Rule of five (radical cyclization selectivity)

A removable amide substituent decides whether light gives you a bicyclo[2.1.1] or a bicyclo[2.2.0]

A visible-light [2+2] on acyclic aza-1,5-dienes beats the rule of five on electronics alone. The switch has to buy back roughly 2.3 kcal/mol, and then survive a strain penalty.