Generation and Wagner-Meerwein Rearrangement of α-Carbonyl Cations from Brønsted Acid-Catalyzed Alkyne Oxidations with Pyridine N-Oxides

α-Carbonyl cations are the umpolung forms of the synthetically fundamental α-carbonyl carbanions. They are highly reactive yet rarely studied and utilized species and their precursors were rather limited. Herein, we report the generation of α-carbonyl cations from HNTf2-catalyzed alkyne oxidations with pyridine N-oxides and the interception of them via Wagner-Meerwein rearrangement. Broad spectrum of alkynes including aryl alkyne, ynamide, and alkynyl sulfide could be utilized and the migrating priorities of different groups in the Wagner-Meerwein rearrangement step was elucidated. DFT calculations further supported the generation of α-carbonyl cations via the N-O bond cleavage. Another key feature of this methodology was the fragmentation of synthetically inert tert-butyl groups into readily transformable olefin functionalities. The synthetic potential was highlighted by the scale-up reaction and the downstream diversifications including the formal synthesis of nicotlactone B and galbacin.