[Telamond]

Hi, my exam is coming up, and I'm doing some old exams.
I've reached a question where I don't know how to solve.

The teacher wants us to present a reasonable mechanism for the reaction below and describe the cyclization in terms of Baldwins Rules.

Additional information: "The reaction might proceed to the same product via an alternative route which involves rearrangement."

This is what I've done. After radical initiation, the radical can now chose to do the ring closing mechanism on the double bond. Via Baldwin's rules it should rather want to do it via 7-exo-trig than an 8-endo-trig. So why is the 8 membered ring formed over the 7? Am I missing a step?

What's the rearrangement reaction that the teacher is hinting about?

[movies]

Well, first off Baldwin's guidelines are for polar ring-closures (there are some different rules for radical cyclization, although the associated names escape my memory right now).  Nonetheless, follow the 7-exo-trig pathway and think about what reactivity you might expect from that intermediate.

[Telamond]

Ah, yes, after a 3-exo-trig you get this intermediate, which probably decomposes to the target molecule?

Which rearrangement are they talking about? Can't really figure it out...

[movies]

Yep, you got it.

The radical adjacent to a cyclopropyl group is often referred to as a cyclopropylcarbinyl radical, and it rapidly isomerizes to a homoallylic radical to relieve ring strain.  You can think of it just as you figured out: first there is the predicted radical cyclization, and then this rearrangement via the cyclopropylcarbinyl radical.  It's related to the Dowd–Beckwith reaction, although it's probably more generally known as a homoallyl radical rearrangement.  It's quite useful for a lot of different things and the cyclopropylcarbinyl radical has been studied very extensively and used several times in total synthesis.

By the way, Beckwith is generally credited for working out the radical cyclization guidelines analogous to Baldwin's for polar ring-formation.

[Telamond]

Thank you for the information and help. I'll remember the cyclopropylcarbinyl radical thing.
I never knew radical chemistry could be this useful, never heard of it being applied for anything until this course.

I have my exams tomorrow. 7 hours of organic synthesis from Carey Sundberg. Hurray.

[movies]

Radical reactions are incredibly useful, but sometimes the reactivity can be difficult to tame.  There are many great examples in synthesis if you take the time to look for them!

Good luck on your exam – I would dread having to face another exam based on Carey & Sundberg!