Discovery of competing anaerobic and aerobic pathways in umpolung amide synthesis allows for site-selective amide 18O-labeling.

The mechanism of umpolung amide synthesis was probed by interrogating potential sources for the oxygen of the product amide carbonyl that emanates from the α-bromo nitroalkane substrate. Using a series of (18)O-labeled substrates and reagents, evidence is gathered to advance two pathways from the putative tetrahedral intermediate. Under anaerobic conditions, a nitro-nitrite isomerization delivers the amide oxygen from nitro oxygen. The same homolytic nitro-carbon fragmentation can be diverted by capture of the carbon radical intermediate with oxygen gas (O(2)) to deliver the amide oxygen from O(2). This understanding was used to develop a straightforward protocol for the preparation of (18)O-labeled amides in peptides by simply performing the umpolung amide synthesis reaction under an atmosphere of 18O2.