Oxaazabicyclooctene Oxides, Another Type of Bridgehead Nitrones: Diastereoselective Assembly from Acetylene Gas, Ketones, and Hydroxyl Amine

Unique
bridgehead nitrones, 8-oxa-6-aza­bi­cyclo­[3.2.1]­oct-6-ene
6-oxides, have been assembled diastereoselectively via acetyldihydropyrans,
products of one-pot self-organization of two molecules of ketones
and two molecules of acetylene, which after oximation undergo acid-catalyzed
ring closure. The proposed mechanism includes the enol double-bond
protonation, followed by intramolecular cyclization involving the
interaction of the carbocation formed with a nitrogen atom. A broad
range of substrates tolerate this facile transformation, in which
the bridgehead nitrones were isolated in high yields.