Thermodynamically controlled reactions acylations

Basic to protease catalyzed oligopeptide synthesis is equilibrium- or thermodynamic control to direct reversal of proteolysis . Difficulties encountered include low reaction rates, high stoichiometric amounts of enzyme, and the need to apply direct approaches to shift the reactions towards formation of desired products. Reaction conditions that lead to product precipitation or extraction increase efficiency of the reverse reaction. Kinetically controlled syntheses has proved useful for serine and cysteine proteases that form activated acyl enzyme intermediates during catalysis. This approach generally involves use of activated acyl moieties, such as esters, as donor components which significantly accelerate the reaction rate. This study makes use of principles from both kinetic and thermodynamically controlled reactions in that, reactants are activated by formation of esters and products precipitate fi om reactions. 

On reaction with acyl chlorides and acid anhydrides phenols may undergo either acylation of the hydroxyl group (O acylation) or acylation of the ring (C acylation) The product of C acylation is more stable and predominates under conditions of thermodynamic control when alu mmum chloride is present (see entry 6 m Table 24 4 Section 24 8) O acylation is faster than C acylation and aryl esters are formed under conditions of kinetic control... 

The enolates of ketones can be acylated by esters and other acylating agents. The products of these reactions are [Tdicarbonyl compounds, which are rather acidic and can be alkylated by the procedures described in Section 1.2. Reaction of ketone enolates with formate esters gives a P-ketoaldehyde. As these compounds exist in the enol form, they are referred to as hydroxymethylene derivatives. Entries 1 and 2 in Scheme 2.16 are examples. Product formation is under thermodynamic control so the structure of the product can be predicted on the basis of the stability of the various possible product anions. 

However, considerable amounts of 2,3-dihydrofuran 50 and tetrahydro-furan-2-carbaldehyde 53 were present because of an isomerization process. The isomerization takes place simultaneously with the hydroformylation reaction. When the 2,5-dihydrofuran 46 reacts with the rhodium hydride complex, the 3-alkyl intermediate 48 is formed. This can evolve to the 2,3-dihydrofuran 50 via /3-hydride elimination reaction. This new substrate can also give both 2- and 3-alkyl intermediates 52 and 48, respectively. Although the formation of the 3-alkyl intermediate 48 is thermodynamically favored, the acylation occurs faster in the 2-alkyl intermediates 52. Regio-selectivity is therefore dominated by the rate of formation of the acyl complexes. The modification of the phosphorus ligand and the conditions of the reaction make it possible to control the regioselectivity and prepare the 2- or 3-substituted aldehyde as the major product [78]. As far as we know, only two... 

Formation of an amide bond (peptide bond) will take place if an amine and not an alcohol attacks the acyl enzyme. If an amino acid (acid protected) is used, reactions can be continued to form oligo peptides. If an ester is used the process will be a kinetically controlled aminolysis. If an amino acid (amino protected) is used it will be reversed hydrolysis and if it is a protected amide or peptide it will be transpeptidation. Both of the latter methods are thermodynamically controlled. However, synthesis of peptides using biocatalytic methods (esterase, lipase or protease) is only of limited importance for two reasons. Synthesis by either of the above mentioned biocatalytic methods will take place in low water media and low solubility of peptides with more than 2-3 amino acids limits their value. Secondly, there are well developed non-biocatalytic methods for peptide synthesis. For small quantities the automated Merrifield method works well. 

When H2O deacetylates the acyl-enzyme, phenylacetic acid is formed. When nucleophiles other than H2O deacylate the acyl-enzyme, a new condensation product, in this case phenylacetyl-O-R or phenylacetyl-NH-R is formed. By definition the hydrolysis of these condensation products can be catalyzed by the same enzyme that catalyzes their formation in equation 10.1. Thus, when the acyl-enzyme is formed from phenylacetyl-glycine or phenylacetyl-O-Me, this gives rise to an alternative process to produce Penicillin G, in addition to the thermodynamically controlled (= equilibrium controlled) condensation of phenylacetic acid and 6-aminopenicillanic acid (6-APA). This reaction that involves an activated side chain is a kinetically controlled (= rate controlled) process where the hydrolase acts as a transferase (Kasche, 1986 1989). 

Benzoylation of benzoylmethylene triphenylarsorane (20) with benzoyl bromide gave a kinetically controlled acylated product, which on treatment with sodium acetate in chloroform afforded thermodynamically controlled dibenzoylmethylene triphenylarsorane (21) (56). Acylation with carbonic acid anhydride (32, 56), phenylisocyanate (32), or chloroformic ester (32) gave in no case O-acylated product. Similarly, reaction with acetic anhydride afforded l,3-dioxo-l-phenyl-butylidene-(2)-triphenylar-sorane (56). 

Both alkaline proteases form an intermediate, the acyl-enzyme complex, on the reaction coordinate from the amino acid component to the dipeptide, which is formed by the triad Ser-(or Cys-)-His-Asp (or -Glu) (see Chapter 9, Section 9.5). The acyl-enzyme complex can be formed with the help of an activated amino acid component such as an amino acid ester. The complex can react either with water to the undesired hydrolysis product, the free amino acid, or with the amine of the nucleophile, such as an amino acid ester or amide, to the desired dipeptide. The particular advantage of enzyme-catalyzed peptide synthesis rests in the biocatalyst specificity with respect to particular amino acids in electrophile and nucleophile positions. Figure 7.26 illustrates the principle of kinetically and thermodynamically controlled peptide synthesis while Table 7.3 elucidates the specificity of some common proteases. 

It is therefore not surprising that, although N-alkylation, acylation, protonation etc. undoubtedly occurs, the reactions of the majority of electrophilic reagents result in substitution at the -position. The higher electron density favours initial reaction at this position under conditions of kinetic control. However, where initial reaction at the nitrogen and the jff-carbon is reversible, or when the transition state is product-like in character, the operation of thermodynamic control may result in preferential or even... 

Unfortunately, several important classes of a-diazo ketones cannot be prepared in good yield via these standard methods. a -Diazo derivatives of a.p-unsaturated ketones, for example, have previously proved to be particularly difficult to prepare.1113 12 The acylation of diazomethane with a.p-unsaturated acid chlorides and anhydrides is generally not a successful reaction because of the facility of dipolar cycloaddition to conjugated double bonds, which leads in this case to the formation of mixtures of isomeric pyrazolines. Also problematic are diazo transfer reactions involving base-sensitive substrates such as certain a,p-enones and heteroaryl ketones. Finally, the relatively harsh conditions and lack of regioselectivity associated with the thermodynamically controlled Claisen formylation step in the "deformylative" diazo transfer procedure limit the utility of this method when applied to the synthesis of diazo derivatives of many enones and unsymmetrical saturated ketones. 

A significant advance in the use of Friedel-Crafts acylation of alkenes to prepare divinyl ketones was the employment of vinylsilanes to control the site of electrophilic substitution. Two groups have developed this approach to cyclopentenone annulation using slightly different strategies. In the method described by Magnus the reagent vinyltrimethylsilane (80) is used primarily as an ethylene equivalent (equation 44). The construction of bicyclic systems followed readily as Nazarov cyclization proceeded under the reaction conditions. Tin(lV) chloride was found to be the most effective promoter of the overall transformation. As expected the position of the double bond is thermodynamically controlled. 

Acylation has frequently been used to convert arsonium ylides into other more stabilized ylides, reagents used being acid chlorides ° acid anhyd-rides " or esters These examples all involve C-acylation but complications can arise, as shown in equation 17 It was suggested that reaction with benzoyl bromide gives a kinetically controlled product which, in the presence of acetate, is converted into the thermodynamically controlled product In this case delocalization of the negative charge leads to acylation at a site other than the ylidic carbon atom. 

An extension of this tin chemistry to the regioselective acylation of unprotected sugars bound to a resin shows the possibility of using solid-phase techniques for the preparation of 0-acyl derivatives of carbohydrates [231]. Very recently, it has been reported that organotin-mediated multiple carbohydrate esterifications can be controlled by the acylating reagent and the solvent polarity. When acetyl chloride is used, the reactions are under thermodynamic control, whereas when acetic anhydride is employed, kinetic control takes place (O Scheme 31) [232]. 

Figure 1 shows the influence of the solution pH on the yield and initial rate of synthesis of CBZ-Lys-Gly-OMe at a temperature of 25°C and [CBZ-Lys] = [Gly-OMe] = 20 mM. The maximum yield is achieved for pH values around 6-6.5. Under thermodynamically controlled conditions, the peptide synthesis occurs between the non-ionic forms of the acyl-donor (CBZ-Lys) and the nucleophile (Gly-OMe). The concentration of these nonionic forms depends on the pH, since an intermediate value between both pK (pHopt = V IpKa +pKb]) is needed in order to achieve high synthetic yields. On the other hand, the reaction rate increases up to pH 7, which is in agreement with the results obtained in the synthesis of the peptide benzoylarginine-leucinamide catalyzed by immobilized trypsin (10), where the authors suggest the nucleophihc attack of the non-ionic form of the nucleophile on the acyl-enzyme complex as the controlling step of the peptide reaction. 

Reaction of phenol derivatives with acyl chlorides in acidic conditions may result in O-acylation or C-acylation, either directly or via a Fries rearrangement. Studies of acylations in acetonitrile using trifluoromethanesulfonic acid (trifllc acid) have shown that O-acylation is favoured at a low acid concentration, while a high acid concentration favours C-acylation. It has also been shown that A-hydroxysuccinimidyl and phenyl esters of benzoic acids are activated by triflic acid and can be used to acylate electron-rich arenes such as ferrocene or pyrene the reactive acylating intermediate is likely to be an acyl triflate or its protonated form. In a polyphosphoric acid medium, the rearrangement of 1,5- 1,8- and 9,10-diacetylanthracenes leads to the formation of the ring-closed product (56). DFT calculations support the conclusion that the reaction involves the intermediacy of 1,9-diacetylanthracene formed under kinetic rather than under thermodynamic control. ... 

The reversibility and thermodynamic control of product formation found for the high-pressure reaction between glycals and tosyl isocyanate indicated that the [2+2]cycloaddition of isocyanates to glycals could occur at atmospheric pressure under specific reaction conditions including an excess of isocyanate, as well as proper selection of solvent and substrates. Acyl isocyanates are generally less reactive in [2+2]cycloaddition reactions than sulfonyl isocyanates, except for trichloro- and trifluoroacetyl isocyanate 10,12 addition, acyl isocyanates are problematic because of the competitive formation of [4+2]cycloadducts, which are usually thermodynamically preferred over the [2+2]cycloadducts. 

This principal reaction mechanism is widely believed to apply to most S Ar reactions irrespective of the electrophilic reagent. There are however a number of experimental observations that indicate exceptions to this mechanism. There are examples of thermodynamically controlled Friedel-Crafts reactions, when using reaction conditions like polyphosphoric acid and elevated temperatures [27,28]. In iodination and some cases of Friedel-Crafts acylation, the last step of the reaction, the proton abstraction, has been shown to have a substantial kinetic isotope effect, which indicates that this step is at least partially rate limiting [29-31]. There are also still open questions regarding the exact nature of the reaction intermediates, and we will focus on these issues in the remaining part of the chapter. 

The synthesis continued with reduction of the cyclohexanone to the alcohol oxidation state, taking it out of play for a series of reactions that constructed the sidechain (49 54). The sidechain ketone was then protected as an acetal, and the cyclohexanone was reinstalled by deprotection and oxidation of the cyclohexanol. Regioselective acylation of 55 under conditions of thermodynamic control, followed by reduction of the intermediate /3-ketoester, gave 56 (for comparison see 3 —> 14 on Steroids-3). Formation of the tosylate, a /3-elimination, and ketal hydrolysis completed the synthesis of 14. 

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