Ring economy

One area of heterocycle chemistry that interests me the most is related to ring expansion and ring contraction reactions. What can be better than taking an existing ring and converting it into a new one, with different connectivity, yet mostly containing the original atoms? I think we should call this strategy “ring economy”… Just joking (I am poking at atom economy, and the like). Ring expansion processes relate to my lab’s interests in cyclic peptide expansion (which I hope to disclose soon). Speaking of small unsaturated heterocycles, I always pay attention to any reaction that allows one to transform existing molecules into larger rings. Smaller are fine too (contractions). Invariably, the mechanisms are complex and demanding because one needs to get used to the idea of disrupting an aromatic ring, followed by breaking the core, making space for new atoms, and restoring the order. A telling example is, for instance, the good old Ciamcian-Dennstedt rearrangement shown below:

The dichlorocarbene is generated from chloroform. It then acts on pyrrole and, voila, you have a one-step route to the pyridine ring. These kinds of reactions are powerful, yet there is not too many of them. While attending the Heterocycle conference here in Russia, I learned about a new addition to this class of reactions. It was described in Professor Aksenov’s presentation. Incidentally, he is the organizer of this conference and I have to admit that I have not met anyone else who has so many jokes up his sleeve. Aksenov’s lab has been active in the utilization of polyphosphoric acid, which deserves another post one day. I still can’s wrap my head around how versatile this reagent is. Here is a representative ring-expansion example and a link to the corresponding ChemComm article: