Transition-Metal-Free Alkylation and Acylation of Benzoxazinones with 1,4-Dihydropyridines

Visible-light induced photocatalyst-free synthesis of β-enaminones

An environmentally friendly and efficient synthesis of β-enaminones through photo-acylation of vinyl azides is described herein, utilizing 4-acyl-1,4-dihydropyridines (4-acyl-1,4-DHPs) as dual-function reagents. These reagents simultaneously serve as reductants and radical initiators. The method accommodates a broad range of vinyl azides, demonstrating excellent functional group tolerance and enabling potential application. Comprehensive mechanistic exploration was conducted employing TEMPO trapping, on-off experiments, and isotopic labelling studies. Key to our understanding was the integration of dual-parameter Hammett analysis and kinetic isotope effect (KIE) studies. The Hammett analysis, using quadratic linear regression with dual radical parameters, σmb and σjj·, highlighted the involvement of radical mechanisms and anionic intermediates. In conjunction with KIE studies, these analyses revealed that the rate-limiting step in the transformation is the single-electron reduction of the conjugated enamine radical to the corresponding anion. This finding offers valuable insights into the reaction dynamics and could guide the development of related synthetic strategies.

Cobalt-Catalyzed Regio- and Stereoselective C(sp2)-H Alkylation of Enamides with 4-Alkyl-1,4-dihydropyridines

We report herein an efficient cobalt-catalyzed regioselective and stereoselective alkenyl C(sp2)-H alkylation of structurally diverse enamides with 4-alkyl-1,4-dihydropyridines (DHPs) as the alkyl radical precursors. This DHPs-based approach features a broad substrate scope and excellent compatibility with diverse functionalities, giving rise to geometrically defined β-alkylated enamides with an exclusive E configuration. Preliminary mechanistic studies suggest a radical-involved pathway.

Asymmetric [3+2] Cycloannulation of Benzoxazinones for the Synthesis of Imidazo[5,1-c]oxazinones

The asymmetric catalytic [3+2] cycloannulation of benzoxazinones with isatin-derived ketimines for the efficient construction of imidazo[5,1-c]oxazinones has been developed, which realized the first asymmetric reaction of benzoxazinones with excellent stereoselectivities. A series of imidazo[5,1-c]oxazinones containing three stereogenic centers with one gem-diamine-type spiro tetrasubstituted center were obtained in this organocatalytic reaction with good yields and high functional group tolerance.

Manganese-Catalyzed Cycloalkene Ring Expansion Synthesis of Azaheterocycles

Herein, a Mn catalytic protocol has been developed for the cycloalkene ring expansion synthesis of azaheterocycles, allowing broad-substrate-scope access to pyridine and isoquinoline derivatives. The initial monoaddition of an azidyl radical to alkene and further as-generated C-radical addition to O2, followed by intramolecular rearrangement and aromatization, showcase a distinct Mn-catalyzed radical reactivity mode. The reaction features a short reaction time and a broad substrate scope, with applications demonstrated in complex structure elaboration and gram-scale vismodegib synthesis.

Acyl-1,4-Dihydropyridines: Universal Acylation Reagents for Organic Synthesis

Acyl-1,4-dihydropyridines have recently emerged as universal acylation reagents. These easy-to-make and bench-stable NADH biomimetics play the dual role of single-electron reductants and sources of acyl radicals. This review article discusses applications of acyl-1,4-dihydropyridines in organic synthesis since their introduction in 2019. Acyl-1,4-dihydropyridines, activated by photochemical, thermal or electrochemical methods, have been successfully applied as radical sources in multiple diverse organic transformations such as acyl radical addition to olefins, alkynes, imines and other acceptors, as well as in the late-stage functionalisation of natural products and APIs. Release of acyl radicals and an electron can be performed under mild conditions-in green solvents, under air and sunlight, and without the use of photocatalysts, photosensitizers or external oxidants-which makes them ideal reagents for organic chemists.

Metal-free direct C-6-H alkylation of purines and purine nucleosides enabled by oxidative homolysis of 4-alkyl-1,4-dihydropyridines at room temperature

Herein we report the application of 4-alkyl-1,4-dihydropyridines (DHPs), which are easily prepared from inexpensive aldehydes in one step, for the direct site-specific C-H alkylation of purines and purine nucleosides. Despite there being three active C(sp²)-H bonds (C-2-H, C-6-H, and C-8-H) in the structure, the reactions still show high regioselectivity at the purinyl C-6-H position. Importantly, the reactions successfully avoid the use of transition metal catalysts and additional acids. Meanwhile, the protocols are not sensitive to moisture and require only persulfate as an oxidant. Besides, this method displays broad functional group compatibility and is easy to scale up. Notably, pharmaceutical purines, e.g. the natural product 6-hydroxymethyl nebularine isolated from basidiomycetes, can be smoothly prepared using this protocol.

Metal-Free Alkylation of Quinoxalinones with Aryl Alkyl ketones

The first metal-free method for alkylation of quinoxalinones using cheap and stable aryl alkyl ketones as nucleophilic alkylation reagents is reported. This strategy greatly broadens the application channels of aryl...