Macrocyclic Oxindole Peptide Epoxyketones-A Comparative Study of Macrocyclic Inhibitors of the 20S Proteasome.

Peptide macrocycles have recently gained attention as protease inhibitors due to their metabolic stability and specificity. However, the development of peptide macrocycles with improved binding potency has so far been challenging. Here we present macrocyclic peptides derived from the clinically applied proteasome inhibitor carfilzomib with an oxindole group that mimics the natural product TMC-95A. Fluorescence kinetic activity assays reveal a high potency of the oxindole group (IC50 = 0.19 µM) compared with agents lacking this motif. X-ray structures of the ligands with the ß5-subunit of the yeast 20S proteasome illustrate that the installed macrocycle forces strong hydrogen bonding of the oxindole group with ß5-Gly23NH. Thus, the binding of our designed oxindole epoxyketones is entropically and enthalpically favored in contrast to more flexible proteasome inhibitors such as carfilzomib.

Electrophotocatalytic C-H Functionalization of Ethers with High Regioselectivity.

The highly regioselective electrophotocatalytic C-H functionalization of ethers is described. These reactions are catalyzed by a trisaminocyclopropenium (TAC) ion at mild electrochemical potential with visible light irradiation. Ethers undergo oxidant-free coupling with isoquinolines, alkenes, alkynes, pyrazoles, and purines with typically high regioselectivity for the less-hindered α-position. The reaction is proposed to operate via hydrogen atom transfer (HAT) from the substrate to the photoexcited TAC radical dication, thus demonstrating a new reactivity mode for this electrophotocatalyst.

Selective Conversion of Carbon Dioxide to Formaldehyde via a Bis(silyl)acetal: Incorporation of Isotopically Labeled C1 Moieties Derived from Carbon Dioxide into Organic Molecules.

The conversion of carbon dioxide to formaldehyde is a transformation that is of considerable significance in view of the fact that formaldehyde is a widely used chemical, but this conversion is challenging because CO2 is resistant to chemical transformations. Therefore, we report here that formaldehyde can be readily obtained from CO2 at room temperature via the bis(silyl)acetal, H2C(OSiPh3)2. Specifically, formaldehyde is released from H2C(OSiPh3)2 upon treatment with CsF at room temperature. H2C(OSiPh3)2 thus serves as a formaldehyde surrogate and provides a means to incorporate CHx (x = 1 or 2) moieties into organic molecules. Isotopologues of H2C(OSiPh3)2 may also be synthesized, thereby providing a convenient means to use CO2 as a source of isotopic labels in organic molecules.

Electrophotocatalysis with a Trisaminocyclopropenium Radical Dication.

Visible-light photocatalysis and electrocatalysis are two powerful strategies for the promotion of chemical reactions. Here, these two modalities are combined in an electrophotocatalytic oxidation platform. This chemistry employs a trisaminocyclopropenium (TAC) ion catalyst, which is electrochemically oxidized to form a cyclopropenium radical dication intermediate. The radical dication undergoes photoexcitation with visible light to produce an excited-state species with oxidizing power (3.33 V vs. SCE) sufficient to oxidize benzene and halogenated benzenes via single-electron transfer (SET), resulting in C-H/N-H coupling with azoles. A rationale for the strongly oxidizing behavior of the photoexcited species is provided, while the stability of the catalyst is rationalized by a particular conformation of the cis-2,6-dimethylpiperidine moieties.

Oxidizable Ketones: Persistent Radical Cations from the Single-Electron Oxidation of 2,3-Diaminocyclopropenones.

Single electron oxidation of 2,3-diaminocyclopropenones is shown to give rise to stable diaminocyclopropenium oxyl (DACO) radical cations. Cyclic voltammetry reveals reversible oxidations in the range of +0.70-1.10 V (vs. SCE). Computational, EPR, and X-ray analysis support the view that the oxidized species is best described as a cyclopropenium ion with spin density located on the heteroatom substituents, including 23.5 % on oxygen. The metal-ligand behavior of the DACO radical is also described.

Synthesis and Evaluation of Macrocyclic Peptide Aldehydes as Potent and Selective Inhibitors of the 20S Proteasome.

This research explores the first design and synthesis of macrocyclic peptide aldehydes as potent inhibitors of the 20S proteasome. Two novel macrocyclic peptide aldehydes based on the ring-size of the macrocyclic natural product TMC-95 were prepared and evaluated as inhibitors of the 20S proteasome. Both compounds inhibited in the low nanomolar range and proved to be selective for the proteasome over other serine and cysteine proteases, particularly when compared to linear analogues with similar amino acid sequences. In HeLa cells, both macrocycles efficiently inhibited activation of nuclear factor-κB (NF-κB) transcription factor by blocking proteasomal degradation of the inhibitor protein IκBα after cytokine stimulation. Due to their covalent mechanism of binding these compounds represent a 1000-fold increase in inhibitory potency over previously reported noncovalently binding TMC-95 analogues. Molecular modeling of the macrocyclic peptides confirms the preference of the large S3 pocket for large, hydrophobic residues and the ability to exploit this to improve selectivity of proteasome inhibitors.