Enantioselective Synthesis of 3,4-Dihydropyran-2-ones via Phase-Transfer-Catalyzed Addition–Cyclization of Acetylacetone to Cinnamic Thioesters

Recent Advances in the Synthesis of 3,4-Dihydropyran-2-Ones Organocatalyzed by N-Heterocyclic Carbenes

In recent years, N-heterocyclic carbenes (NHC) have gained recognition as versatile molecules capable of acting as organocatalysts in various reactions, particularly through the activation of aldehydes via Breslow-type adducts. This organocatalytic activation has enabled the production of numerous 3,4-dihydropyran-2-ones and related derivatives. In this review, we provide an overview of the production of 3,4-dihydropyran-2-ones and derivatives via organocatalytic processes involving NHCs over the past eight years. These processes involve the use of a diverse range of substrates, catalysts, and reaction conditions, which can be classified into [4+2]-and [3+3]-type cycloadditions, primarily aimed at synthesizing this skeleton due to its biological activity and multiple stereocenters. These processes are scaled up to the gram scale, and the resulting products are often directed towards epimerization and functionalization to produce more complex molecules with potential applications in the biological field. Finally, we provide a perspective and the future directions of this topic in organic synthesis.

Synthesis of 3,4-Dihydropyridin-2-ones via Domino Reaction under Phase Transfer Catalysis Conditions

3,4-dihydropyridin-2-ones are of considerable importance due to the large number of these core structures exhibiting a diverse array of biological and pharmacological activities. The Michael-type addition of 1,3-dithiane-2-carbothioates to α,β-unsaturated N-tosyl imines, followed by intramolecular annulation driven by a sulfur leaving group, provides a practical reaction cascade for the synthesis of a variety of substituted 3,4-dihydropyridin-2-ones. In this work, the reaction was carried out under solid–liquid phase transfer catalysis (SL-PTC) conditions at room temperature, in short reaction times in the presence of cheap Bu4N+HSO4– and solid KOH. The new PTC method exhibited adequate functional group tolerance, proving to be a green and reliable method and easy to scale up to furnish rapid access to 3,4-dihydropyridin-2-ones after desulfurization from simple, readily available starting materials.

Study of Ground State Interactions of Enantiopure Chiral Quaternary Ammonium Salts and Amides, Nitroalkanes, Nitroalkenes, Esters, Heterocycles, Ketones and Fluoroamides

Chiral phase-transfer catalysis provides high level of enantiocontrol, however no experimental data showed the interaction of catalysts and substrates. 1 H NMR titration was carried out on Cinchona and Maruoka ammonium bromides vs. nitro, carbonyl, heterocycles, and N-F containing compounds. It was found that neutral organic species and quaternary ammonium salts interacted via an ensemble of catalyst + N-C-H and (sp2 )C-H, specific for each substrate studied. The correspondent BArF salts interacted with carbonyls via a diverse set of + N-C-H and (sp2 )C-H compared to bromides. This data suggests that BArF ammonium salts may display a different enantioselectivity profile. Although not providing quantitative data for the affinity constants, the data reported proofs that chiral ammonium salts coordinate with substrates, prior to transition state, through specific C-H positions in their structures, providing a new rational to rationalize the origin of enantioselectivity in their catalyses.