Synlett

Despite the appeal of organic redox systems as next-generation energy-storage media, achieving high cell voltages with electrolytes based on main-group elements typically comes at the cost of reduced long-term stabilities. In this Synpacts article, we summarize our recent finding that the introduction of phosphine oxide functionalities can unlock the ability of terthiophenes to serve as robust two-electron acceptors at extreme potentials. These investigations uncovered a fundamentally new class of multielectron redox systems, capable of expanding the cell potential range achievable with organic electrolytes without compromising stability.

Herein, we disclose a FeBr2-promoted cycloaddition of readily available 1,3-enynes and nitrosoarenes, providing a promising platform for the synthesis of privileged 4-acylquinoline scaffolds. This simple, one-pot process is characterized by high atom-economy, broad substrate-scope, and excellent functional-group tolerance. A possible reaction mechanism was proposed, involving processes such as [4+2] cycloaddition, ring opening, aromatization, and dehydroaromatization.

Psoralen-conjugated triplex-forming oligonucleotides (Ps-TFOs) have been used to induce DNA mutations or to suppress gene expression through the formation of crosslinked products with DNA in a sequence-specific manner. Psoralen can crosslink with DNA at its furan-ring and/or pyrone-ring side, yielding either a monoadduct or diadduct (interstrand crosslinking) product. The differences in the crosslinked structures of Ps-TFOs with the target DNAs are closely related to the changes in the biological outcomes induced by the Ps-TFOs. However, only a few reports have discussed the photocrosslinking properties of Ps-TFOs. The photocrosslinking properties of Ps-TFOs with structurally diverse psoralen derivatives remain elusive. Herein, we report the modular synthesis of novel methyl-substituted psoralen N-hydroxysuccinimide (NHS) esters. By using these esters, the effect of the methyl substituent of psoralen on the photocrosslinking of the corresponding Ps-TFOs was examined. The amount of the diadduct product was significantly reduced in the presence of methyl substituents at the C-3 and C-4 positions, while the total amount of photocrosslinking product was maintained. This work demonstrates the possibility of controlling the crosslinked product of Ps-TFOs by introducing methyl groups into psoralen: this ability to manipulate the product is an important factor in the biological applications of Ps-TFOs.

Proteins play critical roles in all living organisms, and their properties and functions result directly from their primary sequences. Fluorine, though seldom found in natural organic compounds, has been shown to impart desirable properties to small molecules and proteins alike. However, studies on the impact of this element in enzyme activity and protein–protein interaction are largely absent from the literature. Here we present a microwave-assisted SPPS method for the total synthesis of site-specifically fluorinated barnase variants, as well as characterization of their folding and activity. CD spectroscopy and fluorescence-based activity assays show that the fluorinated amino acids are generally not perturbative of the protein structure and that enzyme activity, albeit reduced, is retained in all variants.

Nucleophilic phosphine-catalyzed annulations are recognized as practical and powerful tools for synthesizing various cyclic compounds. Phosphine acceptors play a key role in nucleophilic phosphine catalysis. The design and synthesis of new phosphine acceptors that are able to introduce new zwitterionic intermediates with new reactivities into phosphine-catalyzed annulations is highly desirable. We recently applied proton-shift principles in the design of new phosphine acceptors, and we synthesized several new acceptors. With the use of these acceptors, we have developed several novel phosphine-catalyzed annulation reactions. In this account, we present a brief introduction to the design and application of a series of acidic-hydrogen-tethered electron-deficient acceptors for phosphine-catalyzed annulation reactions, categorized according to the type of atom (N–H, O–H, C–H) to which the acidic hydrogen is bound.1 Introduction2 Phosphine Acceptors Tethered with an Acidic N–H Group3 Phosphine Acceptors Tethered with an Acidic O–H Group4 Phosphine Acceptors Tethered with an Acidic C–H Group5 Conclusions

A highly effective and selective ortho-directed hydrogen isotope exchange process for aryl carboxylic acids has been achieved by using an iridium(I) N-heterocyclic carbene/phosphine complex under mild and neutral conditions. Good levels of deuterium incorporation have been delivered across a wide array of examples, including a number of biologically active drug compounds.

A series of donor-acceptor chromophores was synthesized bearing a 3-aminoimidazo[1,2-a]pyridine donor motive. Through DFT calculations, different combinations of the ImPy donor motive and different electron acceptors were assessed. In combination with an anthraquinone acceptor, the calculated ΔE
ST values were in range to suggest that these compounds would emit via thermally activated delayed fluorescence. Based on these findings, a series of ImPy-Aq emitters with different geometries and substitution patterns was synthesized through GBB-3CR and Suzuki coupling reactions. According to preliminary experimental data, the compounds were only slightly emissive at ambient temperatures due to a combination of low radiative rates and competing non-radiative deactivation pathways.

The development of an oxa-Diels–Alder reaction between sultines and carbonyl compounds is reported. o-Quinodimethanes, generated from sultines, undergo a [4+2]-cycloaddition with activated aldehydes or ketones in the presence of Cu(OTf)2 to provide a variety of functionalized isochromans, including spiroisochromans, in up to 99% yield. The developed protocol demonstrates broad functional-group compatibility and tolerates unprotected isatins bearing free NH-functionalities.

α-Cyanophosphonates, which are useful reagents for the Horner–Wittig reaction, were synthesized under solvent-free conditions by using a choline chloride–zinc chloride deep-eutectic solvent (DES) as a catalyst. This is only the second report on the synthesis of these compounds. In the previous report, diethyl trimethylsilyl phosphite was used as a reagent and TiCl4 as a catalyst, whereas in this study, both the reagent (triphenylphosphine) and the catalyst (choline chloride–zinc chloride DES) are cheaper, more readily available, and less harmful than those used in the previous work. Moreover, the process involves an interesting cascade reaction between a β-nitrostyrene and two equivalents of triphenyl phosphite, leading to the desired product by a new synthetic route. The products can be used in the pharmaceutical and agricultural industries, in addition to their synthetic applications in the preparation of α,β-unsaturated nitriles. The reactions were completed on using 20 mol% of DES at 80 °C in six hours. Ten different β-nitrostyrenes were synthesized in yields of 55–87% after purification. β-Nitrostyrenes containing electron-donating groups showed higher yields. The reaction failed when aliphatic or heteroaromatic nitroalkenes or β-nitrostyrenes with electron-withdrawing substituents were employed. Finally, three plausible mechanistic routes are proposed for the reaction, starting with the nucleophilic addition of triphenyl phosphite to the carbon, nitrogen, or oxygen atom in the α-position.

A density functional theory (DFT) study was performed to evaluate the reaction mechanism of the C–N bond formation under an integrated hydrogen atom transfer/radical-polar crossover photoredox catalytic cycle. The regioselective activation of a model substrate, including three reactive positions (3° benzylic C–H bond, 2° benzylic C–H bond, and primary C–Cl bond) was addressed to distinguish among the radical C–H activation mechanism and the standard SN2 reaction. We demonstrated that activation of tertiary benzylic C–H bond is the most favored and forms exclusively the experimentally observed product. In addition, the whole photoredox catalytic cycle, including the outer-sphere electron-transfer steps, was characterized computationally.