Sames and coworkers have reported a Lewis-acid catalyzed [1,5] H-shift followed by C-C bond formation via reactive alkenyl oxocarbenium intermediates. Sames and coworkers have previously reported that expensive transition metal Lewis acids promoted cylizations of alpha,beta-unsaturated aldehydes and ketones, but were hampered with low yields and slow reaction times. The preparation of the corresponding acetals and ketals led to an improvement in the yield, reactivity and diastereoselectivity.  Thus, using the standard conditions of BF₃·Et₂O in CH₂Cl₂ at room temperature, a variety of tetrahydropyrans can be prepared, including those from more hindered and complex precursors. Finally, it was demonstrated that the acetals and ketals could be formed catalytically in situ when OHCH₂CH₂OH is added to the reaction mixture and that this method effects an increase in the rate (and in some cases, an increase in yield and diastereoselectivity) of the reaction, while also obviating the need for preparation of these precursors, and resulting in a binary catalytic system composed of BF₃·Et₂O as the catalyst and OHCH₂CH₂OH as the organocatalyst.

McQuaid, K. M.; Sames, D. J. Am. Chem. Soc. ASAP.