N-Mesyloxylactams can undergo ring contraction either by C-3 (usually observed) or C-5 migration. C-5 migration can occur when the C-3 migration product possesses ring strain, but it does not usually compete with C-3 migration. The greater preference for C-3 migration is due to the carbonyl oxygen atom, which greatly stabilizes the intermediate.
can undergo a thermal ring contraction after an in situ triflation. High yields of ring-contraction products are obtained with DBU when the migrating carbon is a methylene, while best results are obtained with Et3N for the migration of quaternary carbons. In some cases, the regiochemical outcome of the reaction can be controlled by changing the base. This novel thermalrearrangement complements a similar