K. A. Woerpel

Ferroptosis: A Regulated Cell Death Nexus Linking Metabolism, Redox Biology, and Disease

Ferroptosis is a form of regulated cell death characterized by the iron-dependent accumulation of lipid hydroperoxides to lethal levels. Emerging evidence suggests that ferroptosis represents an ancient vulnerability caused by the incorporation of polyunsaturated fatty acids into cellular membranes, and cells have developed complex systems that exploit and defend against this vulnerability in different contexts. The sensitivity to ferroptosis is tightly linked to numerous biological processes, including amino acid, iron, and polyunsaturated fatty acid metabolism, and the biosynthesis of glutathione, phospholipids, NADPH, and coenzyme Q10. Ferroptosis has been implicated in the pathological cell death associated with degenerative diseases (i.e., Alzheimers, Huntingtons, and Parkinsons diseases), carcinogenesis, stroke, intracerebral hemorrhage, traumatic brain injury, ischemia-reperfusion injury, and kidney degeneration in mammals and is also implicated in heat stress in plants. Ferroptosis may also have a tumor-suppressor function that could be harnessed for cancer therapy. This Primer reviews the mechanisms underlying ferroptosis, highlights connections to other areas of biology and medicine, and recommends tools and guidelines for studying this emerging form of regulated cell death.

The Effect of Electrostatic Interactions on Conformational Equilibria of Multiply-Substituted Tetrahydropyran Oxocarbenium Ions

The three-dimensional structures of dioxocarbenium ions related to glycosyl cations were determined by an analysis of spectroscopic, computational, and reactivity data. Hypothetical low-energy structures of the dioxocarbenium ions were correlated with both experimentally determined (1)H NMR coupling constants and diastereoselectivity results from nucleophilic substitution reactions. This method confirmed the pseudoaxial preference of C-3 alkoxy-substituted systems and revealed the conformational preference of the C-5 alkoxymethyl group. Although the monosubstituted C-5 alkoxymethyl substituent preferred a pseudoequatorial orientation, the C-5-C-6 bond rotation was controlled by an electrostatic effect. The preferred diaxial conformer of the trans-4,5-disubstituted tetrahydropyranyl system underscored the importance of electrostatic effects in dictating conformational equilibria. In the 2-deoxymannose system, although steric effects influenced the orientation of the C-5 alkoxymethyl substituent, the all-axial conformer was favored because of electrostatic stabilization.

Electrostatic interactions in cations and their importance in biology and chemistry

Electrostatic effects exert strongly stabilizing influences on cations, in many cases controlling the conformational preferences of these cations. The lowest energy conformers are ones where the positive charge is brought closest to substituents bearing partial negative charges. These conformational biases, along with stereoelectronic effects, can control the stereoselectivity of reactions involving carbocationic intermediates.

Correlations Between Nucleophilicities and Selectivities in the Substitutions of Tetrahydropyran Acetals

Selectivities that deviate from S(N)1 stereoelectronic models in the nucleophilic substitutions of tetrahydropyran acetals were investigated. When weak nucleophiles were employed, stereoselectivities conformed to known S(N)1 stereoelectronic models. In contrast, stereoselectivities in the substitutions of acetals with strong nucleophiles depended on reaction conditions. Erosions in selectivities were observed when strong nucleophiles were employed in the absence of coordinating counterions. These erosions in selectivities are attributed to rates of nucleophilic additions to oxocarbenium ion intermediates that approach the diffusion limit. When triflate counterions were present, however, S(N)2-like pathways became accessible with strong nucleophiles. In most cases examined, the major stereoisomers formed from reactions that proceeded through S(N)2-like pathways were opposite to the major stereoisomers formed from the analogous reactions that proceeded through S(N)1 pathways.

Erosion of Stereochemical Control with Increasing Nucleophilicity: O-Glycosylation at the Diffusion Limit

Nucleophilic substitution reactions of 2-deoxyglycosyl donors indicated that the reactivity of the oxygen nucleophile has a significant impact on stereoselectivity. Employing ethanol as the nucleophile resulted in a 1:1 (alpha:beta) ratio of diastereomers under S(N)1-like reaction conditions. Stereoselective formation of the 2-deoxy-alpha-O-glycoside was only observed when weaker nucleophiles, such as trifluoroethanol, were employed. The lack of stereoselectivity observed in reactions of common oxygen nucleophiles can be attributed to reaction rates of the stereochemistry-determining step that approach the diffusion limit. In this scenario, both faces of the prochiral oxocarbenium ion are subject to nucleophilic addition to afford a statistical mixture of diastereomeric products. Control experiments confirmed that all nucleophilic substitution reactions were performed under kinetic control.

Phosphine-catalyzed reductions of alkyl silyl peroxides by titanium hydride reducing agents: development of the method and mechanistic investigations.

A method that allows for the reduction of protected hydroperoxides by employing catalytic amounts of phosphine is presented. The combination of a titanium(IV) alkoxide and a siloxane allowed for the chemoselective reduction of phosphine oxides in the presence of alkyl silyl peroxides. Subsequent reduction of the peroxide moiety by phosphine provided the corresponding silylated alcohols in useful yields. Mechanistic experiments, including crossover experiments, support a mechanism in which the peroxide group was reduced and the silyl group was transferred in a concerted step. Labeling studies with (17)O-labeled peroxides demonstrate that the oxygen atom adjacent to the silicon atom is removed from the silyl peroxide.

Continuum of mechanisms for nucleophilic substitutions of cyclic acetals.

The effect of nucleophile strength on diastereoselectivity in the nucleophilic substitution of cyclic acetals was explored. Stereoselectivity remained constant and high as nucleophilicity increased until a threshold value was reached. Beyond this point, however, selection of Lewis acid determined whether stereochemical inversion or erosion was observed.

C-glycosylation reactions of sulfur-substituted glycosyl donors: evidence against the role of neighboring-group participation.

Nucleophilic substitution reactions of C-4 sulfur-substituted tetrahydropyran acetals revealed that neighboring-group participation does not control product formation. Spectroscopic evidence for the formation of an intermediate sulfonium ion is provided, as are data from nucleophilic substitution reactions demonstrating that products are formed from oxocarbenium ion intermediates. The selectivity was not sensitive to solvent or to which Lewis acid was employed. The identity of the heteroatom at the C-4 position also did not significantly impact diastereoselectivity. Consequently, neighboring-group participation was not responsible for the formation of either the major or the minor products. These studies implicate a Curtin-Hammett kinetic scenario in which the formation of a low-energy intermediate does not necessitate its involvement in the product-forming pathway.

Copper(I)-Catalyzed Oxidation of Alkenes Using Molecular Oxygen and Hydroxylamines: Synthesis and Reactivity of α-Oxygenated Ketones

The copper(I)-catalyzed oxidation of alkenes with molecular oxygen and N-hydroxyphthalimide (NHPI) or N-hydroxybenzotriazole (HOBt) provided α-oxygenated ketones. The reaction proceeded under a balloon of O2 at room temperature to furnish the dioxygenated products in 50-90% yield. These compounds, particularly the HOBt derivatives, can be further functionalized with phosphorus, nitrogen, and sulfur nucleophiles to give synthetically useful products.

13C NMR Spectroscopy for the Quantitative Determination of Compound Ratios and Polymer End Groups

13C NMR spectroscopic integration employing short relaxation delays was evaluated as a quantitative tool to obtain ratios of diastereomers, regioisomers, constitutional isomers, mixtures of unrelated compounds, peptoids, and sugars. The results were compared to established quantitative methods such as 1H NMR spectroscopic integration, gas chromatography, and high-performance liquid chromatography and were found to be within <3.4% of 1H NMR spectroscopic values (most examples give results within <2%). Acquisition of the spectra took 2–30 min on as little as 10 mg of sample, proving the general utility of the technique. The simple protocol was extended to include end group analysis of low molecular weight polymers, which afforded results in accordance with 1H NMR spectroscopy and matrix-assisted laser desorption-ionization time-of-flight spectrometry.