Intramolecular 1,8-Hydrogen Atom Transfer. Stereoselectivity of the Hexopyranos-5′-yl Radical Reactions in Hexp-(1→4)-Hexp Disaccharide Systems

Synthesis of Indolyl Phenyl Diketones through Visible-Light-Promoted Ni-Catalyzed Intramolecular Cyclization/Oxidation Sequence of Ynones

The combination of visible light catalysis and Ni catalysis has enabled the synthesis of indolyl phenyl diketones through the cyclization/oxidation process of ynones. This reaction proceeded under mild and base-free conditions and showed a broad scope and feasibility for gram-scale synthesis. Several natural products and biologically interesting molecules could be readily postfunctionalized by this method.

Access to carbonyl compounds via the electroreduction of N-benzyloxyphthalimides: Mechanism confirmation and preparative applications

A method to access carbonyl compounds using reductive conditions was evaluated via electrochemical reduction of their corresponding N-benzyloxyphthalimide derivatives (NBOPIs). The mechanism of this originally reported electrochemical reaction was proposed based on DFT calculation and is experimentally confirmed herein, contrasting simulated and experimental cyclic voltammetry data. The reaction scope studied in a preparative scale and using redox sensitive functional groups showed good selectivity and tolerance toward oxidation under the reaction conditions with a moderate to good yield (50-71%). Nevertheless, some restrictions with reducible functional groups, like benzyl-brominated and nitro-aromatic derivatives, were observed. The present approach can be considered a self-sustainable electrochemical catalysis for the synthesis of aromatic carbonylic compounds passing through anion radical intermediates produced by a cathodic reaction.

1,5-Hydrogen Atom Transfer/Surzur-Tanner Rearrangement: A Radical Cascade Approach for the Synthesis of 1,6-Dioxaspiro[4.5]decane and 6,8-Dioxabicyclo[3.2.1]octane Scaffolds in Carbohydrate Systems

The 1,5-HAT–1,2-(ester)alkyl radical migration (Surzur–Tanner rearrangement) radical/polar sequence triggered by alkoxyl radicals has been studied on a series of C-glycosyl substrates with 3-C-(α,β-d,l-glycopyranosyl)1-propanol and C-(α-d,l-glycopyranosyl)methanol structures prepared from chiral pool d- and l-sugar. The use of acetoxy and diphenoxyphosphatoxy as leaving groups provides an efficient construction of 10-deoxy-1,6-dioxaspiro[4.5]decane and 4-deoxy-6,8-dioxabicyclo[3.2.1]octane frameworks. The alkoxyl radicals were generated by the reaction of the corresponding N-alkoxyphthalimides with group 14 hydrides [n-Bu₃SnH(D) and (TMS)₃SiH], and in comparative terms, the reaction was also initiated by visible light photocatalysis using the Hantzsch ester/fac-Ir(ppy)₃ procedure. Special attention was devoted to the influence of the relative stereochemistry of the centers involved in the radical sequence on the reaction outcome. The addition of BF₃•Et₂O as a catalyst to the radical sequence resulted in a significant increase in the yields of the desired bicyclic ketals.

Radical Dehydroxymethylative Fluorination of Carbohydrates and Divergent Transformations of the Resulting Reverse Glycosyl Fluorides

A mild and convenient method for the synthesis of reverse glycosyl fluorides (RGFs) has been developed that is based on the silver-promoted radical dehydroxymethylative fluorination of carbohydrates. A salient feature of the reaction is that furanoid and pyranoid carbohydrates furnish structurally diverse RGFs bearing a wide variety of functional groups in good to excellent yields. Intramolecular hydrogen atom transfer experiments revealed that the reaction involves an underexploited radical fluorination that proceeds via β-fragmentation of sugar-derived primary alkoxyl radicals. Structurally divergent RGFs were obtained by catalytic C-F bond activation, and our method thus offers a concise and efficient strategy for the synthesis of reverse glycosides by late-stage diversification of RGFs. The potential of this method is showcased by the preparation and diversification of sotagliflozin, leading to the discovery of a promising SGLT2 inhibitor candidate.

A new indole glycoside from Kitasatospora sp. MG372-hF19 carrying a 6-deoxy-α-L-talopyranose moiety

Small cell lung cancer (SCLC) is a severe malignancy with early and widespread metastasis, and novel therapeutic drugs are needed. To identify cytotoxic natural compounds against SCLC, we screened libraries of microbial fermentation broths using several lung cancer cell lines. We found that the actinomycete strain MG372-hF19 produces a compound that has not been isolated from natural sources but previously chemically synthesized, 6-chloro-1H-indole-3-carboxaldehyde (1), and an entirely new compound, named 6-deoxy-α-L-talopyranose 1-(6-chloro-1H-indole-3-carboxylate) (2), together with leptomycins. The molecular formulas of the compounds were established as C₉H₆ClNO and C₁₅H₁₆ClNO₆, respectively, via high-resolution electrospray ionization mass spectrometry, and their structures were determined using detailed NMR. Absolute configurational analysis of the sugar unit of compound 2 revealed that the compound incorporates the rare deoxyhexose 6-deoxy-α-L-talopyranose. Both compounds exhibited weak growth-inhibiting activities against human lung cancer cell lines.

Transition-Metal-Free Synthesis of C-Glycosylated Phenanthridines via K2S2O8-Mediated Oxidative Radical Decarboxylation of Uronic Acids

We have developed an efficient protocol for the synthesis of C-glycosylated phenanthridines. Tetrafuranos-4-yl and pentapyranos-5-yl radicals, generated from K₂S₂O₈-mediated oxidative decarboxylation of furan- and pyranuronic acids, undergo attack to 2-isocyanodiphenyls and ensuing homolytic aromatic substitution to provide diverse C-glycosylated phenanthridines in satisfactory yields without resort to transition metals. This reaction tolerates various functional groups, and enables ready synthesis of complex oligosaccharide-based phenanthridines. The C-glycosylated phenanthridine derived from β-cyclodextrin has been prepared, which might be potential in medicinal and biological chemistry due to its flexible conformation.