Kinetic Resolution of β-Branched Aldehydes through Peptide-Catalyzed Conjugate Addition Reactions

The catalytic kinetic resolution of racemic β-branched aldehydes offers a straightforward stereoselective entry to aldehydes and addition products. Yet, control over stereoselectivity is difficult due to the conformational flexibility of β-branched aldehydes. Here, we show that the peptide catalyst H-dPro-αMePro-Glu-NH2 resolves β-branched aldehydes through reaction with nitroolefins and provides γ-nitroaldehydes with three consecutive stereogenic centers in high yields and stereoselectivities. Kinetic, NMR spectroscopic, and computational studies provided insights into the selectivity-determining step and origins of the kinetic resolution.

Machine Learning to Develop Peptide Catalysts-Successes, Limitations, and Opportunities

Peptides have been established as modular catalysts for various transformations. Still, the vast number of potential amino acid building blocks renders the identification of peptides with desired catalytic activity challenging. Here, we develop a machine-learning workflow for the optimization of peptide catalysts. First-in a hypothetical competition-we challenged our workflow to identify peptide catalysts for the conjugate addition reaction of aldehydes to nitroolefins and compared the performance of the predicted structures with those optimized in our laboratory. On the basis of the positive results, we established a universal training set (UTS) containing 161 catalysts to sample an in silico library of ∼30,000 tripeptide members. Finally, we challenged our machine learning strategy to identify a member of the library as a stereoselective catalyst for an annulation reaction that has not been catalyzed by a peptide thus far. We conclude with a comparison of data-driven versus expert-knowledge-guided peptide catalyst optimization.

Overcoming Deactivation of Amine-Based Catalysts: Access to Fluoroalkylated γ-Nitroaldehydes

Organocatalytic conjugate addition reactions of aldehydes to fluoroalkylated nitroolefins with chiral amine catalysts offer a straightforward stereoselective path to fluoroalkylated γ-nitroaldehydes and downstream derivatives. However, amine-based catalysts suffer from deactivation by reaction with electron-poor fluoroalkylated nitroolefin. Here, we show that catalyst deactivation can be overcome by catalysts that bear an intramolecular acid for protonation and release of the alkylated catalyst through ß-elimination of the nitroolefin. NMR spectroscopic, kinetic, and molecular modeling studies provided detailed structural and mechanistic insights into the factors that control reversible catalyst alkylation and facilitate efficient catalysis.

Synthesis of Bicyclic Hemiacetals Catalyzed by Unnatural Densely Substituted γ-Dipeptides

The asymmetric synthesis of bicyclic highly substituted tetrahydropyrans is described. The reaction is catalyzed by unnatural γ-dipeptides based on densely substituted l- and d-proline derivatives. This organocatalytic one-pot reaction takes place among a ketone, a nitroalkene, and an aldehyde to yield an octahydro-2H-chromene scaffold. Monomeric species, from which the corresponding γ-dipeptides are synthesized, cannot catalyze the reaction, thus confirming the emergent nature of the catalytic behavior of these dimeric species.

Peptide-Catalyzed Stereoselective Conjugate Addition Reaction of Aldehydes to C-Substituted Maleimides

Catalytic stereoselective additions with maleimides are useful one-step reactions to yield chiral succinimides, molecules that are widespread among therapeutically active compounds but challenging to prepare when the maleimide is C-substituted. We present the tripeptide H-Pro-Pro-Asp-NHC12 H25 as a catalyst for conjugate addition reactions between aldehydes and C-substituted maleimides to form succinimides with three contiguous stereogenic centers in high yields and stereoselectivities. The peptidic catalyst is so chemoselective that no protecting group is needed at the imide nitrogen of the maleimides. Derivatization of the succinimides was straightforward and provided access to chiral pyrrolidines, lactones, and lactams. Kinetic studies, including a Hammett plot, provided detailed insight into the reaction mechanism.

Stereoselective Peptide Catalysis in Complex Environments – From River Water to Cell Lysates

Many stereoselective peptide catalysts have been established. They consist, like nature's catalysts, of amino acids but have significantly lower molecular weights than enzymes. Whereas enzymes operate with exquisite chemoselectivity in complex biological environments, peptide catalysts are used in pure organic solvents and at higher concentrations. Can a peptide catalyst exhibit chemoselectivity reminiscent of enzymes? Here, we investigated the properties of tripeptide catalysts in complex mixtures in hydrophobic and aqueous solvents. We challenged the catalysts with biomolecules bearing functional groups that could interfere by coordination or reaction with the peptide, the substrates, or intermediates. H-dPro-αMePro-Glu-NHC₁₂H₁₅ emerged through tailoring of the trans/cis ratio of the tertiary amide as a conformationally well-defined tripeptide that catalyzes C–C bond formations with high reactivity and stereoselectivity – regardless of the solvent and compound composition. The chemoselectivity of the tripeptide is so high that it even catalyzes reactions in cell lysates. The findings provoke the question of the potential role of peptide catalysis in nature and during the evolution of enzymes.

The reactivity, stereo-, and chemoselectivity of a tripeptide are so high that it catalyzes conjugate addition reactions with high stereoselectivity in complex compound mixtures—even in cell lysates.

Additive and Emergent Catalytic Properties of Dimeric Unnatural Amino Acid Derivatives: Aldol and Conjugate Additions

Different densely substituted L- and D-proline esters were prepared by asymmetric (3+2) cycloaddition reactions catalyzed by conveniently selected EhuPhos chiral ligands. The γ-nitro-2-alkoxycarbonyl pyrrolidines thus obtained in either their endo or exo forms were functionalized and coupled to yield the corresponding γ-dipeptides. The catalytic properties of these latter dimers were examined using aldol and conjugate additions as case studies. When aldol reactions were analyzed, an additive behavior in terms of stereocontrol was observed on going from the monomers to the dimers. In contrast, in the case of the conjugate additions between ketones and nitroalkenes, the monomers did not catalyze this reaction, whereas the different γ-dipeptides promoted the formation of the corresponding Michael adducts. Therefore, in this latter case emergent catalytic properties were observed for these novel γ-dipeptides based on unnatural proline derivatives. Under certain conditions stoichiometric amounts of ketone, acid and nitroalkene), formation of N-acyloxy-2-oxooctahydro-1H-indoles was observed.

Understanding the Diastereopreference of Intermediates in Aminocatalysis: Application to the Chiral Resolution of Lactols

Downstream intermediates are crucial for the reactivity and selectivity of aminocatalytic reactions. We present an analysis of the stereopreference in aminocatalytic downstream intermediates, which reveals an inconspicuous mechanism of chiral recognition between the catalyst and the rest of the molecule. We delineate a stereoelectronic model to rationalize the mode of chiral transmission. We also exploit it for the resolution of chiral lactols relevant in organic synthesis as well as in the flavor and fragrance industry.

Amine Catalysis with Substrates Bearing N-Heterocyclic Moieties Enabled by Control over the Enamine Pyramidalization Direction

Stereoselective organocatalytic C-C bond formations that tolerate N-heterocycles are valuable since these moieties are common motifs in numerous chiral bioactive compounds. Such transformations are, however, challenging since N-heterocyclic moieties can interfere with the catalytic reaction. Here, we present a peptide that catalyzes conjugate addition reactions between aldehydes and nitroolefins bearing a broad range of different N-heterocyclic moieties with basic and/or H-bonding sites in excellent yields and stereoselectivities. Tuning of the pyramidalization direction of the enamine intermediate enabled the high stereoselectivity.

Deactivation of Secondary Amine Catalysts via Aldol Reaction-Amine Catalysis under Solvent-Free Conditions

Despite intense interest in amine-catalyzed stereoselective reactions, high catalyst loadings of ≥10 mol % are still common and either due to low reactivity or catalyst deactivation. Yet, few deactivation pathways are well understood. Here, we unraveled the deactivation of secondary amines by undesired aldol reaction. Mechanistic studies with peptide and prolinol silyl ether catalysts showed the generality of this so-far underappreciated catalyst deactivation pathway. The insights enabled conjugate addition reactions between aldehydes and nitroolefins on a multigram scale in the absence of solvent-conditions that are attractive as environmentally benign processes-with excellent product yields and stereoselectivities in the presence of as little as 0.1 mol % of a chemoselective peptidic catalyst.

Peptide-Catalyzed Highly Asymmetric Cross-Aldol Reaction of Aldehydes to Biomimetically Synthesize 1,4-Dicarbonyls

β-Turn tetrapeptides were demonstrated to catalyze asymmetric aldol reaction of α-branched aldehydes and α-carbonyl aldehydes, i.e. glyoxylates and α-ketoaldehydes, to biomimetically synthesize acyclic all-carbon quaternary center-bearing 1,4-dicarbonyls in high yield and excellent enantioselectivity under mild conditions. The spatially restricted environment of the tetrapeptide warrants high enantioselectivity and yield with broad substrates. Using this protocol, (R)-pantolactone, the key intermediate of vitamin B5, was readily accessed in a practical, efficient, and environmentally benign process from inexpensive starting materials.

Effect of the enamine pyramidalization direction on the reactivity of secondary amine organocatalysts

Chiral secondary amines are valuable catalysts for reactions that proceed through an enamine intermediate. Here, we explored the importance of the pyramidalization direction of the enamine-N on the reactivity of chiral enamines with a combination of computational, NMR spectroscopic, and kinetic experiments. Studies with peptidic catalysts that bear cyclic amines with different ring sizes revealed that endo-pyramidalized enamines are significantly more reactive compared to exo-pyramidalized analogs. The results show that the pyramidalization direction can have a greater effect than n→π* orbital overlap on the reactivity of chiral enamines. The data enabled the development of a catalyst with higher reactivity compared to the parent catalyst.

Endo-pyramidalisation at nitrogen bestows enamines derived from α-substituted amines with higher reactivity compared to exo-pyramidalisation.

Organocatalytic Asymmetric Addition of Aldehyde to Nitroolefin by H-d-Pro-Pro-Glu-NH2: A Mechanistic Study

The mechanism of the asymmetric addition of aldehyde (butanal) to nitroolefin (β-nitrostyrene) catalyzed by H-d-Pro-Pro-Glu-NH₂ (dPPE-NH₂; 1) was explored using density functional theory methods in chloroform. By conformational search, it was confirmed that catalyst 1 and its enamine intermediate adopted a dominant conformation with a βI structure stabilized by a C₁₀ H-bond between the C=O of d-Pro1 and C-terminal NH₂ proton and by an additional H-bond between the side chain and the backbone of Glu3. This βI turn structure was conserved all along the catalytic cycle. Consistently with the kinetic studies, the C-C bond formation between the enamine and electrophile was also confirmed as the rate-determining step. The stereoselectivity results from a re → re prochiral approach of enamine and β-nitrostyrene with a gauche^- orientation of the double bonds. Although it was suggested as the possible formation of dihydrooxazine oxide species, this process was confirmed to be kinetically less accessible than the formation of acyclic nitronate. In particular, our calculated results supported that the carboxylic acid group of Glu3 in 1 played a central role by acting as general acid/base all along the catalytic cycle and orienting the asymmetric C-C bond formation.

Prospect of Thiazole-based γ-Peptide Foldamers in Enamine Catalysis: Exploration of the Nitro-Michael Addition

As three-dimensional folding is prerequisite to biopolymer activity, complex functions may also be achieved through foldamer science. Because of the diversity of sizes, shapes and folding available with synthetic monomers, foldamer frameworks enable a numerous opportunities for designing new generations of catalysts. We herein demonstrate that heterocyclic γ-peptide scaffolds represent a versatile platform for enamine catalysis. One central feature was to determine how the catalytic activity and the transfer of chiral information might be under the control of the conformational behaviours of the oligomer.

Conformational Properties of a Peptidic Catalyst: Insights from NMR Spectroscopic Studies

Peptides have become valuable as catalysts for a variety of different reactions, but little is known about the conformational properties of peptidic catalysts. We investigated the conformation of the peptide H-dPro-Pro-Glu-NH₂, a highly reactive and stereoselective catalyst for conjugate addition reactions, and the corresponding enamine intermediate in solution by NMR spectroscopy and computational methods. The combination of nuclear Overhauser effects (NOEs), residual dipolar couplings (RDCs), J-couplings, and temperature coefficients revealed that the tripeptide adopts a single predominant conformation in its ground state. The structure is a type I β-turn, which gains stabilization from three hydrogen bonds that are cooperatively formed between all functional groups (secondary amine, carboxylic acid, amides) within the tripeptide. In contrast, the conformation of the enamine intermediate is significantly more flexible. The conformational ensemble of the enamine is still dominated by the β-turn, but the backbone and the side chain of the glutamic acid residue are more dynamic. The key to the switch between rigidity and flexibility of the peptidic catalyst is the CO₂H group in the side chain of the glutamic acid residue, which acts as a lid that can open and close. As a result, the peptidic catalyst is able to adapt to the structural requirements of the intermediates and transition states of the catalytic cycle. These insights might explain the robustness and high reactivity of the peptidic catalyst, which exceeds that of other secondary amine-based organocatalysts. The data suggest that a balance between rigidity and flexibility, which is reminiscent of the dynamic nature of enzymes, is beneficial for peptidic catalysts and other synthetic catalysts.

Amide rotation trajectories probed by symmetry

Amide rotation of peptidyl-prolyl fragments is an important factor in backbone structure organization of proteins. Computational studies have indicated that this rotation preferentially proceeds through a defined transition-state structure (syn/exo). Here, we complement the computational findings by determining the amide bond rotation barriers for derivatives of the two symmetric proline analogues, meso and racemic pyrrolidine-2,5-dicarboxylic acids. The rotations around these residues represent syn/exo-syn/exo and anti/endo-syn/exo hybrid transition states for the meso and racemic diastereomer, respectively. The rotation barriers are lower for the former rotation by about 9 kJ mol^-1 (aqueous medium), suggesting a strong preference for the syn/exo (clockwise) rotation over the anti/endo (anticlockwise) rotation. The results show that both hybrid rotation processes are enthalpically driven but respond differently to solvent polarity changes due to the different transition state dipole-dipole interactions.

Recent Advances in Asymmetric Organocatalyzed Conjugate Additions to Nitroalkenes

The asymmetric conjugate addition of carbon and heteroatom nucleophiles to nitroalkenes is a very interesting tool for the construction of highly functionalized synthetic building blocks. Thanks to the rapid development of asymmetric organocatalysis, significant progress has been made during the last years in achieving efficiently this process, concerning chiral organocatalysts, substrates and reaction conditions. This review surveys the advances in asymmetric organocatalytic conjugate addition reactions to α,β-unsaturated nitroalkenes developed between 2013 and early 2017.

Is more better? A comparison of tri- and tetrapeptidic catalysts

From an enzymatic perspective, there is a general notion that the bigger and more complex a catalytically active peptide is the more enzyme-like and the better it should become. But is this really true?

Remote-Stereocontrol in Dienamine Catalysis: Z-Dienamine Preferences and Electrophile-Catalyst Interaction Revealed by NMR and Computational Studies

Catalysis with remote-stereocontrol provides special challenges in design and comprehension. One famous example is the dienamine catalysis, for which high ee values are reported despite insufficient shielding of the second double bond. Especially for dienamines with variable Z/E-ratios of the second double bond, no correlations to the ee values are found. Therefore, the structures, thermodynamics, and kinetics of dienamine intermediates in SN-type reactions are investigated. The NMR studies show that the preferred dienamine conformation provides an effective shielding if large electrophiles are used. Calculations at SCS-MP2/CBS-level of theory and experimental data of the dienamine formation show kinetic preference for the Z-isomer of the second double bond and a slow isomerization toward the thermodynamically preferred E-isomer. Modulations of the rate-determining step, by variation of the concentration of the electrophile, allow the conversion of dienamines to be observed. With electrophiles, a faster reaction of Z- than of E-isomers is observed experimentally. Calculations corroborate these results by correlating ee values of three catalysts with the kinetics of the electrophilic attack and reveal the significance of CH-π and stacking interactions in the transition states. Thus, for the first time a comprehensive understanding of the remote stereocontrol in γ-functionalization reactions of dienamines and an explanation to the "Z/E-dilemma" are presented. The combination of bulky catalyst subsystems and large electrophiles provides a shielding of one face and causes different reactivities of E/Z-dienamines in nucleophilic attacks from the other face. Kinetic preferences for the formation of Z-dienamines and their unfavorable thermodynamics support high ee values.

Mass Spectrometric Back Reaction Screening of Quasi-Enantiomeric Products as a Mechanistic Tool

In this account, we discuss a mass spectrometric method that enables unambiguous identification of intermediates involved in the enantioselective step of a catalytic cycle. This method, which we originally developed for rapid evaluation of chiral catalysts, is based on monitoring the back reaction of mass-labeled quasi-enantiomeric products by ESI-MS. In this way, the intrinsic enantioselectivity of a chiral catalyst can be determined directly by quantification of catalytically relevant intermediates. By comparing the results from the forward and back reaction, direct evidence for the involvement of a catalytic intermediate in the enantioselective step can be obtained. In addition, insights about the energy profile of the catalytic cycle may be gained. The potential of back reaction screening as a mechanistic tool is demonstrated for organocatalytic aldol reactions, 1,4-additions of aldehydes to nitroolefins, Diels-Alder reactions, Michael additions, and Morita-Baylis-Hillman reactions.

Importance of dipole moments and ambient polarity for the conformation of Xaa-Pro moieties - a combined experimental and theoretical study

NMR spectroscopic studies with a series of proline derivatives revealed that the polarity of the environment has a significant effect on the trans : cis isomer ratio of Xaa-Pro bonds. Computational studies showed that this effect is due to differences in the overall dipole moments of trans and cis conformers. Comparisons between the conformational properties of amide and ester derivatives revealed an intricate balance between polarity effects and n → π* interactions of adjacent carbonyl groups. The findings have important implications for protein folding and signaling as well as the performance of proline-based stereoselective catalysts.

Organocatalytic asymmetric Michael addition of aldehydes and ketones to nitroalkenes catalyzed by adamantoyl l-prolinamide

A series of adamantoyl l-prolinamides have been synthesized.

Peptidomimetic organocatalysts: efficient Michael addition of ketones onto nitroolefins with very low catalyst loading

The syntheses and applications of peptidomimetic triazole-based catalysts are described as efficient organocatalysts in Michael reaction with low loading.