PREPARATION OF BRIDGEHEAD SUBSTITUTED BICYCLO[3.3.0]OCTANE DERIVATIVES BY THE BF3-CENTER-DOT-OET(2)-CATALYZED REARRANGEMENT OF BICYCLO[3.2.1]OCT-6-EN-8-OLS

6-t-Butyldimethylsilyl-7-methylthiobicyclo[3.2.1]oct-6-en-8-ol derivatives prepared from the [2+2] cycloadducts of 1-t-butyldimethylsilyl-2-methylthioacetylene and 2-cyclohexenones are rearranged by the treatment with BF3.OEt(2) into two types of bicycle[3.3.0]octane compounds, bicyclo[3.3.0]oct-3-en-2-one or 2-methylenebicyclo[3.3.0]oct-3-ene derivatives selectively, depending on the substituent at C-1 position.