Stannyl radical-mediated synthesis of 6H-1,3-oxazin-6-ones from 2-acyloxyazirines or whether free radicals can open the azirine ring?

Abstract

A fundamentally new radical cascade reaction of 2-acyloxyazirines provides an effective one-step method for the preparation of 5-hydroxy-6H-1,3-oxazin-6-ones from methyl 2-acyloxy-2H-azirine-2-carboxylates using a Bu₃SnH/ACHN system as a source of stannyl radicals. The method provides high product yields from a variety of 2-aroyloxy-, 2-hetaroyloxy-, and 2-tert-alkylcarbonyloxy-substituted methyl azirine-2-carboxylates. The decreasing volume of the acyloxy substituent causes a side reaction leading to the formation of methyl oxazole-2-carboxylates. According to the DFT calculations, the switch between these reaction pathways occurs when changing the C1-substituent in the 2-azabuta-1,3-dien-4-oxyl intermediate, the volume of which controls further stabilization routes: radical–radical recombination or Z,E isomerization followed by recombination. The hydroxyl group in 5-hydroxy-6H-1,3-oxazin-6-ones can be easily replaced, through a triflation step, with aryl, pyridyl, alkenyl, alkynyl, and cyano groups by palladium-catalyzed cross-coupling reactions. 5-Hydroxy-6H-1,3-oxazin-6-ones were converted to pyridine-2,3(1H,4H)-diones in good yields via copper-catalyzed transannulation with 3-tolyl-2H-azirine. The key step of this new domino reaction is the copper-catalyzed pyrrolooxazine–oxazolopyridine isomerization, which, according to the DFT calculations, proceeds via a concerted mechanism.