[Heory]

 

[azmanam]

There is certainly a lot of functionality in this system.  I'd wager the acetal is definitely the fastest process based on relative nucleophilicity of alcohols over alkenes.  From there, the 6-membered transition state in the Prins rxn[1] is fast due to proximity.  The pinacol[2] (good job on the naming, btw) occurs based on the chair geometry of the THP.  I believe only the alkyl group has sufficient overlap with the p-orbital to migrate.  While there is a lot of functionality, Overman sure tames it well, as the product is isolated in near 80% yield as a single isomer.[3]

The Prins-Pinacol is a recurring theme in the Overman lab, as they have numerous papers on the subject[4].  It is certainly a powerful too for building significant molecular complexity (3 new stereocenters and a new ring) on their way to Briarellin E.  Well done.

[1] http://www.organic-chemistry.org/namedreactions/prins-reaction.shtm
[2] http://www.organic-chemistry.org/namedreactions/pinacol-rearrangement.shtm
[3] http://dx.doi.org/10.1021/ja035445c (comm)
     http://dx.doi.org/10.1021/jo9010156 (full paper)
[4] for example:
     http://dx.doi.org/10.1021/jo0522862
     http://dx.doi.org/10.1021/jo034982c

[Heory]

finally work it out! But it's quite a hard work for me to predict the product without any hints. How great overmann's idea is!

[Heory]

BTW, I just started to learn organosilicon in my advanced organic chemistry class, and just after class this problem happened to occur to me, which made me itch for a reaction related to the silyl groups. However, the C≡C-TMS groups didn't react, so I get the concept that C≡C-TMS is not so active as C=C-TMS, which usually acts as a nucleophilic reagent.
I still have some doubt. When the alcohol OH bound to BF₃, which weakened the B-F bond and released F^- (in equilibration), why wasn't the protected alkyne or the hydroxyl deprotected? As we know, Si-F bond is one of the most strongest sigma bond. On the other hand, the resulted water would react with BF₃ thus leading to the formation of HF and B(OH)₃, which undoubtedly would not only make the two groups deprotected but also destroy the catalyst of the reaction. But it seems that water was not removed from the reaction system, why?

[Ulfsaar]

But why does the tow hydroxyl group only one can take part in the reaction?

[Heory]

azmanam, could you please answer my questions? Thanks very much.

[azmanam]

re: heory, I don't have a great answer to your question, but I would not expect the equilibrium with F- to be very favorable.  We don't usually think of F- as a leaving group, at least not in the same way we think of Cl- or Br- or I- as leaving groups. 

Also of interest, I copied the question from Evans' problem database.  In Overman's actual papers, though, the published route uses tin tetrachloride.  so maybe sometime between the posting of the question to the database and the publication they did encounter such problems.  They also preform the acetal with PPTS and MgSO4 (to soak up the water), so the actual prins/pinacol pathway maintains anhydrous conditions.  I think your comments are quite insightful, but Overman does not seem to expand on the point much in either the comm or the full paper.

re: ulfsaar, I don't know.  You can form a diffrent 6-membered TS with the other olefin to give a tertiary carbocation... maybe sterics of that TS preclude that pathway.  Interesting question I don't have a good answer for off the top of my head.