Conjugate Addition Reactions Of Aldehydes Research Articles

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.

Impact of the β-turn hydrogen bond on the trans/cis ratio and the performance of the peptide catalyst H-dPro-Pro-Glu-NH2

Abstract The effect of the β-turn hydrogen bond on the trans/cis conformer ratio and the catalytic performance of the peptidic catalyst H- d Pro-Pro-Glu-NH2 was investigated. Different electron withdrawing and donating substituents at the C-terminus of peptide analogues caused changes in the trans/cis ratio and the enantioselectivities of the peptides in conjugate addition reactions of aldehydes to nitroolefins. The results imply that the β-turn H-bond contributes but is not critical for the catalytic performance of H- d Pro-Pro-Glu-NH2 and related peptidic catalysts.

Organocatalysed conjugate addition reactions of aldehydes to nitroolefins with anti selectivity

Catalytic reactions that enable access to different diastereoisomers are important but often difficult to achieve. One long-standing unsolved challenge is the anti-selective conjugate addition reaction between aldehydes and nitroolefins. This organocatalytic transformation is a versatile method for the synthesis of γ-nitroaldehydes and downstream compounds such as pyrrolidines and γ-butyrolactams, which are key moieties in bioactive molecules. Numerous amine catalysts have been developed for this transformation, but all provide syn-configured products. Here, we present a tripeptide as a general catalyst for the synthesis of anti-configured γ-nitroaldehydes. Key to the anti selectivity is the installation of substituents at Cδ of the reactive pyrrolidine of proline to enable the formation of s-cis enamine intermediates. The peptidic catalyst converts different aldehyde and nitroolefin combinations into the products in high yields and stereoselectivities. Conformational and mechanistic studies revealed a Curtin–Hammett scenario for the catalytic system. The strategy provides long-sought-after access to anti-configured γ-nitroaldehydes and a guide for reversing the diastereoselectivity of amine-based organocatalysts. The organocatalysed addition of aldehydes to nitroolefins is an extremely well-studied reaction that almost exclusively provides the syn-configured products. Here a general method to reverse the diastereoselectivity is reported, whereby a tripeptide catalyst consistently provides the anti product with high selectivity.

Effect of β3‐Amino Acids on the Performance of the Peptidic Catalyst H‐dPro‐Pro‐Glu‐NH2

AbstractThe effect of β3‐amino acids on the conformation and catalytic performance of the peptidic catalyst H‐dPro‐Pro‐Glu‐NH2 was investigated. Analogues of the peptidic catalyst bearing instead of the α‐amino acids the respective β3‐amino acids were prepared and their reactivity and stereoselectivity was investigated in conjugate addition reactions of aldehydes to nitroolefins. Additional computational studies provided insights into the preferred conformations of the peptidic catalysts. The results show that conformational flexibility at the N‐terminus has a severe effect on the stereoselectivity but is tolerated at the C‐terminus.

ChemInform Abstract: Peptide‐Catalyzed Stereoselective Conjugate Addition Reactions of Aldehydes to Maleimide.

AbstractThe tripeptide H‐dPro‐Pro‐Asn‐NH2 is presented as catalyst for asymmetric conjugate addition reactions of aldehydes to maleimide.

Peptide‐Catalyzed Stereoselective Conjugate Addition Reactions of Aldehydes to Maleimide

AbstractThe tripeptide H‐dPro‐Pro‐Asn‐NH2 is presented as a catalyst for asymmetric conjugate addition reactions of aldehydes to maleimide. The peptidic catalyst promotes the reaction between various aldehydes and unprotected maleimide with high stereoselectivities and yields. The obtained products were readily derivatized to the corresponding pyrrolidines, lactams, lactones, and peptide‐like compounds. 1H NMR spectroscopic, crystallographic, and computational investigations provided insight into the conformational properties of H‐dPro‐Pro‐Asn‐NH2 and revealed the importance of hydrogen bonding between the peptide and maleimide for catalyzing the stereoselective C−C bond formation.

Peptide-Catalyzed Stereoselective Conjugate Addition Reactions of Aldehydes to Maleimide.

The tripeptide H-dPro-Pro-Asn-NH2 is presented as a catalyst for asymmetric conjugate addition reactions of aldehydes to maleimide. The peptidic catalyst promotes the reaction between various aldehydes and unprotected maleimide with high stereoselectivities and yields. The obtained products were readily derivatized to the corresponding pyrrolidines, lactams, lactones, and peptide-like compounds. (1) H NMR spectroscopic, crystallographic, and computational investigations provided insight into the conformational properties of H-dPro-Pro-Asn-NH2 and revealed the importance of hydrogen bonding between the peptide and maleimide for catalyzing the stereoselective C-C bond formation.

Peptide catalysis in aqueous emulsions

Amphiphilic tripeptides catalyze conjugate addition reactions of aldehydes to nitroolefins in water with high yields and stereoselectivities. The amphiphilic nature shields the peptidic catalyst from the water and stabilizes the emulsion formed by the reactants in the aqueous environment. The findings have intriguing implications for the chemical evolution of enzymes.

ChemInform Abstract: Peptide‐Catalyzed 1,4‐Addition Reactions of Aldehydes to Nitroolefins

AbstractReview: 40 refs.

Peptide-catalyzed 1,4-Addition Reactions of Aldehydes to Nitroolefins

Conjugate addition reactions of aldehydes to nitroolefins provide synthetically useful gamma-nitroaldehydes. Here we summarize our research on peptide-catalyzed conjugate addition reactions of aldehydes to differently substituted nitroolefins. We show that peptides of the general type Pro-Pro-Xaa (Xaa = acidic amino acid) are not only highly active, robust and stereoselective catalysts but have also remarkable chemoselectivities.

Adapting to Substrate Challenges: Peptides as Catalysts for Conjugate Addition Reactions of Aldehydes to α,β‐Disubstituted Nitroolefins

Conjugate addition reactions of aldehydes to α,β-disubstituted nitroolefins are important because they provide synthetically useful γ-nitroaldehydes bearing three consecutive stereogenic centers. Such reactions are challenging due to the drastically lower reactivity of α,β-disubstituted nitroolefins compared to, for example, β-monosubstituted nitroolefins. The testing of a small collection of peptides of the type Pro-Pro-Xaa (Xaa=acidic amino acid) led to the identification of H-Pro-Pro-D-Gln-OH and H-Pro-Pro-Asn-OH as excellent stereoselective catalysts for this transformation. In the presence of 5 mol% of these peptides different combinations of aldehydes and α,β-disubstituted nitroolefins react readily with each other providing γ-nitroaldehydes in good yields and diastereoselectivities as well as excellent enantioselectivities. Chiral pyrrolidines as well as fully substituted γ-butyrolactams and γ-amino acids are easily accessible from the γ-nitroaldehydes. Mechanistic studies demonstrate that the configuration at all three stereogenic centers is induced by the peptidic catalysts. Only a minimal amount of products from homo-aldol reactions is observed demonstrating the high chemoselectivity of the peptidic catalysts.

Peptide-Catalyzed Conjugate Addition Reactions of Aldehydes to Nitroolefins

The tripeptide H-d-Pro-Pro-Glu-NH2 is an efficient catalyst for conjugate addition reactions of aldehydes to nitroolefins. Chiral γ-nitroaldehydes are obtained in excellent yields and stereoselectivities in the presence of as little as 1 mol% of the tripeptide catalyst. Under further optimized reaction conditions the product was obtained in essentially perfect stereoselectivity (syn/anti, >99:1, 99% ee).

Tripeptides of the Type H‐D‐Pro‐Pro‐Xaa‐NH2 as Catalysts for Asymmetric 1,4‐Addition Reactions: Structural Requirements for High Catalytic Efficiency

Analysis of the structural and functional requirements within the asymmetric peptidic catalyst H-D-Pro-Pro-Asp-NH(2) led to the development of the closely related peptide H-D-Pro-Pro-Glu-NH(2) as an even more efficient catalyst for asymmetric conjugate addition reactions of aldehydes to nitroolefins. In the presence of as little as 1 mol % of H-D-Pro-Pro-Glu-NH(2), a broad range of aldehydes and nitroolefins react readily with each other. The resulting gamma-nitroaldehydes were obtained in excellent yields and stereoselectivities at room temperature. Within the structure of the peptidic catalysts, the D-Pro-Pro motif is the major contributor to the high stereoselectivities. The C-terminal amide and the spacer to the carboxylic acid in the side-chain of the C-terminal amino acid are responsible for the fine-tuning of the stereoselectivity. The peptidic catalysts not only allow for highly effective asymmetric catalysis under mild conditions, but also function in the absence of additives.

ChemInform Abstract: Peptide Catalyzed Asymmetric Conjugate Addition Reactions of Aldehydes to Nitroethylene — A Convenient Entry into γ2‐Amino Acids.

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Peptide Catalyzed Asymmetric Conjugate Addition Reactions of Aldehydes to Nitroethylene—A Convenient Entry into γ2-Amino Acids

The peptide H-D-Pro-Pro-Glu-NH2 is a highly effective catalyst for conjugate addition reactions between aldehydes and nitroethylene. Only 1 mol % of H-d-Pro-Pro-Glu-NH2 and a 1.5-fold excess of aldehyde with respect to nitroethylene suffice to obtain gamma-nitroaldehydes and, after reduction, monosubstituted gamma-nitroalcohols in excellent yields and optical purities. The products can be readily converted into gamma2-amino acids, thereby opening an effective direct entry into this important class of compounds.