Photocatalyzed reductive fluoroalkylation of 2-acetoxyglycals towards the stereoselective synthesis of α-1-fluoroalkyl-C-glycosyl derivatives

Perfluoroalkylation of Triarylamines by EDA Complexes and Ulterior Sensitized [6π]-Electrocyclization to Perfluoroalkylated Endo-Carbazoles. Mechanistic and Photophysical Studies

Blue LEDs-irradiation of a mixture of N,N,N',N'-tetramethylethylenediamine (TMEDA) and perfluoroalkyl iodides (RF-I) - Electron Donor Acceptor (EDA)-complex - in the presence of triphenylamines (TPAs) in an aqueous solvent mixture afforded mono-perfluoroalkylated triphenylamines (RF-TPA) in good yields. These RF-TPA were further subjected to acetone-sensitized [6π]-electrocyclization at 315 nm-irradiation affording exclusively perfluoroalkylated endo-carbazole derivatives (RF-CBz) in quantitative yields. Mechanistic studies and photophysical properties of products are studied.

Transformations of carbohydrate derivatives enabled by photocatalysis and visible light photochemistry

This review article highlights the diverse ways in which recent developments in the areas of photocatalysis and visible light photochemistry are impacting synthetic carbohydrate chemistry. The major topics covered are photocatalytic glycosylations, generation of radicals at the anomeric position, transformations involving radical formation at non-anomeric positions, additions to glycals, processes initiated by photocatalytic hydrogen atom transfer from sugars, and functional group interconversions at OH and SH groups. Factors influencing stereo- and site-selectivity in these processes, along with mechanistic aspects, are discussed.

Synthesis of N-glycosyl amides: conformational analysis and evaluation as inhibitors of β-galactosidase from E. coli

The synthesis of N-glycosyl amides typically involves the use of glycosyl amines as direct precursors, resulting in low yields due to hydrolysis and the loss of stereocontrol through anomerization processes. In this study, a sequential synthesis of N-glycosyl amides is proposed, employing glycosyl amines as intermediates obtained from glycosyl azides. Derivatives with gluco, galacto, or xylo configurations were synthesized. Hexose derivatives were obtained under stereocontrol to give only the β anomer, while the xylo derivatives provided a mixture of α and β anomers. Conformational analysis revealed that all β anomers adopted the 4C1 conformation, while α anomers were found in the 1C4 chair as the major conformer. After de-O-acetylation, the derivatives containing a galactose unit were evaluated as inhibitors of β-galactosidase from E. coli and were found to be moderate inhibitors.

Photocatalyzed Perfluoroalkylation of Endoglycals

The visible light-induced perfluoroalkyl (RF) radical reactions on peracetylglycals derived from hexoses and pentoses (galactal, glucal, arabinal, and xylal derivatives) were investigated. Various photocatalysts and perfluoroalkyl iodides (RF-I) were employed as sources of RF radicals with LEDs as the irradiation source. Particularly noteworthy was the use of an Iridium photocatalyst, Ir[dF(CF3)ppy]2(dtbpy))PF6, which yielded two distinct product types when applied to glucal. On the one hand, the 2-RF-substituted glucal was formed, a trend observed even when utilizing organic dyes as photocatalysts. On the other hand, the unexpected addition product, namely the 1-RF-2-iodo-α-manno-configured C-glycosyl derivative, was also obtained, as a result of a highly regioselective addition reaction of the RF moiety into the anomeric carbon, followed by attachment of the iodine atom on C-2 in axial disposition. This result contrasted with other radical reactions carried out on 2-unsubstituted glycals, where the incipient radical adds to C-2, generating a stabilized 1-glycosyl radical. The photocatalyzed radical perfluoroalkylations of peracetyl glycals derived from galactose, arabinose, and xylose all afforded the 2-RF-substituted glycals in good yields as a result of the expected vinylic substitution reaction. Mechanistic studies revealed that the 1-RF-2-iodo-α-manno-configured C-glycosyl derivatives arise from a radical chain reaction, whereas the 2-RF-substituted glycals proceed from inefficient chain processes.

Drug Discovery Based on Fluorine-Containing Glycomimetics

Glycomimetics, which are synthetic molecules designed to mimic the structures and functions of natural carbohydrates, have been developed to overcome the limitations associated with natural carbohydrates. The fluorination of carbohydrates has emerged as a promising solution to dramatically enhance the metabolic stability, bioavailability, and protein-binding affinity of natural carbohydrates. In this review, the fluorination methods used to prepare the fluorinated carbohydrates, the effects of fluorination on the physical, chemical, and biological characteristics of natural sugars, and the biological activities of fluorinated sugars are presented.

Visible-Light Promoted Radical Fluoroalkylation of O- and N-Substituted Alkenes

Interaction of enol ethers enol acetates, enamides and enamines with fluorinated reagents may be considered as a reliable method for the synthesis of organofluorine compounds. While classic nucleophile/electrophile substitution or addition mechanisms cannot be realized for coupling of these components, their intrinsic reactivities are revealed with the aid of photoredox catalysis. A combination of these electron donating and accepting components provides a perfect balance needed for individual redox steps, which in some cases may proceed even without a photocatalyst. The same electronic factors also support the key C,C-bond forming event involving addition of fluorinated radical at the electron rich double bond.

Rose Bengal-photocatalyzed perfluorohexylation reactions of organic substrates in water. Applications to late-stage syntheses

The Rose Bengal-photocatalyzed perfluorohexylation of olefins, alkynes, and electron-rich aromatic compounds in water was achieved employing perfluorohexyl iodide as fluoroalkyl source and TMEDA as sacrificial donor under green LED irradiation. Alkenes and alkynes rendered products derived from the atom transfer radical addition (ATRA) pathway, and in the case of alkynes, exclusively as E-stereoisomers. These are the first examples of photocatalyzed ATRA reactions carried out excursively in water alone. The reactions of aromatic compounds under the current protocol in water present the advantage of employing a perfluoroalkyl iodide (C₆F₁₃-I) as source of perfluorohexyl radicals. Examples of photocatalytic late-stage incorporations of fluoroalkyl moieties into two commercial drugs of widespread use are reported.

Approaches to the Synthesis of Perfluoroalkyl-Modified Carbohydrates and Derivatives: Thiosugars, Iminosugars, and Tetrahydro(thio)pyrans

Fluoroalkyl-substituted carbohydrates play relevant roles in diverse areas such as supramolecular chemistry, glycoconjugation, liquid crystals, and surfactants, with direct applications as wetting, antifreeze, and coating agents. In light of these promising applications, new methodologies for the late-stage incorporation of fluoroalkyl R_F groups into carbohydrates and derivatives are herein presented as they are relevant to the synthetic carbohydrate community. Previously reviewed protocols for the installation of R_F groups onto carbohydrates and derivatives will be succinctly summarized in the light of the new achievements. Fluoroalkyl-substituted iminosugars, on the other hand, are also interesting glycomimetic derivatives with prominent roles as glycosidases and glycosyltransferases inhibitors, as has recently been demonstrated. Also, they positively contribute to the study of sugar-protein interactions and enzyme mechanisms. New advances in the syntheses of fluoroalkyl-substituted iminosugars will also be presented here.