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When I was an undergrad, roughly around 2005, I took a grad level class that covered various topics in advanced organic synthesis. I don't remember much clearly from the class, but one thing that I can't forget was this one reaction where a single chain compound was, in one step, cyclized to a multi-ring system (maybe 4 rings) via radical cyclization. This cyclization was precipitated by the release of N_2 from an azido group.

It was similar to the cyclization of squalene into lanosterol, but it was not enzymatically catalyzed. I can remember being absolutely astounded that nature had figured out how to do this, and then to later learn that some crazy bastard had done something similar without protein scaffolding was extremely humbling.

I unfortunately can't find my notes from that class, and my google searching hasn't helped me, either. I will try to reach out the original professor, but I think that's a long shot. And it's of course possible, in all honestly, that I could be misremembering it. But, I still have a fuzzy photo in my memory of the lower left hand side of a page of a journal article where this reaction was drawn out. If only I had enough detail to draw it!

So, I guess I'll just ask a more general question to the community. What is the largest single step cyclization that you are aware of? By "largest", I mean "number of atoms that end up as part of a ring that were not originally part of the ring". Very large diameter, single bond/ring radical formations are entropically unlikely, so that should be a fair metric.

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