Copper(II)-Catalyzed Synthesis of Pyrazinones from α-Azido-N-allylamides under an Oxygen Atmosphere

Abstract

A copper(II)-catalyzed reaction of α-azido-N-allylamide synthetic under an oxygen atmosphere resulted in the formation of 2-formyl pyrazinones. The present transformation was characterized by the following steps: 1) 1,3-dipolar cycloaddition of the azido part onto the intramolecular alkene to give bicyclic aziridine intermediates; 2) further copper(II)-catalyzed oxygenation-oxidation of the aziridines to give 2-formyl pyrazinones.

Key words

pyrazinone - organic azides - 1,3-dipolar cycloaddition - copper - oxygen

References and Notes

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7

Other bases, such as K₃PO₄ and NaOAc exhibited similar reactivity, while MgO was not a viable catalyst for this transformation.

8

The structures of 3b and 3i were secured by X-ray crystallographic analysis (see Supporting Information). The supplementary crystallographic data of these molecules are contained in CCDC 824717 and 824717, respectively. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/conts/retrieving.html.

10

General Procedure for the Cu(II)-Catalyzed Synthesis of Pyrazinones from α-Azido- N -allylamides
To a solution of N-allyl-2-azido-N-benzyl-2-phenyl-acetamide (1b, 156.1 mg, 0.510 mmol) in DMF (5.1 mL) were added Cu(OAc)₂ (18.9 mg, 0.104 mmol) and K₂CO₃ (70.4 mg, 0.509 mmol), and the mixture was stirred at 80 ˚C for 2 h under an O₂ atmosphere (1 atm). After cooling to r.t., the solid was filtered through a Celite pad. To the mixture,
1 M aq HCl was added, and the organic materials were extracted twice with Et₂O. The combined extracts were then washed with H₂O, brine, and dried over MgSO₄. Filtration and removal of the solvent under reduced pressure afforded a crude mixture, which was subjected to flash column chromatography (hexane-EtOAc = 90:10) to afford 2-formyl pyrazinone 3b (91.8 mg, 0.316 mmol) in 62% yield.

11

For the synthesis of α-azido-N-allylamides 1 as well as characterization of all new compounds, see the Supporting Information.