Stump Your Friends! A Devious Diels-Alder

You may have noticed a lot of chatter about the "turbo-charged" Diels-Alder (Chemistry World, C&EN) recently reported by Prof. Thomas Hoye and colleagues at UMN. With all the fuss, another equally exciting, albeit subtler D-A development might have been overlooked.

Introducing: the metathesis-modified Himbert reaction!

Wait...the what?

Browse on over to Org. Lett. ASAP for a crazy two-stepper you're sure to love. Here's the opening scheme, so sharpen those pencils, and try some electron-pushing before I tell you the secret:

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Give up? I tried all sorts of pi-arene cyclizations, and drew out amide tautomers until my eyes bled. But the real answer, clues author Prof. Chris Vanderwal, lies 30 years in the past. A brief (2/3 page!) 1982 communication in ACIEE related a rare bird indeed: an unactivated benzene ring participating as a diene, with no added metal catalysts, pressure, acids, or bases. Upon heating, the tethered alkyne isomerizes to an allenamide, which gladly serves as the dienophile (somewhere, Richard Hsung is smiling).

Source: Himbert, Henn ACIEE 1982, p. 620

Vanderwal's insight comes next: seeing a strained [2.2.2] diene, he figures he can pop it open with some metathesis magic. A touch of Hoveyda-Grubbs, some ethylene, and a little heat rearrange the strained system to a complex 7-6-5 tricyclic lactam. His group prepares quite a few of these scaffolds, and hopes to utilize them to access several of the Erythrina alkaloids. 

P.S. First one to use this problem for 'mechanisms' group meeting wins!

Time's Terpenoids Need Chem Context

The latest issue of Time discussed drought impact on food flavoring molecules: Health & Science section, p. 26. Seeing words like sucrose, sulfur, and capsaicin in a major magazine perked up my chem "spider sense," but I balked at the description under "bitter" vegetables like dill, fennel, or carrots. The exact passage reads:

"Chock-full of terpenoids, or aromatic compounds, that become especially concentrated during drought conditions"

OK, Time, what did you mean, exactly? Context clues are key - the author wants the word "aromatic" to describe sense impact, or how these compounds taste and smell. Of course, when I think "aromatic compounds," I imagine cyclic molecules with lots of pi electrons, stabilized through conjugation.

Can't we both be right? Of course! Certain compounds fit both criteria - they have both an aromatic ring and also smell good. Then, of course, there's terpenoids that don't fit either rule. Most play to the middle: they're odiferous, but would still make Huckel cry himself to sleep.

Either way, a little context goes a long way to helping people understand the wide array of flavor compounds in their food...without causing chemistry consternation.

Update (8/9/12) - Fixed image, deleted THC, added thymol as a more representative volatile terpenoid