Enantiopure 5-CF3-Proline: Synthesis, Incorporation in Peptides, and Tuning of the Peptide Bond Geometry

Proline Analogues

Within the canonical repertoire of the amino acid involved in protein biogenesis, proline plays a unique role as an amino acid presenting a modified backbone rather than a side-chain. Chemical structures that mimic proline but introduce changes into its specific molecular features are defined as proline analogues. This review article summarizes the existing chemical, physicochemical, and biochemical knowledge about this peculiar family of structures. We group proline analogues from the following compounds: substituted prolines, unsaturated and fused structures, ring size homologues, heterocyclic, e.g., pseudoproline, and bridged proline-resembling structures. We overview (1) the occurrence of proline analogues in nature and their chemical synthesis, (2) physicochemical properties including ring conformation and cis/trans amide isomerization, (3) use in commercial drugs such as nirmatrelvir recently approved against COVID-19, (4) peptide and protein synthesis involving proline analogues, (5) specific opportunities created in peptide engineering, and (6) cases of protein engineering with the analogues. The review aims to provide a summary to anyone interested in using proline analogues in systems ranging from specific biochemical setups to complex biological systems.

Late-stage gem-difluoroallylation of phenol in bioactive molecules and peptides with 3,3-difluoroallyl sulfonium salts

An efficient method for the late-stage selective O-fluoroalkylation of tyrosine residues with a stable yet highly reactive fluoroalkylating reagent, 3,3-difluoroallyl sulfonium salts (DFASs), has been developed. The reaction proceeds in a mild basic aqueous buffer (pH = 11.6) with high efficiency, high biocompatibility, and excellent regio- and chemoselectivity. Various oligopeptides and phenol-containing bioactive molecules, including carbohydrates and nucleosides, could be selectively O-fluoroalkylated. The added vinyl and other functional groups from DFASs can be valuable linkers for successive modification, significantly expanding the chemical space for further bioconjugation. The synthetic utility of this protocol has been demonstrated by the fluorescently labeled anti-cancer drug and the synthesis of O-link type 1,4,7,10-tetraazacyclododecane-N,N',N,N'-tetraacetic acid-tyrosine3-octreotate (DOTA-TATE), showing the prospect of the method in medicinal chemistry and chemical biology.

Fluorine-Containing Prolines: Synthetic Strategies, Applications, and Opportunities

Fluorinated prolines play an important role in peptide studies, protein engineering, medicinal chemistry, drug discovery, and agrochemistry. Since the first synthesis of 4-fluoroprolines by Gottlieb and Witkop in 1965, their popularity started to grow exponentially. For example, during the past two decades, all isomeric trifluoromethyl-substituted prolines have been synthesized. In this Perspective, chemical properties and applications of fluorinated prolines are discussed. Synthetic approaches to all known fluorine-containing prolines are also discussed and analyzed. This analysis unexpectedly revealed an unsolved problem: in strict contrast to fluoro- and trifluoromethyl-substituted prolines, the corresponding analogues with fluoromethyl and difluoromethyl groups are mostly unknown. At the end of the paper, structures of several interesting, yet unknown, fluorinated prolines are disclosed─a good opportunity for chemists to make them.

Nickel-Catalyzed Cross-Coupling of Amino-Acid-Derived Alkylzinc Reagents with Alkyl Bromides/Chlorides: Access to Diverse Unnatural Amino Acids

Unnatural α-amino acids are important synthetic targets in the field of peptide science. Herein we report an efficient, versatile, and straightforward strategy for the synthesis of homophenylalanine derivatives via the nickel-catalyzed Csp3-Csp3 cross-coupling of (fluoro)benzyl bromides/chlorides with natural α-amino-acid-derived alkylzinc reagents. The current protocol features the advantages of a low-cost nickel catalyst system, synthetic convenience, and the tolerance of rich functionality and stereochemistry.

Constrained Dipeptide Surrogates: 5- and 7-Hydroxy Indolizidin-2-one Amino Acid Synthesis from Iodolactonization of Dehydro-2,8-diamino Azelates

The constrained dipeptide surrogates 5- and 7-hydroxy indolizidin-2-one N-(Boc)amino acids have been synthesized from L-serine as a chiral educt. A linear precursor ∆⁴-unsaturated (2S,8S)-2,8-bis[N-(Boc)amino]azelic acid was prepared in five steps from L-serine. Although epoxidation and dihydroxylation pathways gave mixtures of hydroxy indolizidin-2-one diastereomers, iodolactonization of the ∆⁴-azelate stereoselectively delivered a lactone iodide from which separable (5S)- and (7S)-hydroxy indolizidin-2-one N-(Boc)amino esters were synthesized by sequences featuring intramolecular iodide displacement and lactam formation. X-ray analysis of the (7S)-hydroxy indolizidin-2-one N-(Boc)amino ester indicated that the backbone dihedral angles embedded in the bicyclic ring system resembled those of the central residues of an ideal type II’ β-turn indicating the potential for peptide mimicry.

Synthesis of enantiopure α-Tfm-proline and α-Tfm-pipecolic acid from oxazolo-pyrrolidines and -piperidines

Enantiopure α-Tfm-proline and α-Tfm-pipecolic acid were synthesized starting from commercially available diesters and ethyl trifluoroacetate. A Strecker type reaction on intermediate chiral Tfm-oxazolo-pyrrolidine and -piperidine provided the corresponding nitrile precursor of enantiopure (R) and (S) α-Tfm-proline and α-Tfm-pipecolic acid. The C-terminal peptide coupling reaction of α-Tfm-pipecolic acid has been successfully achieved.