I’ll see your Diels-Alder and raise you one 1,3-dipolar cycloaddition
On Monday, See Arr Oh over at Just Like Cooking posted on this non-obvious Diels-Alder reaction recently published by the Vanderwal group, suggesting that it'd make good problem session fodder. And I agree:
Fortunately, this tied in perfectly with my plans to run our group problem session next week on a pericyclic theme and so it was duly incorporated. If you're interested in what else featured, I also included a question on the origin of the metastability of Dewar Benzene (which I've blogged about before).
After a few easier questions I finished up by asking people to suggest a mechanism for this interesting sequence published a few years back.
Have a think and then read on for a possible answer!
Okay, here's one plausible mechanism that the authors themselves put forward (J. Org. Chem., 2006, 71, 2417). I find the step that breaks the N-O bond a bit troubling (bottom right), but other than that it's not too bad. The halomethylcyclopropane-halocyclobutane rearrangement is apparently quite a well studied thing.
October 20th, 2012 - 12:22
Great mech! I love rearrangments like these, and when I saw the “bow-tie” of gem cyclopropanes, I knew de Meijere had to be involved.
One (small) criticism: I think your alkoxide has moved. In the SM, it’s in the “4″ position, and it looks to have migrated to “3″ on the indolizinone. What do you think?
October 20th, 2012 - 13:30
Ah yes, so it has! I will fix that later when I get home. Thanks for the feedback.