Upon treatment with BF3·Et2O at low temperature, enantiopure benzyl-type ethers of arylglycidols with electron withdrawing substituents at the skeletal aryl group and electron donating substituents at the benzyl group undergo stereospecific rearrangements of Friedel-Crafts type, leading to enantiopure 4-diarylmethyl-1,3-dioxolanes (2) or to enantiopure trans-4,5 disubstituted tetrahydrobenzo[c]oxepin-4-ols (5). The course of the reactions is controlled by the substitution pattern at the benzyl ether: While benzylic systems activated toward ipso substitution afford diarylmethanes 2 through a Friedel-Crafts reaction followed by fragmentation, benzylic systems activated toward ortho attack lead to enantiopure oxepinols 5 through a 7-endo-tet ring closure of Friedel-Crafts type.
![Boron trifluoride-induced, new stereospecific rearrangements of chiral epoxyethers. A ready access to enantiopure 4-diarylmethyl-1,3-dioxolanes and 4,5-disubstituted tetrahydrobenzo[c]oxepin-4-ols](https://www.iciq.org/wp-content/uploads/2014/03/Boron-trifluoride-induced_eng_noticies_articles_2006_67.gif)