Rh-Catalyzed Asymmetric Hydrogenation of 2-Substituted 4H-(Thio)chromenes for Synthesis of Chiral (Thio)chromanes

Rh-catalyzed asymmetric hydrogenation of 2-substituted 4H-thiochromenes and 4H-chromenes was successfully developed. This method provided highly efficient access to a series of chiral 2-substituted thiochromanes and chromanes in high yields with excellent enantioselectivities (up to 99% yield, 86-99% ee). The obtained chiral 2-substituted thiochromane products were also successfully transformed to corresponding chiral α-substituted sulfoxides and sulfones with excellent enantioselectivities. Furthermore, this highly enantioselective hydrogenation process could be successfully applied to the concise and practical synthesis of the chiral pharmaceutical BW683C.

Radical Reactions in Organic Synthesis: Exploring in-, on-, and with-Water Methods

Radical reactions in water or aqueous media are important for organic synthesis, realizing high-yielding processes under non-toxic and environmentally friendly conditions. This overview includes (i) a general introduction to organic chemistry in water and aqueous media, (ii) synthetic approaches in, on, and with water as well as in heterogeneous phases, (iii) reactions of carbon-centered radicals with water (or deuterium oxide) activated through coordination with various Lewis acids, (iv) photocatalysis in water and aqueous media, and (v) synthetic applications bioinspired by naturally occurring processes. A wide range of chemical processes and synthetic strategies under different experimental conditions have been reviewed that lead to important functional group translocation and transformation reactions, leading to the preparation of complex molecules. These results reveal how water as a solvent/medium/reagent in radical chemistry has matured over the last two decades, with further discoveries anticipated in the near future.

Thiyl Radicals: Versatile Reactive Intermediates for Cyclization of Unsaturated Substrates

Sulfur centered radicals are widely employed in chemical synthesis, in particular for alkene and alkyne hydrothiolation towards thioether bioconjugates. The steadfast radical chain process that enables efficient hydrothiolation has been explored in the context of cascade reactions to furnish complex molecular architectures. The use of thiyl radicals offers a much cheaper and less toxic alternative to the archetypal organotin-based radical methods. This review outlines the development of thiyl radicals as reactive intermediates for initiating carbocyclization cascades. Key developments in cascade cyclization methodology are presented and applications for natural product synthesis are discussed. The review provides a chronological account of the field, beginning in the early seventies up to very recent examples; a span of almost 50 years.

BCl3-Mediated C-N, C-S, and C-O Bond Formation of Imidazo[1,2-a]pyridine Benzylic Ethers

An efficient BCl3-mediated reaction of imidazo[1,2-a]pyridines has been developed for the C-N, C-S, and C-O bond formation. The salient features of this method correspond to the substitution of different nucleophiles via in situ unconventional debenzylation. The developed process is applicable for the synthesis of a wide variety of ((3-amino/thio/alkoxy)-methyl)-imidazo[1,2-a]pyridines.

Radical "On Water" Addition to the C═N Bond of Hydrazones: A Synthesis of Isoindolinone Derivatives

A radical "on water" addition to the C═N bond of hydrazones has been described. Hydrazone, diphenylsilane, alkyl iodide, and triethylborane afforded the corresponding addition products "on water" in good yields. A significant solvent effect was observed from the water. The developed protocol can be applied to the synthesis of 3-substituted isoindolinone derivatives. Moreover, the process offers environmentally benign tin-free radical reaction conditions.

Highly mono-selective ortho-methylthiolation of benzamides via cobalt-catalyzed sp2 C–H activation

A highly mono-selective ortho-methylthiolation of benzamides was achieved via Co-catalyzed coupling of benzamides with DMSO.

Aqueous-Medium Carbon-Carbon Bond-Forming Radical Reactions Catalyzed by Excited Rhodamine B as a Metal-Free Organic Dye under Visible Light Irradiation

The utility of rhodamine B as a water-soluble organic photocatalyst was studied in the cascade radical addition-cyclization-trapping reactions under visible light irradiation. In the presence of (i-Pr)2NEt, the electron transfer from the excited rhodamine B to perfluoroalkyl iodides proceeded smoothly to promote the carbon-carbon bond-forming radical reactions in aqueous media. When i-C3F7I was employed as a radical precursor, the aqueous-medium radical reactions proceeded even in the absence of (i-Pr)2NEt. In these reactions, the direct electron transfer from the excited singlet state of rhodamine B would take place. Furthermore, the cleavage of the C-I bond in less reactive i-PrI could be achieved by the reductive electron transfer from the excited rhodamine B, which was confirmed by the fluorescence quenching of rhodamine B with the addition of i-PrI.

Stereoselective synthesis of organosulfur compounds incorporating N-aromatic heterocyclic motifs and quaternary carbon centers via a sulfa-Michael triggered tandem reaction

A novel sulfa-Michael addition (SMA)-triggered tandem reaction was developed by combining a SMA reaction with a simultaneous rearomatization process utilizing a less reactive carbonyl group as an intramolecular electrophile partner, which provided a unique synthetic route to access various organosulfur compounds incorporating an N-aromatic heterocyclic motif and quaternary carbon centers.

Asymmetric Synthesis of Dihydronaphthoquinones Containing Adjacent Stereocenters via a Sulfa-Michael Addition Triggered Ring-Expansion Approach

A novel asymmetric synthetic approach for the construction of enantioenriched functionalized dihydroquinones incorporating adjacent quaternary and tertiary stereocenters has been reported, in which enantioenriched 3-allylic phthalides engaged in an unprecedented sulfa-Michael addition-triggered stereoselective ring-expansion reaction, and furnished the desired sulfur-incoporated dihydronaphthoquinones with high stereoselectivity.

Ag@TiO2 nanocomposite; synthesis, characterization and its application as a novel and recyclable catalyst for the one-pot synthesis of benzoxazole derivatives in aqueous media

A Ag@TiO₂ nanocomposite/water as a novel catalytic system is used for the synthesis of benzoxazole derivatives. A shorter reaction time along with high product yield, catalyst stability and recyclability are the merits of this novel protocol.

Catalyst-free photoredox addition-cyclisations: exploitation of natural synergy between aryl acetic acids and maleimide

Suitably functionalised carboxylic acids undergo a previously unknown photoredox reaction when irradiated with UVA in the presence of maleimide. Maleimide was found to synergistically act as a radical generating photoxidant and as a radical acceptor, negating the need for an extrinsic photoredox catalyst. Modest to excellent yields of the product chromenopyrroledione, thiochromenopyrroledione and pyrroloquinolinedione derivatives were obtained in thirteen preparative photolyses. In situ NMR spectroscopy was used to study each reaction. Reactant decay and product build-up were monitored, enabling reaction profiles to be plotted. A plausible mechanism, whereby photo-excited maleimide acts as an oxidant to generate a radical ion pair, has been postulated and is supported by UV/Vis. spectroscopy and DFT computations. The radical-cation reactive intermediates were also characterised in solution by EPR spectroscopy.