[movies]

Not really.  I'm curious though.  Does it have to do with the placement of the carbocations?

Sort of, but really as a consequence of something else that would cause some of the carbocations to be in other places.

[Scatter]

As in a hydride shift?  We've only began talking about this stuff...   

[Scatter]

movies?  I really do want to know what's missing...

[movies]

I believe that this is a Nazarov cyclization and therefore would be a pericyclic reaction with all four electrons moving at once.

The wiki mechanism is very lacking.  I have most commonly seen it drawn like this:

[Scatter]

Thanks, movies. 

[srihari]

wat is PPA doing here???
is it used only to activate the carbonyl ??
if so can't we use any other acid ??

[movies]

In principle, yes you could use another acid.  PPA is quite a strong acid and tends to be very effective for a lot of Brønsted acid-catalyzed transformations.  It has some advantages as well since it is very cheap, relatively easy to handle, is able to absorb trace amounts of water without ill effects, and can be removed in workup easily because it is soluble in large amounts of water.

[milo2112]

PPA also provides a highly polar environment which can have a dramatic effect on reactivity. It can be a challenge to work with though, it is very viscous and the workup often involes lots and lots of hot water (to hydrolyse and dissolve the various phospho species), but it is highly effective, especially in Friedel-Crafts and Nazarov type reactions.

[eureka123]

So whose mechnaism is better ??
orgopete's or movies' ??

or are both exactly same ??

[orgopete]

Movies is better in that it is a mechanism. I was reading from the original posts that there was some reasonable expectation of what was going to happen, but errantly included an incorrect number of atoms. For me, that has always been a source of frustration as this seemed to be the simplest level of checking whether an answer is reasonable or not. When I posted, I did not offer up a mechanism per se. It was just a listing of probably intermediates that might lead to a likely product. Movies foresaw this problem as having more mechanistic value and noted this, hence even posting a mechanism.

[eureka123]

kk..
thanx