Synthesis of substituted tetrahydron-1H-carbazol-1-one and analogs via PhI(OCOCF3)2-mediated oxidative C–C bond formation

Upgrading furanic platforms to α-enaminones: tunable continuous flow hydrogenation of bio-based cyclopentenones

Continuous flow hydrogenation of trans-4,5-diamino cyclopentenones to diamino cyclopentanones and base-promoted elimination originates α-enaminones. The strategy is applied to the short synthesis of reported ATP-sensitive potassium channel agonist.

Can Primary Arylamines Form Enamine? Evidence, α-Enaminone, and [3+3] Cycloaddition Reaction

The formation of enamine from primary arylamines was detected and confirmed by nuclear magnetic resonance spectroscopy. The presence of a radical quencher, e.g., (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl, was found to be essential for the detection of enamine formation. A direct synthesis of α-enaminones from primary arylamines and ketones was also developed. Mechanistic investigation of α-enaminone formation suggests that an amine radical cation generated through O₂ singlet energy transfer was involved in initiating α-enaminone formation. The reactivity and utility of α-enaminones were explored with a [3+3] cycloaddition reaction of enones affording dihydropyridines in good yields (58-85%). α-Enaminones displayed a set of reactivities that is different from that of enamines. The knowledge gained in this work advances our basic understanding of organic chemistry, providing insights and new opportunities in enamine catalysis.

Differentiation between enamines and tautomerizable imines in the oxidation reaction with TEMPO

Enamine and imine represent two of the most common reaction intermediates in syntheses, and the imine intermediates containing α-hydrogen often exhibit the similar reactivity to enamines due to their rapid tautomerization to enamine tautomers. Herein, we report that the minor structural difference between the enamine and the enamine tautomer derived from imine tautomerization results in the different chemo- and regioselectivity in the reaction of cyclohexanones, amines and TEMPO: the reaction of primary amines furnishes the formal oxygen 1,2-migration product, α-amino-enones, while the reaction of secondary amines under similar conditions generates exclusively arylamines via consecutive dehydrogenation on the cyclohexyl rings. The ¹⁸O-labeling experiment for α-amino-enone formation revealed that TEMPO served as oxygen transfer reagent. Experimental and computational studies of reaction mechanisms revealed that the difference in chemo- and regioselectivity could be ascribed to the flexible imine-enamine tautomerization of the imine intermediate containing an α-hydrogen.

Tautomerization of imines into enamines is the basis of their similar reactivity; however, minor structural changes may lead to different outcomes. Here, the authors show that the reaction of cyclohexanone and amines in presence of TEMPO affords either α-amino-enones or arylamines depending on the intermediate imine structure.

Multifaceted α-Enaminone: Adaptable Building Block for Synthesis of Heterocyclic Scaffolds Through Conceptually Distinct 1,2-, 1,3-, 1,4-, and C-O Bond Forming Annulations

The new reactivity of α,β-unsaturated enaminones driven by their "dual electronic attitude" is reported. We introduce unexplored, α-enaminone synthones and reveal the unusual functionalities of these building blocks. The feasibility of this new concept is demonstrated in the direct functionalization of enaminone precursors, such as alkylation; 1,2- 1,3-, or 1,4-addition; and C-O bond formation. The general and potential applicability is presented through the collective synthesis of several important classes of heterocycles via controlled cyclizations of easily accessible common precursors. The rapid composition of novel key α-enaminone synthones yields an assembly of oxazines, azaspirones, quinolinones, and quinolinols in a regio- and chemoselective fashion.