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

Photochemical Synthesis of S,N,O-Containing Polyheterocycles via an α-C(sp3)-H Functionalization/Radical Cyclization Cascade

A highly effective approach based on an organophotocatalytic α-C(sp3)-H functionalization/radical cyclization cascade has been developed. This method enables the synthesis of various tricyclic heterocycles containing S, O, and N atoms with excellent site selectivity and diastereoselectivity. Mechanistic investigations have confirmed that the reaction involves photoredox-triggered C(sp3)-H cleavage followed by a radical cyclization and aromatization process. These findings are expected to pave the way for developing cost-effective tandem radical reactions and synthesizing heterocyclic drugs.

Overcoming Back Electron Transfer in the Electron Donor-Acceptor Complex-Mediated Visible Light-Driven Generation of α-Aminoalkyl Radicals from Secondary Anilines

An additive-free, visible light-driven annulation between N-aryl amino acids and maleimide to form tetrahydroquinolines (THQs) is disclosed. Photochemical activation of an electron donor-acceptor (EDA) complex between amino acids and maleimides drives the reaction, and aerobic oxygen acts as the terminal oxidant in the net oxidative process. A range of N-aryl amino acids and maleimides have been investigated as substrates to furnish the target THQ in good to excellent yield. Mechanistic investigations, including titration and UV-vis studies, demonstrate the key role of the EDA complex as the photoactive species.

Nitrative bicyclization of 1,7-diynes for accessing skeletally diverse tricyclic pyrroles

A novel metal-free nitrative bicyclization of 1,7-diynes with ^tBuONO in the presence of H₂O is reported, producing three types of skeletally diverse tricyclic pyrroles, namely pyrrolo[3,4-c]quinolines, chromeno[3,4-c]pyrroles and benzo[e]isoindoles, with moderate to good yields by simply tuning the linkers of the 1,7-diynes. This domino protocol demonstrates remarkable compatibility regarding 1,7-diynes with different linkers, such as nitrogen and oxygen atoms and a hydroxymethyl group, and ^tBuONO plays dual roles as a nitro precursor as well as a nitrogen atom source.

Photochemical synthesis of an epigenetic focused tetrahydroquinoline library

Discovery of epigenetic chemical probes is an important area of research with potential to deliver drugs for a multitude of diseases. However, commercially available libraries frequently used in drug discovery campaigns contain molecules that are focused on a narrow range of chemical space primarily driven by ease of synthesis and previously targeted enzyme classes (e.g., kinases) resulting in low hit rates for epigenetic targets. Here we describe the design and synthesis of a compound collection that augments current screening collections by the inclusion of privileged isosteres for epigenetic targets.

Preparative semiconductor photoredox catalysis: An emerging theme in organic synthesis

Heterogeneous semiconductor photoredox catalysis (SCPC), particularly with TiO2, is evolving to provide radically new synthetic applications. In this review we describe how photoactivated SCPCs can either (i) interact with a precursor that donates an electron to the semiconductor thus generating a radical cation; or (ii) interact with an acceptor precursor that picks up an electron with production of a radical anion. The radical cations of appropriate donors convert to neutral radicals usually by loss of a proton. The most efficient donors for synthetic purposes contain adjacent functional groups such that the neutral radicals are resonance stabilized. Thus, ET from allylic alkenes and enol ethers generated allyl type radicals that reacted with 1,2-diazine or imine co-reactants to yield functionalized hydrazones or benzylanilines. SCPC with tertiary amines enabled electron-deficient alkenes to be alkylated and furoquinolinones to be accessed. Primary amines on their own led to self-reactions involving C-N coupling and, with terminal diamines, cyclic amines were produced. Carboxylic acids were particularly fruitful affording C-centered radicals that alkylated alkenes and took part in tandem addition cyclizations producing chromenopyrroles; decarboxylative homo-dimerizations were also observed. Acceptors initially yielding radical anions included nitroaromatics and aromatic iodides. The latter led to hydrodehalogenations and cyclizations with suitable precursors. Reductive SCPC also enabled electron-deficient alkenes and aromatic aldehydes to be hydrogenated without the need for hydrogen gas.

Photocatalysis with TiO2 Applied to Organic Synthesis

Titanium dioxide is a versatile heterogeneous catalyst. Absorption of light by a TiO2 particle leads to the formation of an electron–hole pair. Electron transfer from or to the particle induces redox reactions. Although mainly applied in the context of environmental chemistry, these processes are also used to selectively transform organic compounds. Oxidations and reductions have been carried out. Applications to the synthesis of heterocycles have been reported. Many C–C bond formation reactions have been performed. Owing to adsorption of the substrates or by different surface modifications, visible light can be used to excite the catalytic system, which generates mild reaction conditions.