The Role of the Chiralcis-1,3-Disubstituted 2,2-Dimethylcyclobutane Motif in the Conformational Bias of Several Types of γ-Peptides

Direct Access to Unnatural Cyclobutane α-Amino Acids through Visible Light Catalyzed [2+2]-Cycloaddition

In this work, we report the first selective, photocatalyzed [2+2]-cycloaddition of dehydroamino acids with styrene-type olefins. This simple, mild, and scalable approach relies on the use of the triplet energy transfer catalyst [Ir(dFCF₃ppy₂)dtbpy]PF₆ under visible light irradiation and provides fast access to value-added substituted strained cyclobutane α-amino acid derivatives.

Asymmetric Catalysis Mediated by Synthetic Peptides, Version 2.0: Expansion of Scope and Mechanisms

Low molecular weight synthetic peptides have been demonstrated to be effective catalysts for an increasingly wide array of asymmetric transformations. In many cases, these peptide-based catalysts have enabled novel multifunctional substrate activation modes and unprecedented selectivity manifolds. These features, along with their ease of preparation, modular and tunable structures, and often biomimetic attributes make peptides well-suited as chiral catalysts and of broad interest. Many examples of peptide-catalyzed asymmetric reactions have appeared in the literature since the last survey of this broad field in Chemical Reviews (Chem. Rev. 2007, 107, 5759-5812). The overarching goal of this new Review is to provide a comprehensive account of the numerous advances in the field. As a corollary to this goal, we survey the many different types of catalytic reactions, ranging from acylation to C-C bond formation, in which peptides have been successfully employed. In so doing, we devote significant discussion to the structural and mechanistic aspects of these reactions that are perhaps specific to peptide-based catalysts and their interactions with substrates and/or reagents.

Stereoselectivity of Proline/Cyclobutane Amino Acid-Containing Peptide Organocatalysts for Asymmetric Aldol Additions: A Rationale

Several α,β,α- or α,γ,α-tripeptides, consisting of a central cyclobutane β- or γ-amino acid being flanked by two d- or l-proline residues, have been synthesized and tested as organocatalysts in asymmetric aldol additions. High yields and enantioselectivities have been achieved with α,γ,α-tripeptides, being superior to peptides containing a cyclobutane β-amino acid residue. This is probably due to their high rigidity, which hinders some of the peptide catalysts to adopt the proper active conformation. This reasoning correlates with the major conformation of the peptides in the ground state, as suggested by ¹H NMR and computational calculations. The configuration of the aldol products is controlled by the proline chirality, and consequently, the R/S configuration of aldol products can be tuned by the use of either commercially available d- or l-proline. The enantioselectivity in the aldol reactions is reversed if the reactions are carried out in the presence of water or other protic solvents such as methanol. Spectroscopic and theoretical investigations revealed that this effect is not the consequence of conformational changes in the catalyst but rather caused by the participation of a water molecule in the rate determining transition state, in such a way that the preferential nucleophilic attack is oriented to the opposite enantiotopic aldehyde face.

Conformational Effects through Hydrogen Bonding in a Constrained γ-Peptide Template: From Intraresidue Seven-Membered Rings to a Gel-Forming Sheet Structure

A series of three short oligomers (di-, tri-, and tetramers) of cis-2-(aminomethyl)cyclobutane carboxylic acid, a γ-amino acid featuring a cyclobutane ring constraint, were prepared, and their conformational behavior was examined spectroscopically and by molecular modeling. In dilute solutions, these peptides showed a number of low-energy conformers, including ribbonlike structures pleated around a rarely observed series of intramolecular seven-membered hydrogen bonds. In more concentrated solutions, these interactions defer to an organized supramolecular assembly, leading to thermoreversible organogel formation notably for the tripeptide, which produced fibrillar xerogels. In the solid state, the dipeptide adopted a fully extended conformation featuring a one-dimensional network of intermolecularly H-bonded molecules stacked in an antiparallel sheet alignment. This work provides unique insight into the interplay between inter- and intramolecular H-bonded conformer topologies for the same peptide template.