[Singingintain]

I need to come up with a mechanism for the reaction in the attached image. Thanks a bunch!

[discodermolide]

I need to come up with a mechanism for the reaction in the attached image. Thanks a bunch!

This is just a series of ring closures sparked off by coordination of the Lewis acid to the OH group. Look for Core's proposed mechanism leading to the formation of squalene

[Åke]

This is just a series of ring closures sparked off by coordination of the Lewis acid to the OH group. Look for Core's proposed mechanism leading to the formation of squalene

Are you maybe thinking of squalene oxide to dammaradienol cyclization? Note that squalene is an acyclic compound, itself biosynthesized from acyclic precursors.

As a matter of fact, Singingintain’s problem is an example of a cation-olefin (polyene) cyclization and it constitutes the key step in Johnson-Semmelhack classic route to steroids^1,2, in particular dl-16,17-dehydro-progesterone. The mechanism is really simple, Lewis acid forms a tertiary carbocation (electron sink), this event goes on to trigger a remarkable, entirely stereospecific cation-TT cyclization:



1. Johnson, W. S.; Semmelhack, M. F.; Sultanbawa, M. U. S.; Dolak, L. A. J. Am. Chem. Soc. 1968, 90, 2994–2996.
2. Johnson, W. S.; Li, T.; Harbert, C. A.; Bartlett, W. R.; Herrin, T. R.; Staskun, B.; Rich, D. H.  J. Am. Chem. Soc. 1970, 92, 4461-4463.

Also, check out Overman's article on contiguous stereogenic quaternary carbons (and references therein), as it happens it is available for free: http://www.pnas.org/content/101/33/11943.full.pdf+html