There are many atom transfer reactions that generate three-membered heterocycles from multiple bonds. Yet, if you really think about it, there are not too many diverse mechanisms that describe these reactions. Or, more precisely, a few well-established mechanisms describe the vast majority of known processes. For instance, oxene and nitrene-like species (typically associated with a metal) is what we expect to see in epoxidation and aziridination reactions. I keep my eyes peeled for rare cases where I see some razzle-dazzle (to use the term Jon Soderquist uses to describe some interesting bond movements). You look at such reactions while sifting through TOCs and you go “wait a second, what happened here?”. Take, for instance, a really neat example described by a group from Merck several years ago. In this sequence, a cycloaddition initially generates a 5-membered ring adduct that later rearranges into an aziridine-containing structure. The most interesting features of this reaction, in my view, are the net transfer of oxygen to the alkyne carbon (generating the ketone you see) and the arrows themselves. If you think about it, it is not entirely obvious how something like this can be described using frontier molecular orbitals. I need to think about it (and maybe ask Dean Tantillo of UC Davis)…

http://pubs.acs.org/doi/abs/10.1021/ol1007923