Catalytic strategies

The enzyme-mediated Baeyer-Villiger oxidation to chiral lactone intermediates has received considerable attention in recent years as it offers several advantages in chemo-, regio-, and stereoselectivity compared to other catalytic strategies... 

Kurz LC, Fite B, Jean J, Park J, Erpelding T, Callis P (2005) Photophysics of tryptophan fluorescence link with the catalytic strategy of the citrate synthase from Thermoplasma acidophilum. Biochemistry 44(5) 1394-1413... 

Catalytic strategies for improving specific fuel properties... 

Compared with the artificial neural network (ANN) approach used in previous work to predict CN12 the linear regression model by QSAR is as good or better and easier to implement. The predicted CN values, some of which are tabulated in Table 1, will be employed below to evaluate the different catalytic strategies to optimize the fuel. 

As shown above, CN and ON are important properties of diesel and gasoline fuels, respectively. Although a set of large databases are available, cetane and octane values of many individual compounds are still missing. Therefore, the prediction tools we have developed are very useful in correlating the molecular structures with properties of fuels. In this section, the predicted CN and ON values are implemented to construct the most effective catalytic strategies to optimize CN for diesel and ON for gasoline. 

Cech TR, Herschlag D (1996) Group I ribozyme substrate recognition, catalytic strategies and comparative mechanistic analysis. In Eckstein F, Lilley DMJ (eds) Nucleic acids and molecular biology, vol 10, catalytic RNA. Springer, Berlin Heidelberg New York, P 1... 

In the initial report by MacMillan, use of the imidazolidinonium salt 12 HC1 to generate iminium ion intermediates identified a new catalytic strategy for the activation of a,P-unsaturated carbonyl compounds towards cycloaddition [3]. Inherent... 

Catalytic strategies for making high octane gasolines include ... 

A number of catalytic processes in current use make use of these strategies including reforming, isomerization, dimerization, alkylation and fluid catalytic cracking (FCC). The object of this paper is to discuss the catalytic strategies available to produce octane in the FCC unit. 

Most enzymes employ a combination of several catalytic strategies to bring about a rate enhancement. A good example of the use of both covalent catalysis and general acid-base catalysis is the reaction catalyzed by chymotrypsin. The first step is cleavage of a peptide bond, which is accompanied by formation of a covalent linkage between a Ser residue on the enzyme and part... 

Induced fit is only one aspect of the catalytic mechanism of hexokinase—like chymotrypsin, hexokinase uses several catalytic strategies. For example, the active-... 

This type of catalytic strategy has recently been extended to enantio-selective addition of alkyllithiums to certain prochiral imines (Scheme 18) (35). Relevantly, in the presence of a small amount of a chiral ether ligand, 1-naphthyllithium reacts with a sterically hindered imine of l-fluoro-2-naphthaldehyde (conjugate addition/elimination) to afford a binaphthyl compound in greater than 80% ee. 

Chapman CJ, Frost CG (2007) Tandem and domino catalytic strategies for enan-tioselective synthesis. Synthesis 2007 1-21... 

The catalytic strategy that an enzyme develops over evolutionary time is dictated by the chemistry of the reaction being catalyzed. The prolyl isomerases that have been studied to date are able to simply stabilize the nonenzymatic transition state without formation of covalent intermediates. Based on a value of lO" sec for CyP (Harrison and Stein, 1992 Kofron et ai, 1991) and a of sec" for the cis-to-trans isomerization of Suc-Ala-Ala-cis-Pro-Phe-pNA, we calculate an acceleration factor, of 10 , which corresponds to a transition state... 

Shih IH, Been MD. Catalytic strategies of the hepatitis delta virus ribozymes. Annu. Rev. Biochem. 2002 71 887—917. 

The hepatitis delta virus (HDV) ribozyme is a member of the class of small ribozymes and functions as a self-cleaving RNA sequence critical to the replication of the virus RNA genome (1, 8, 40). HDV ribozymes are proposed to employ several catalytic strategies that include an important example of general acid/base catalysis that involves a specific cytosine residue in the active site. Indeed, a milestone in our understanding of RNA catalysis was the observation that HDV and other small ribozymes could function in the absence of divalent metal ion cofactors, provided that high (molar) concentrations of monovalent ions are present (41, 42). These high monovalent ion concentrations are believed to stabilize the active RNA conformation, which implies that the primary role of divalent metal ions is in structural stabilization (42). 

A number of proteolytic enzymes participate in the breakdown of proteins in the digestive systems of mammals and other organisms. One such enzyme, chymotrypsin, cleaves peptide bonds selectively on the carboxylterminal side of the large hydrophobic amino acids such as tryptophan, tyrosine, phenylalanine, and methionine (Figure 91). Chymotry psin is a good example of the use of covalent modification as a catalytic strategy. The enzyme employs a powerful nucleophile to attack the unreactive carbonyl group of the substrate. This nucleophile becomes covalently attached to the substrate briefly in the course of catalysis. 

Thus, the catalytic triad in proteases has emerged at least three times in the course of evolution. We can conclude that this catalytic strategy must be an especially effective approach to the hydrolysis of peptides and related bonds. 

Less than 10 years after the discovery of carbonic anhydrase in 1932, this enzyme was found to contain bound zinc, associated with catalytic activity. This discovery, remarkable at the time, made carbonic anhydrase the first known zinc-containing enzyme. At present, hundreds of enzymes are knovm to contain zinc. In fact, more than one-third of all enzymes either contain bound metal ions or require the addition of such ions for activity. The chemical reactivity of metal ions—associated with their positive charges, with their ability to form relatively strong yet kinetically labile bonds, and, in some cases, with their capacity to be stable in more than one oxidation state—explains why catalytic strategies that employ metal ions have been adopted throughout evolution. 

Other proteases employ the same catalytic strategy. Some of these proteases, such as trypsin and elastase, are homologs of chymotrypsin. In other proteases, such as subtilisin, a very similar catalytic triad has arisen by convergent evolution. Active-site structures that differ from the catalytic triad are present in a number of other classes of proteases. These classes employ a range of catalytic strategies but, in each case, a nucleophile is generated that is sufficiently powerful to attack the peptide carbonyl group. In some enzymes, the nucleophile is derived from a side chain whereas, in others, an activated water molecule attacks the peptide carbonyl directly. 

Cinchona alkaloids possess a nucleophilic quinuclidine structure and can perform as versatile Lewis bases to react with ketenes generated in situ from acyl halides in the presence of an acid scavenger. The resulting ketene enolates can react with electrophilic C=0 or C=N bonds to deliver chiral [i-lactones [5] or [i-lactams [6], respectively, in a [2 + 2] cycloaddition manner, which is discussed in Chapter 5 in detail. Gaunt et al. also developed practical one pot cydopropanation processes mediated by the modified cinchona alkaloids via ammonium ylide intermediates [7]. Although the catalytic strategy has been well established, the utilization of ammonium enolate based [4 + 2] cycloaddition is rare probably because of the relative unreactivity of the... 

As outlined in Scheme 10.23, this catalytic strategy could be applied for the synthesis of multifunctionalized cyclohexanes 18 and cyclopentanes 19 with two quaternary stereocenters in excellent enantioselectivities (up to >99% ee) and high diastereoselectivities (93 7-99 1). 

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