Structural characterization of N-protonated amides: regioselective N-activation of medium-bridged twisted lactams.

The straightforward protonation of lactams by treatment with acid and the full structural characterization of three resulting N-protonated lactams are disclosed. This work provides experimental evidence that N-protonation of amide bonds results in a dramatic increase in nonplanarity about the C-N amide bond. The resulting compounds are discussed in structural, spectroscopic, and reactivity terms. The data suggest that distortion of these amide bonds by approximately 50 degrees is sufficient for their effective N-activation.