[Telamond]

So I'm kind of stuck on three mechanism. They're random questions from practice exams that the teachers let us practice on.

(Pic named Question 1)
This is what I came up. I figured out that it had to be some kind of rearrangement since you only added acidic to the starting material.
I also thought that perhaps it was an irreversible reaction from an aldol reaction, but that didn't really work.
So I thought of this rearrangement. It has a bensyllic carbocation as an intermediate which should be stable enough.
But I don't really see why this product would be a lot more stable than the starting material...

(Pic named Question 2)
I finished the first part of the mechanism. A to B. That was pretty easy. Hope it's correct. 
The formation C I'm having a bit problem with. (Also had a hard time drawing the structure on Symyx),
I can see that it's a Diels Alder cycloaddition reaction. But the substituents on the benzene ring are now different. Also, do benzene tend to do Diels-Alder since they would lose the aromaticity?

(Pic named Question 3)
What does CSA do? I've never heard of such a reagent, I googled it... I think it stands for Camphor sulfonic acid. What does that do?
We've used something called TsOH, and it looks similar to that. Does it do the same thing (make OH into a better leaving group?)

Thank you for any help.

[James Newby]

Qn1 Have you studied alkenes next to carbonyls? Alkenes here are electrophilic so are unlikely to react with H+.  The acid will protonate the carbonyl in my opinion

[aldoxime_amine]

IN question number 2, i find that the total number of hydrogens on both sides are unequal (index of hydrogen deficiency is not matching)

Probably you wanted to draw the structure i have attached. I have drawn it in 2 equivalent forms.

[aldoxime_amine]

I think that the mechanism number one is correct....

[azmanam]

Re: 3.  CSA is just a proton source.  Treat it as you would any catalytic acid.  Hint: this isn't how the mechanism actually goes, but first pretend like you premix CSA and dimethoxypropane.  What do you think might happen?  Once that reaction is complete, pretend you add in the triol.  there will be a subsequent reaction between the intermediate formed from the reaction of acid with DMP and the triol.

[azmanam]

Re 1: I think protonation of the carbonyl then Michael add'n to the protonated enone is a more plausible mechanism.  the alpha carbon usually isn't nucleophilic until after the Michael add'n (and you form an intermediate enol).

[James Newby]

alkenes close to carbonyls are electrophilic, a strong acid should protonate the carbonyl.  The tethered OH group can then undergo nucleophilic attack on the alkene and push electrons onto the carbonyl.  The resulting product can tautomerise to the ketone.

Page 229 in Clayden Organic Chemistry

Page 234 has a similar reaction

The starting material has both a nulceophile and an electrophile near each other so would readily form the 5 membered ring.

[Telamond]

Reaction 1:
I attached the new mechanism for reaction 1.
The same product is formed, but I do understand now that the first mechanism I posted is very unlikely. (due to it being electrophilic)

Reaction 2:
I'm uncertain if the first image is the one of the product, but the second one seems to be it. Do you have any ideas on how that one is formed?

Reaction 3:
Adding CSA and DMP I'm thinking that perhaps its an acid catalysed decomposition of an acetal? 
Product would then be a ketone? But how would that react with the starting material? :S

[azmanam]

Re2: brush up on your pericyclic reactions

Re3: circular reasoning, my friend.  And yes, step one can be thought of as acid cat decomp of an acetal.

[Telamond]

Re3: circular reasoning, my friend. 

Probably why I never draw any conclusions when I do these excercises.   

[azmanam]

no... it was a hint.  it's not a coincidence that you decomposed an acetal in the first step...  What do ketones and polyols do under acidic conditions?

[Telamond]

So this is what I've come up with:

Reaction 2:
I did a [3-3] sigmatropic rearrangement to form the ketone. But It needs to react in another pericyclic reaction to give the correct product. But I don't know which... 

Reaction 3:
Can it react further? Seems pretty anticlimactic.

[aldoxime_amine]

Read up on Claisen rearrangement for reaction number 2.

[Telamond]

Thanks for giving me the mechanism aldoxime_amine. I had done that last night, but I didn't get the same product. I'll do it again.

Now the question I have left is: Why does it happen? Usually when I've done these rearrangements where aromaticity has been broken, they've always restored it. Is the TM worth losing aromaticity? O.o