Microwave-Enhanced Reactivity of Non-Activated Dienophiles Towards Pyrazine o-Quinodimethanes

Abstract

Microwave irradiation in solvent-free conditions induces cycloaddition reactions of pyrazine o-quinodimethane intermediates with electron-rich dienophiles within 10-15 min to afford quinoxaline derivatives in acceptable yields.

alkynes - heterocycles - Diels-Alder reactions - microwaves - o-quinodimethanes

References

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Experimental procedure: A mixture of 2,3-bis(dibromomethyl)pyrazine [5] (1) (150 mg, 0.35 mmol), NaI (265 mg, 1.77 mmol), DMF (0.1 mL) and the corresponding dienophile (1.25 mmol) was placed in an open vessel and irradiated at 60 W in a focused microwave reactor (CixMO model RF46280) [8] for 10-15 min (final temperature 90 ºC). The crude reaction product was purified by flash column chromatography (silica gel, Merck type 60 230-400 mesh) using hexane/ethyl acetate as the eluent to obtain the adduct. For example:
Data for 13: yellow oil; IR (film, cm^-1) 2950, 2850, 1580, 1490, 1460; ¹H NMR (CDCl₃, 300 MHz) δ (ppm) 1.90 (m, 4 H, H-7 and -8), 3.06 (m, 4 H, H-6 and -9), 7.79 (s, 2 H, H-5 and -10), 8.73 (s, 2 H, H-2 and -3); ¹³C NMR (CDCl₃, 75 MHz) δ (ppm) 22.7 (C-7 and -8), 29.8 (C-6 and -9), 127.8 (C-5 and -10), 141.3 and 141.5 (C-4a, -5a, -9a and -10a), 144.1 (C-2 and -3); EM (EI) m/z 184 (M⁺).

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Focused microwave reactor model RF46280 has total control of the power and temperature by an infrared sensor and has been developed by CixMO (www.cixmo.com).