 55 2499-2504. 

Chauhan A, RK Jain (2000) Degradation of o-nitrobenzoate via anthranilic acid (o-aminobenzoate) hyArthro-acterprotophormiae a plasmid-encoded new pathway. Biochem Biophys Res Commun 267 236-244. 

Goszczynski S, A Paszczynski, MB Pasti-Grigsby, RL Crawford, DL Crawford (1994) New pathways for degradation of sulfonated azo dyes by microbial peroxidases of Phanerochaete chrysosporium and Streptomyces chromofuscus. J Bacterial YIU. 1339-1347. 

Roman-Franco, A.A. (1982). Non-enzymic extramicrosomal activation of chemical carcinogens by phagocytes a proposed new pathway. J. Theor. Biol. 97, 543-555. 

H. M. Colquhoun, J. Holton, D. J. Thomson, and M. V. Twigg, New Pathways for Organic Synthesis, Plenum Press, New York, 1984. 

We have been recently studying new pathways leading to polysilanes with low polydispersity and controlled structures. Our research is focused on three areas. The first one is low temperature reductive coupling in the presence of ultrasound. This leads to monomodal polymers with molecular weights in the range from Mn=50,000 to Mn=300,000 and polydispersities as low as Mw/Mn=1.20 (in addition to usually formed cyclic oligomers). 

Marian CM (2005) A new pathway for the rapid decay of electronically excited adenine. J Chem Phys 122 104314... 

Liu YC, Lin LH (2004) New pathway for the synthesis of ultrafine silver nanoparticles from bulk silver substrates in aqueous solutions by sonoelectrochemical methods. Electrochem Commun 6 1163-1168... 

Particularly important to the pathways of modular synthases is the incorporation of novel precursors, including nonproteinogenic amino acids in NRP systems [17] and unique CoA thioesters in PK and fatty acid synthases [18]. These building blocks expand the primary metabolism and offer practically unlimited variability applied to natural products. Noteworthy within this context is the contiguous placement of biosynthetic genes for novel precursors within the biosynthetic gene cluster in prokaryotes. Such placement has allowed relatively facile elucidation of biosynthetic pathways and rapid discovery of novel enzyme mechanisms to create such unique building blocks. These new pathways offer a continued expansion of the enzymatic toolbox available for chemical catalysis. 

The efficiency of a transition path sampling simulation crucially depends on how new pathways are generated from old ones. Various schemes to do that are possible. 

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