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DOI: 10.1055/s-2002-35593
Microwave-Enhanced Reactivity of Non-Activated Dienophiles Towards Pyrazine o-Quinodimethanes
Publikationsverlauf
Publikationsdatum:
20. November 2002 (online)
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.
Key words
alkynes - heterocycles - Diels-Alder reactions - microwaves - o-quinodimethanes
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References
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+).
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).