[azmanam]

Thanks to everyone who participated in last week's PotW.  Here's this week's problem:

QUESTION: Provide a complete, arrow-pushing mechanism to account for the following transformation (account for stereochemistry where appropriate).  Bonus:  Two of the intermediate steps in the mechanism are named reactions.  Name them.

[Promedol]

May be it is something like this...
First named reaction is pinacol rearrangement, second is aldol condensation.

[azmanam]

Good work.  I'm ok with the first half.  The skeletal rearrangement is indeed a pinacol rearrangement.  You have H2O leave to form a distinct carbocation... I did it in one step (i.e., electrons come down from OMe oxygen, carbon-carbon bond migrates, and H2O leaves all in one step).  No problems there, though.

The second half, alas, is not an aldol.  Give it another look and try again.  Your scheme is correct up to the first structure on the 4th line (the neutral, rearranged, keto-aldehyde).

[Promedol]

Good work.  I'm ok with the first half.  The skeletal rearrangement is indeed a pinacol rearrangement.  You have H2O leave to form a distinct carbocation... I did it in one step (i.e., electrons come down from OMe oxygen, carbon-carbon bond migrates, and H2O leaves all in one step).  No problems there, though.

The second half, alas, is not an aldol.  Give it another look and try again.  Your scheme is correct up to the first structure on the 4th line (the neutral, rearranged, keto-aldehyde).

Oh! Of course it is not aldol. It is something close to aldol but not it. Ok, I'll try again =)

[Squirmy]

Looks like an Ene to me.

[azmanam]

It's not easy being ene

[macman104]

I can't post my own drawing (no scanner) but I drew the carbonyl-ene occuring spontaneously (just under heat?).  Does the acid play a role in that part?

I also can see that the alcohol ends up back, but only because I can't envision how the ene would attack from the opposite side.  I can't actually figure out how to draw a good transition state to show why it produces that stereochemistry.

[Promedol]

That is the only probable variant which I came up with

[azmanam]

The ene will happen spontaneously under heat - no acid necessary.  Promedol, your arrows are close but not quite right. 

You might have to rotate the compound immediately preceding the ene to correctly predict the stereochem.

[azmanam]

*Ignore me, I am impatient*.

anyone come up with a good way to draw that last transition state which leads to the observed stereochemistry?

[Dan]

I had to make a model for this one. What do you think:

[azmanam]

Yup. Good work.  The published answer draws it as I have below. 

This cool step was a key step in Kim's synthesis of (±)-perhydrohistrionicotoxin.  doi: http://dx.doi.org/10.1039/a706193e