Boron Trifluoride-Induced, New Stereospecific Rearrangements of Chiral Epoxy Ethers. Ready Access to Enantiopure 4-(Diarylmethyl)-1,3-dioxolanes and 4,5-Disubstituted Tetrahydrobenzo[c]oxepin-4-ols

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.