 <unk> have been found to undergo <unk> reactions via a radical mechanism when <unk> with <unk> light or in the presence of a single electron transfer reagent such as <unk> <unk> <unk> undergo ring expansions to the corresponding <unk> . <unk> , the migrating <unk> is determined by a <unk> effect where the group trans to the lone pair on the nitrogen will always be the predominant migration product . In light of this effect , it is possible to take advantage of the <unk> nitrogen due to high <unk> barrier to direct the <unk> . This phenomenon is demonstrated by observed <unk> in the <unk> below . In the <unk> on the left the <unk> unfavorable product is observed exclusively , while in the reaction on the right the product derived from the less stable radical intermediate is favored . 
