The Construction of Polycyclic Pyridones via Ring-Opening Transformations of 3-hydroxy-3,4-dihydropyrido[2,1-c][1,4]oxazine-1,8-diones

This work describes the synthesis of 3-hydroxy-3,4-dihydropyrido[2,1-c][1,4]oxazine-1,8-diones, their tautomerism, and reactivity towards binucleophiles. These molecules are novel and convenient building-blocks for the direct construction of biologically important polycyclic pyridones via an oxazinone ring-opening transformation promoted with ammonium acetate or acetic acid. In the case of o-phenylenediamine, partial aromatization of the obtained heterocycles proceeded to form polycyclic benzimidazole-fused pyridones (33-91%).

Transition-Metal-, Additive-, and Solvent-Free [3 + 3] Annulation of RCF2-Imidoyl Sulfoxonium Ylides with Cyclopropenones to Give Multifunctionalized CF3-Pyridones

An efficient and practical strategy was developed to synthesize 1,3,4-triaryl-6-trifluoromethylpyridones from CF₃-imidoyl sulfoxonium ylides and cyclopropenones in good to excellent yields. This stepwise [3 + 3] annulation reaction was carried out under transition-metal-, additive-, and solvent-free conditions, generating 1 equiv of dimethyl sulfoxide as byproduct and tolerating a series of functional groups.

Controllable Synthesis of Two Isomers 4H-Chromene and 2,8-Dioxabicyclo[3.3.1]nonane Derivatives under Catalyst-Free Conditions

A one-pot method for the selective synthesis of two isomers 4H-chromene and 2,8-dioxabicyclo[3.3.1]nonane derivatives was developed without a catalyst and using EtOH/H₂O (4:1, v/v) as the solvent. The reaction was conducted under mild conditions, with forming multiple chemical bonds in one pot and high atom economy, and only a stoichiometric amount of H₂O is produced as the byproduct. Its selectivity was controlled by thermodynamics and kinetics, and the reasons for the transformation of the two structures are also discussed.

Bifunctional Urea/Hg(OAc)2-Mediated Synthesis of 4-Aryl-6-oxycarbonyl-2-pyrones and 2-Pyridones from Dithiomalonate and β,γ-Unsaturated α-Keto Esters

Disubstituted 2-pyrones and 2-pyridones were obtained by bifunctional urea-catalyzed Michael addition/lactonization or lactamization followed by a Hg(OAc)₂- or Hg(OAc)₂/DBU-mediated hydrolysis/decarboxylation/dehydrogenation process. This one-pot two-stage protocol enabled the rapid synthesis of 4,6-disubstituted 2-pyrones and 2-pyridones from dithiomalonate and β,γ-unsaturated α-keto esters in practical yields under mild reaction conditions. Additionally, the obtained 2-pyridones were facilely transformed to 2,4,6-trisubstituted pyridines in excellent yields.

Construction of Polyfunctionalized 2,4-Dioxa-8-azaspiro[5.5]undec-9-enes and 2,4,8-Triazaspiro[5.5]undec-9-enes via a Domino [2+2+2] Cycloaddition Reaction

The three-component reaction of α,β-unsaturated N-arylaldimines, dialkyl but-2-ynedioates, and 2-arylidene Meldrum acids in DCM at room temperature gave mixtures of cis/trans-11-aryl-7-styryl-2,4-dioxa-8-azaspiro[5.5]undec-9-enes in satisfactory yields. The similar three-component reaction with 2-arylidene-N,N'-dimethylbarbituric acids afforded cis-11-phenyl-7-styryl-2,4,8-triazaspiro[5.5]undec-9-enes as major products. On the other hand, the three-component reaction of N-arylaldimines, dialkyl but-2-ynedioates, and 2-arylidene Meldrum acids or 2-arylidene-N,N'-dimethylbarbituric acids afforded cis/trans-isomeric spirocompounds in satisfactory yields with high diastereoselectivity. This domino [2+2+2] cycloaddition reaction proceeded with sequential nucleophilic addition of N-arylaldimine to an electron-deficient alkyne, Michael addition, and annulation process. The stereochemistry of all cis/trans isomeric spirocompounds was clearly elucidated by the determination of 33 single-crystal structures. The diastereoselectivity of the three-component reaction was correlated by DFT calculations.

Selective Construction of Diverse Polycyclic Spirooxindoles via a Three-Component Reaction of Cyclic Mercapto-Substituted β-Enamino Esters, Isatins, and Cyclic 1,3-Diketones

The three-component reaction of alkyl 2-(benzo[b][1,4]thiazin-3-ylidene)acetates, isatins, and 1,3-indanedione (1,3-cyclopentanedione) in ethanol in the presence of acetic acid conveniently afforded spiro[indeno[1,2-b]phenothiazine-6,3'-indolines] or spiro[cyclopenta[b]phenothiazine-4,3'-indolines] in good yields and with high diastereoselectivity. More interestingly, a similar three-component reaction with 4-hydroxychromen-2-one resulted in the unexpected polycyclic spiro[benzo[b]chromeno[3',4':5,6]pyrano[2,3-e][1,4]thiazine-7,3'-indolines] in satisfactory yields. A plausible reaction mechanism was rationally proposed for formation of different kinds of the spiro compounds, and the stereochemistries of the various spiro compounds were clearly elucidated.