The reaction is this: 
The only idea I have thus far is if we have a chloride acid that can form an ester, then we might be able to find a suitable mechanism. This would require a chloride reagent.
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$\begingroup$ You'd think it all starts with protonation at the oxetane oxygen. $\endgroup$– Abel FriedmanCommented Oct 7, 2014 at 0:07
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1$\begingroup$ You might want to look up non-classical carbocations. $\endgroup$– Geoff HutchisonCommented Oct 7, 2014 at 3:24
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$\begingroup$ Oh, I'd also suggest making a 3D model of the species, either physically using a modeling kit or using a molecular visualization tool. $\endgroup$– Geoff HutchisonCommented Oct 7, 2014 at 3:25
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1$\begingroup$ And look at Wagner-Meerwein rearrangements, too! $\endgroup$– Abel FriedmanCommented Oct 7, 2014 at 3:43
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2 Answers
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I have drawn a mechanism but I haven't checked for any references as I am at home now, so it's more like paper chemistry. The chlorine atoms should play some role and possibly disfavour formation of certain carbocations that you would expect in a standard Wagner - Meerwein. It is possible to draw a concerted mechanism (not shown here) but seems very unlikely
**EDIT: I have changed my initial mechanism so it explains better the methyl ester formation (see the comments below). The stereochemistry at C* has also been corrected.
As a general comment you cannot propose a non trivial mechanism without drawing one, it is handwaving. So I did.
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$\begingroup$ Can you comment on your thought process? Much appreciated. $\endgroup$– AyeshaCommented Oct 8, 2014 at 12:07
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$\begingroup$ Is there a specific step or steps that you are asking about? In general I tried to use the most stable (looking to me) intermediates and ... get there. Remember also that this is only a suggestion not based on any literature. I did have a quick look today but couldn't find anything relevant, if you or anyone else have any references please post $\endgroup$– K_PCommented Oct 8, 2014 at 21:41
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$\begingroup$ @ K_P: I don't think step 3 is correct; you show methanol attacking the oxocarbenium ion, but the reaction takes place in water/sulfuric acid $\endgroup$ Commented Oct 9, 2014 at 0:48
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$\begingroup$ I don't love this either but you have to end up with the methyl ester somehow, and the most reasonable source is MeOH produced by the initial hydrolysis of the dimethyl ketal. You could probably make it more "believable" by a mechanism that involves an intramolecular attack by the methoxy. I'll have a look later as I'm on my mobile now. $\endgroup$– K_PCommented Oct 9, 2014 at 7:50
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$\begingroup$ Considering your second proposed mechanism, I find the 3-membered ring closure unlikely. Essentially it's a cationic enol ether, which shouldn't be nucleophilic, attacking a carbocation. Put another way, why doesn't water just attack the carbocation directly instead of through an alkene? $\endgroup$ Commented Oct 10, 2014 at 20:57
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I haven't looked at the literature either, but it seems to go like this:
- protonation at the oxetane
- ring-opening of the oxetane. This gives a tertiary carbenium ion and the keto group (elimination of HCl from the $\alpha$-chloroalcohol).
- skeletal rearrangement to yield the dioxocarbenium ion; the resonance stabilization is what provides the driving force
- nucleophilic attack of solvent, to lose a methyl group and form the methyl ester
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1$\begingroup$ It would help if you post a diagram. A picture is worth a thousand words. $\endgroup$– ronCommented Oct 9, 2014 at 0:56
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$\begingroup$ @ron: I know. It's difficult to draw the polycyclic system in an aesthetically pleasing way. $\endgroup$ Commented Oct 9, 2014 at 2:11
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2$\begingroup$ @AbelFriedman please post a picture of your mechanism. I think starting at the oxetane makes sense, but I can't see out how a rearrangement (unless it's more than one) that gives a dioxocarbenium ion also gives the correct carbon skeleton. $\endgroup$ Commented Oct 9, 2014 at 17:34