Synthesis of Substituted Pyridines via Formal (3+3) Cycloaddition of Enamines with Unsaturated Aldehydes and Ketones

Photooxidative Reaction of β-Oxoamides with Amines for the Synthesis of Pyrrolin-4-ones under External Photocatalyst-Free Conditions

The incorporation of oxygen atoms from air under aerobic conditions plays an important role in organic synthesis. Herein, Brønsted acids are found to be a two-in-one strategic catalyst to transform enamines from β-oxoamides and amines to pyrrolin-4-ones without an external photocatalyst under visible-light conditions. The Brønsted acid can inhibit the C-C bond fragmentation of the [2 + 2] adduct from enamine and 1O2, but most importantly, it can form photosensitizers with enamine and pyrrolin-4-one product by acidochromism to promote the 1O2 generation.

Thioarylation of 6-Amino-2,3,6-trideoxy-d-manno-oct-2-ulosonic Acid (IminoKdo): Access to 3,6-Disubstituted Picolinates and Mechanistic Insights

In this work, we present a metal-free coupling protocol for the regio- and stereoselective C3-thioarylation of 6-amino-2,3,6-trideoxy-d-manno-oct-2-ulosonic acid (iminoKdo). The developed procedure enables the coupling of electron-rich, electron-deficient, and hindered arylthiols, providing a series of C3-modified iminoKdo derivatives in moderate to good yields. Elucidation of active species through controlled experimental studies and time-lapse 31 P NMR analysis provides insights into the reaction mechanism. We demonstrate that, following a tandem Staudinger/aza-Wittig reaction of an azido-containing keto ester, an inseparable equimolar mixture of imine/enamine is formed. The enamine then undergoes a Stork-like nucleophilic attack with the in situ-formed disulfide reagent, resulting in the formation of the coupling products. Additionally, we describe a rarely reported acid-promoted aromatization of the C3-thioarylated iminoKdo skeleton into 3,6-disubstituted picolinates, which are reminiscent of dichotomines.

Regioselective access to di- and trisubstituted pyridines via a metal-oxidant-solvent-free domino reaction involving 3-chloropropiophenones

A remarkable metal-oxidant-solvent- and base-free domino route for regioselective access to a wide range of 2,4-di- and 2,3,4/6-trisubstituted pyridines including carbo- and heterocyclic fused pyridines is reported. This [3C + 2C + 1N] cyclization reaction occurs between 3-chloropropiophenones (3C units), enolizable acyclic/cyclic ketones (2C sources) and NH₄OAc as a robust N source under neat conditions under an open atmosphere, producing new C=C and C=N-C bonds in highly chemo- and regioselective manners. Interestingly, this eco-friendly method has many positive features: excellent functional group tolerance, broad substrate scope, good to excellent regioselectivities, promising yields, no-unwanted products, neutral reaction conditions and appropriateness for large-scale synthesis. Mechanism studies reveal that the in situ generated β-amino ketone from 3-chloropropiophenone and an ammonium salt undergoes C=N bond formation with a ketone followed by an intramolecular cyclization process (C=C bond), which are the decisive steps for pyridine synthesis.