Scheme 4.

(A) Lewis acid-catalyzed, [4+3] cycloaddition, and (B) a stereochemical model for the observed diastereoselectivity. Black hashes indicate cation–π interactions, while the red line represents a steric repulsion. Black and green dotted lines show the forming C–C bonds, with bond distances indicated in Å. The differential enthalpies of activation are shown in kcal/mol (B3LYP/6-31G(d)). Adapted with permission from ref. 28a. Copyright 2010 American Chemical Society. TMS = trimethylsilyl, TES = triethylsilyl, OTf = trifluoromethanesulfonate