Fluorous Substituent-Based Enantiomer and Diastereomer Separation: Orthogonal Use of HPLC Columns for the Synthesis of Nonproteinogenic Polyfluoro Amino Acids and Peptides

Liquid Chromatographic Enantioseparation of Newly Synthesized Fluorinated Tryptophan Analogs Applying Macrocyclic Glycopeptides-Based Chiral Stationary Phases Utilizing Core-Shell Particles

Due to the favorable features obtained through the incorporation of fluorine atom(s), fluorinated drugs are a group with emerging pharmaceutical importance. As their commercial availability is still very limited, to expand the range of possible candidates, new fluorinated tryptophan analogs were synthesized. Control of enantiopurity during the synthesis procedure requires that highly efficient enantioseparation methods be available. In this work, the enantioseparation of seven fluorinated tryptophans and tryptophan was studied and compared systematically to (i) develop analytical methods for enantioselective separations and (ii) explore the chromatographic features of the fluorotrytophans. For enantioresolution, macrocyclic glycopeptide-based selectors linked to core-shell particles were utilized, applying liquid chromatography-based methods. Application of the polar-ionic mode resulted in asymmetric and broadened peaks, while reversed-phase conditions, together with mobile-phase additives, resulted in baseline separation for all studied fluorinated tryptophans. The marked differences observed between the methanol and acetonitrile-containing eluent systems can be explained by the different solvation abilities of the bulk solvents of the applied mobile phases. Among the studied chiral selectors, teicoplanin and teicoplanin aglycone were found to work effectively. Under optimized conditions, baseline separations were achieved within 6 min. Ionic interactions were semi-quantitatively characterized and found to not influence enantiorecognition. Interestingly, fluorination of the analytes does not lead to marked changes in the chromatographic characteristics of the methanol-containing eluents, while larger differences were noticed when the polar but aprotic acetonitrile was applied. Experiments conducted on the influence of the separation temperature indicated that the separations are enthalpically driven, with only one exception. Enantiomeric elution order was found to be constant on both teicoplanin and teicoplanin aglycone-based chiral stationary phases (L < D) under all applied chromatographic conditions.

Development of Electrophilic Radical Perfluoroalkylation of Electron-Deficient Olefins

Fluorinated organic compounds have attracted significant attention over the past few decades owing to their unique properties and versatility. An established method for the synthesis of fluorinated organic compounds involves radical perfluoroalkylation reactions towards double bonds. In this radical pathway, electrophilic perfluoroalkyl radicals exhibit excellent reactivity towards electron-rich olefins. Therefore, several splendid perfluoroalkylation reactions of electron-rich olefins have been reported. However, there are only a few examples of reaction involving electron-deficient olefins because of their poor electronic compatibility with perfluoroalkyl radicals. This review focuses on the reports that challenge this long-standing issue. Radical perfluoroalkylation/bifunctionalization reactions of electron-deficient olefins are described according to the radical generation methods.

Product Control in Alkene Trifluoromethylation: Hydrotrifluoromethylation, Vinylic Trifluoromethylation, and Iodotrifluoromethylation using Togni Reagent

Hydrotrifluoromethylation, vinylic trifluoromethylation, and iodotrifluoromethylation of simple alkenes have been achieved by using Togni reagent in the absence of any transition metal catalyst. These reactions were readily controllable by selection of appropriate salts and solvents. The addition of K2CO3 afforded the hydrotrifluoromethylation product, with DMF acting not only as a solvent, but also as the hydrogen source. In contrast, the use of tetra-n-butylammonium iodide (TBAI) in 1,4-dioxane resulted in vinylic trifluoromethylation, while the use of KI afforded the iodotrifluoromethylation product. The vinylic trifluoromethylation product was obtained by treatment of the iodotrifluoromethylation product with ammonium 2-iodobenzoate, indicating that it was formed through an elimination reaction of the in-situ-generated iodotrifluoromethylation product, and the solubility of the resulting 2-iodobenzoate salt plays a key role in the product switching. A radical-clock experiment showed that these reactions proceed via radical intermediates.

Trifluoromethylation of alkenes with concomitant introduction of additional functional groups

The trifluoromethyl group is found in many synthetic bioactive compounds, and the difunctionalization of a C=C bond, as a powerful strategy for the construction of compounds with various functional groups, has been intensively investigated. Therefore, the difunctionalizing trifluoromethylation of alkenes has attracted growing interest because of the potential of the products as building blocks for bioactive molecules. In this review, we focus on recent advances in the trifluoromethylation of alkenes with concomitant introduction of additional functional groups.

Polarity-mismatched addition of electrophilic carbon radicals to an electron-deficient acceptor: cascade radical addition-cyclization-trapping reaction

The polarity-mismatched perfluoroalkyl radical addition to electron-deficient alkenes was studied. For this study, several substrates having two polarity-different radical acceptors were employed to investigate the regiochemical courses of cascade reaction. In the case of substrate 1 having a methacryloyl moiety, we found polarity-mismatched perfluoroalkylation giving 15a-e as a major course over the polarity-matched perfluoroalkylation giving 16a-e. Moreover, in the case of substrates 2-7, perfluoroalkyl radicals selectively added to an electron-deficient alkene moiety of 2-7, to give polarity-mismatched perfluoroalkylation products without the formation of regioisomers. Next, the control of enantioselectivity was studied. In the case of substrates 1 and 3, the reaction proceeded with good enantioselectivities by employing a chiral Lewis acid, prepared from chiral box ligand 24 and Zn(OTf)(2). For direct comparison, we also studied the reaction with other carbon radicals, derived from ICH(2)CO(2)Et, ICH(2)CN, BrC(CO(2)Et)(2)Me, and CCl(3)Br, which have electrophilic character.