K-Lewis acidic metal-catalyzed skeletal rearrangement reactions of o-propargylic oximes

In recent years, we have disclosed that O- propargylic oximes, which have a propargyl group on the oxime oxygen atom, undergo skeletal rearrangement reactions by the action of K-Lewis acidic metal catalysts, leading to a wide variety of nitrogenous heterocyclic compounds in an efficient manner with high functional group compatibility. For example, the copper-catalyzed reaction of O-propargylic oximes proceeded via 2,3-rearrangement involving C-O bond cleavage. The resulting V/-allenylnitrone intermediate subsequently underwent intramolecular reactions, such as 4K - electrocyclization, 6K -electrocyclization, and nitrone-amide rearrangement, according to the functional group on the oxime carbon atom. The key intermediate also underwent cascade reactions via intermolecular transformations with various external reagents, such as electron-deficient olefins, tosylcyanates, and azodicarboxylates, affording, oxazepines, dihydropyrimidines, and triazines, respectively. The rhodium-catalyzed reactions of O-propargylic oximes having a cyclopropyl or cyclobutyl group at the oxime moiety afforded the corresponding medium ring-sized nitrogenous heteromonocycles in good yields via 2,3-rearrangement followed by oxidative cyclization of the V-allenylnitrone with rhodium (I) catalyst. In contrast, gold-catalyzed rearrangement reaction of the formaldoximes proceeded via C=N bond cleavage, affording 4-methylenated 2 - isoxazolines in an efficient manner. The mechanistic studies suggest that the reaction proceeds via cyclization — intermolecular methylene transfer sequence. Moreover, gold-catalyzed reactions of O-propargylic oximes, which have an electron-withdrawing p-nitrophenyl group at the oxime moiety, proceeded through N-O bond cleavage, producing 2H- 1,3-oxazines. The use of Bronsted base as cocatalyst was effective for the rearrangement reaction via N-O bond cleavage.