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Metal Templates in Macrocycle Synthesis

 

By: Carol A. Rouzer, VICB Communications
Published:  April 6, 2018

 

Studies of the interaction of macrocycles with metal ions support a template role for the ions during macrocyclooligomerization.

 

The ability to synthesize small molecules that contain large rings (macrocycles) is an ongoing challenge in synthetic organic chemistry. Macrocyclic compounds are of interest in the development of sensors, new materials, and novel therapeutics. Many natural products that exhibit interesting biological activities are macrocyclic, and the ability to synthesize these types of molecules enables chemists to incorporate structural variations that might enhance desired and reduce unwanted properties. Difficulties associated with the synthesis of macrocycles include the need to generate long linear precursors, define reaction conditions that favor cyclization rather than polymerization of those precursors, and dispose of large amounts of waste produced in the application of solid phase techniques to solve these problems. These challenges led Vanderbilt Institute of Chemical Biology investigator Jeffrey Johnston and his doctoral student Suzanne Batiste to use macrocyclooligomerization (MCO) to synthesize libraries of cyclodepsipeptides (Figure 1) from linear dipeptide and tetradepsipeptide precursors [S. M. Batiste & J. N. Johnston, Proc. Natl. Acad. Sci. U.S.A., (2016) 113, 14893]. In that work, they found that the addition of alkali metal salts influenced the relative proportions of the various macrocycles they obtained in the reaction. Now, they report that these changes in product size-distribution can be correlated to relative macrocycle-metal ion binding interactions, thus enabling the rational design of reaction conditions to favor a desired product or mix of products [S. M. Batiste & J. N. Johnston, J. Am. Chem. Soc. (2018) published online March 22, doi: 10.1021/jacs.7b13148].

 

 

FIGURE 1. Depsipeptides are polymers of amino acids and hydroxy acids linked by peptide (red) and ester (blue) linkages. A cyclodepsipeptide results from the cyclization reaction of a linear depsipeptide.

 

 

The effect of metal ions on cyclodepsipeptide formation is most likely the result of the ability of the macrocycles to act as ionophores by binding the metal ions in the center of the ring. Presumably, this interaction should also favor the arrangement of precursor molecules around the metal during the MCO reaction so that the metal ion serves as a template for ring formation. The researchers therefore hypothesized that, given a library of cyclodepsipeptides formed from the MCO of a particular starting molecule, the effects of a metal ion template on the relative amount of each product formed could be correlated to the binding interactions of that product with the alkali metal cation. The researchers noted, however, that macrocycle-metal ion binding interactions are thermodynamic measurements, so their use to predict product formation most commonly applies in the case of a reversible reaction that is thermodynamically driven. The Mitsunobu-based macrocyclooligomerization conditions used in their reactions, however, result in irreversible ester formation. Thus, they are kinetically, rather than thermodynamically, driven. Nevertheless, the investigators argued that the irreversible nature of the reaction suggests that the transition states leading to cyclization of the metal ion-templated linear precursors have conformations that should closely resemble the structure of the product. Thus, the thermodynamic macrocycle-metal binding interactions were hypothesized to reflect the binding of the transition state for that same metal, and therefore provide insight into the metal's ability to stabilize the transition state and accelerate the rate of product formation.

 

To test their hypothesis, the investigators started with data from their prior studies. One of the reactions started with a tetradepsipeptide (compound 2 in Figure 2), which in the absence of a metal template, generated a 24-atom ring dimer, a 36-atom ring trimer, and a 60 atom-ring pentamer (compounds 4, 6, and 7 in Figure 2). Of these, compound 4 predominated, having been generated at 63% yield. A particularly striking observation from this study was the effect of adding sodium ions to the reaction mixture, which increased the yield of compound 4 to 89% and eliminated formation of the other two compounds. Addition of potassium and cesium ions, in contrast, increased formation of the larger products to varying degrees. To attempt to understand these results, the investigators evaluated the interaction of each metal ion with the individual products using isothermal titration calorimetry (ITC). The results revealed that sodium ions interact with compound 4 at a 1:1 ratio with high affinity (>104), consistent with the increase in formation of that product in the presence of a sodium salt. Compound 6 also exhibited fairly high affinity for sodium ions, but the metal binds at a ratio of 2:1, and the rate at which this ternary complex could form should be much lower than that of the sodium-compound 6 complex. Compound 7 exhibits low affinity for sodium ions, forms a 2:1 complex, and the interaction is disfavored by a positive enthalpy of reaction. Thus, the ITC results help to explain why sodium increases compound 4 formation at the expense of the other two products. Similarly, ITC demonstrated that compound 6 is the only product that exhibits high affinity, binding to potassium ions with a favorable 1:1 ratio and negative enthalpy, and compound 6 was the only product that was formed at higher quantities in the presence of potassium ions than its absence. The results with cesium ions were somewhat more complex. The affinity of compound 4 for cesium was too low to be detected. The other two products exhibited low affinity, and in the case of compound 7, a 3:1 complex ratio and positive enthalpy. Despite these relatively poor binding parameters, the cesium salt did promote the formation of both compounds 6 and 7, although the effects were modest.

 

 

 

FIGURE 2. Yields of each product formed by the reaction of compound 2 under MCO conditions (top) in the presence and absence of metal ions and the results of ITC analysis of the interaction of each metal ion with each of the products (bottom). Statistically significant changes in product yields are indicated by bold type on the bar graph. Figure reproduced with permission from S. M. Batiste & J. N. Johnston, J. Am. Chem. Soc. (2018) published online March 22, doi: 10.1021/jacs.7b13148. Copyright 2018, American Chemical Society.

 

 

Additional experiments focused on the reaction of a dipeptide (compound 1 in Figure 3) under MCO conditions, which in the absence of a metal template, produced an 18-atom ring trimer, a 24-atom ring tetramer, a 30-atom ring pentamer, and a 36 atom-ring hexamer (compounds 3, 4, 5, and 6 in Figure 3). Note that compounds 4 and 6 are identical to the compounds of the same number in the prior experiment, but the smaller monomer in this experiment allowed the formation of products (compounds 3 and 5) that could not have formed from compound 2 (Figure 2). A striking finding in this experiment is that addition of a sodium salt to the reaction, which exclusively favored compound 4 formation when the tetradepsipeptide was the starting material, now led to significant increases in formation of both compounds 4 and 6. Inclusion of potassium or cesium salts led to higher levels of only compound 6. Once again, ITC results provided insights into the mechanism behind these observations. The smallest product, compound 3, exhibited high affinity for sodium ions, which might lead one to conclude that its formation should increase in the presence of that metal. However, compound 3 formation disappeared when the sodium salt was added to the reaction mixture. To explain this finding, the investigators noted that compound 3 initially forms a 1:2 complex with sodium ions, a configuration that could easily lead to its dimerization to form compound 6, which was formed at substantially higher levels in the presence of a sodium salt. Formation of compound 4 also increased in the presence of sodium ions, as was observed in the prior experiment. The only product of this reaction that exhibited substantial affinity for potassium or cesium was compound 6, and it was the only compound that showed an increased yield in the presence of either of these ions.




FIGURE 3. Yields of each product formed by the reaction of compound 1 under MCO conditions (top) in the presence and absence of metal ions and the results of ITC analysis of the interaction of each metal ion with each of the products (bottom). Statistically significant changes in product yields are indicated by bold type on the bar graph. Figure reproduced with permission from S. M. Batiste & J. N. Johnston, J. Am. Chem. Soc. (2018) published online March 22, doi: 10.1021/jacs.7b13148. Copyright 2018, American Chemical Society.

 

 

These results and others led the investigators to draw a number of general conclusions (Figure 4). First of all, a strong template binding affinity between a cyclodepsipeptide product and a metal ion correlates with formation of that product in the presence of the metal. A 1:1 metal ion:product ratio favors a more rapid rate of product formation than a 2:1 metal ion:product ratio due to the simplicity of complex formation. Formation of a 1:2 complex inhibits monocyclization and increases dimer formation in the case of smaller rings. For larger rings, this ratio slows the rate of cyclization. Finally, a negative enthalpy of complex formation favors rapid cyclization, whereas a positive enthalpy favors a slower reaction.

 

 

 

FIGURE 4. Insights into the effects of metal ions on MCO reactions obtained from ITC results. Figure reproduced with permission from S. M. Batiste & J. N. Johnston, J. Am. Chem. Soc. (2018) published online March 22, doi: 10.1021/jacs.7b13148. Copyright 2018, American Chemical Society.

 

Based on these principles, the investigators sought to determine if they could predict the effects of metal ions on relative product formation for an entirely new dipeptide (Figure 5). They initially characterized the reaction products generated in the absence of a metal template. Then, they performed ITC analysis of the interaction of each product with sodium, potassium, and cesium ions. Using the ITC results, they predicted the effects of adding each metal on the relative amounts of all products, and then they actually ran the reactions. As seen in Figure 5, the majority of their predictions were correct.

 

 

FIGURE 5. Example used to test the ability of ITC to predict the effects of metal ions on product formation. The products are shown along with ITC results from their interaction with each metal. Comparison of predicted salt effects on product ratios with experimental outcomes is depicted by red and black arrows, respectively.

 

 

From these results, the researchers have concluded that the data acquired from ITC analysis of metal-macrocycle binding interactions provides a basis for the first analytical platform to rationally select the best metal ion template for a targeted size-regime of cyclic oligomeric depsipeptides. Further work will help to identify other reaction characteristics that play a role in determining the course of the reaction and product outcome.

 


 

View J Amer Chem Soc article: Evidence for Ion-Templation During Macrocyclooligomerization of Depsipeptides

 

 

 

 

 

 

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