Influence of the -CH₂X substituent on the regioselectivity of intramolecular meta -photocycloaddition reactions

In studies related to the synthesis of the bicyclo[3.2.1]octane core of enterocin by an intramolecular meta-photocycloaddition, it was found that the regioselectivity of the reaction depends strongly on the substituent -CH₂X in the ortho-position to the tether. Electropositive groups X (X = H, Me, TMS, TES) gave preferentially the linear isomer (regioisomeric ratio = 87/13 to >95/5), whereas electronegative substituents (X = OH, OAc, F) showed a clear preference for the angular isomer (regioisomeric ratio = 75/25 to >95/5). The silylated and fluorinated products were obtained as single isomers in moderate yield.