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

摘要

Upon treatment with BF_3 centre dot Et_2O 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-l,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.