Matthew J. O'Connor

Superelectrophiles and the Effects of Trifluoromethyl Substituents

Trifluoromethyl-substituted superelectrophiles were generated in superacid (CF(3)SO(3)H), and their chemistry was examined. The strong electron-withdrawing properties of the trifluoromethyl group are found to enhance the electrophilic character at cationic sites in superelectrophiles. This leads to greater positive-charge delocalization in the superelectrophiles. These effects are manifested by the unusual chemo-, regio-, and stereoselectivities shown by the superelectrophiles in chemical reactions.

Orthogonal lipid sensors identify transbilayer asymmetry of plasma membrane cholesterol

Controlled distribution of lipids across various cell membranes is crucial for cell homeostasis and regulation. We developed an imaging method that allows simultaneous in situ quantification of cholesterol in two leaflets of the plasma membrane (PM) using tunable orthogonal cholesterol sensors. Our imaging revealed marked transbilayer asymmetry of PM cholesterol (TAPMC) in various mammalian cells, with the concentration in the inner leaflet (IPM) being ∼12-fold lower than that in the outer leaflet (OPM). The asymmetry was maintained by active transport of cholesterol from IPM to OPM and its chemical retention at OPM. Furthermore, the increase in the IPM cholesterol level was triggered in a stimulus-specific manner, allowing cholesterol to serve as a signaling lipid. We found excellent correlation between the IPM cholesterol level and cellular Wnt signaling activity, suggesting that TAPMC and stimulus-induced PM cholesterol redistribution are crucial for tight regulation of cellular processes under physiological conditions.

Simultaneous in situ quantification of two cellular lipid pools using orthogonal fluorescent sensors

Lipids regulate a wide range of biological activities. Since their local concentrations are tightly controlled in a spatiotemporally specific manner, the simultaneous quantification of multiple lipids is essential for elucidation of the complex mechanisms of biological regulation. Here, we report a new method for the simultaneous in situ quantification of two lipid pools in mammalian cells using orthogonal fluorescent sensors. The sensors were prepared by incorporating two environmentally sensitive fluorophores with minimal spectral overlap separately into engineered lipid-binding proteins. Dual ratiometric analysis of imaging data allowed accurate, spatiotemporally resolved quantification of two different lipids on the same leaflet of the plasma membrane or a single lipid on two opposite leaflets of the plasma membrane of live mammalian cells. This new imaging technology should serve as a powerful tool for systems-level investigation of lipid-mediated cell signaling and regulation.

Sequential reactions of trimethylsilyldiazomethane with 4-alkenyl ketones and aldehydes catalyzed by lewis bases

A sequence of a carbonyl addition between 4-alkenyl ketones and aldehydes with trimethylsilyldiazomethane followed by a 1,3-Brook rearrangement and an intramolecular 1,3-dipolar cycloaddition was promoted by Lewis bases tetrabutylammonium triphenyldifluorosilicate (TBAT) or potassium tert-butoxide (KO-t-Bu). Through these concatenated bond-forming processes, a variety of novel bi- and tricyclic Δ(1)-pyrazolines were synthesized.

Synthesis of Amathaspiramides by Aminocyanation of Enoates

Concise routes for the total and formal syntheses of the amathaspiramides were developed through a formal [3+2] cycloaddition between lithium(trimethylsilyl)diazomethane and α,β-unsaturated esters. The effectiveness of this new cycloaddition for the construction of Δ(2)-pyrazolines containing a α-tert-alkylamino carbon center and subsequent facile protonolytic N-N bond cleavage allows the synthesis of a key intermediate of the amathaspiramides and other α,α-disubstituted amino acid derivatives.

Formal Aminocyanation of α,β-Unsaturated Cyclic Enones for the Efficient Synthesis of α-Amino Ketones†

Installation of amino functionality on organic molecules through direct CN bond formation is an important research objective. To achieve this goal, a 1,2-aminocyanation reaction was developed. The reaction occurs through the formation of pyrazolines by means of a formal dipolar cycloaddition of cyclic α,β-unsaturated ketones with lithium trimethylsilyldiazomethane followed by novel protonolytic N-N bond cleavage under mild conditions. This two-step process provides a diverse array of structurally complex free and mono-alkylated α-amino ketones in excellent yields.

Sequential 1,4-/1,2-addition of lithium(trimethylsilyl)diazomethane onto cyclic enones to induce C-C fragmentation and N-Li insertion

α,β-Unsaturated ketones generally undergo addition reactions with nucleophiles with a preference for either 1,2- or 1,4-addition, but rarely both. However, the right combination of reagents allows for consecutive 1,4- and 1,2-additions to occur: Cyclic α,β-unsaturated ketones undergo double additions with lithium(trimethylsilyl)diazomethane, effectively generating various molecular frameworks with complexity and diversity. Owing to the sequential generation of several intermediates of multifaceted reactivity, including diazoalkane derivatives and alkylidene carbenes, it is possible to induce novel Grob-type C-C fragmentations, alkylidene carbene mediated Li-N insertions, and dipolar cycloadditions by controlling the reaction parameters.

DFT Studies on the Stereoselectivity of α-Silyloxy Diazoalkane Cycloadditions

The intramolecular [3+2] cycloaddition (32CA) of alkene-tethered α-silyloxydiazoalkanes provides variable stereoselectivity in generating bicyclic pyrazolines where the silyloxy group is either syn or anti to the newly formed pyrazoline ring. To elucidate the origin of the stereoselectivity, density functional theory (DFT) calculations were carried out for the energy of each transition state structure (TSs) and product. Steric effects were identified as the major determining factors in the diastereoselectivity of the 32CA reaction with regards to substrate structure (cyclic or acyclic α-silyloxydiazoalkanes).