A AgCO/TFA-catalyzed intramolecular annulation approach to imidazo[1,2-][1,3]oxazin-5-one derivatives.

A series of 2,7-disubstituted 3-methylimidazo[1,2-][1,3]oxazin-5-ones were synthesized in good yields AgCO/TFA-mediated intramolecular annulation of -Boc-2-alkynyl-4-bromo(alkynyl)-5-methylimidazoles. This methodology was carried out in the presence of a catalytic amount of silver carbonate and trifluoroacetic acid in dichloroethane at 60 °C. In all experiments, only the six-membered ring product was obtained since the possible five-membered compound was not observed, proving the high regioselectivity of this approach. A complementary computational study was performed in order to rationalize the mechanism of 6- heterocycle formation. In addition, 2-bromo-3-methyl-7-phenylimidazo[1,2-][1,3]oxazin-5-one was used as a building block to synthesize a small library of new 2-substituted imidazo[1,2-][1,3]oxazin-5-one derivatives through the Suzuki, Sonogashira and Heck cross coupling reactions.