USE OF ARENE 1,3-SUBSTITUENTS TO CONTROL CYCLOPROPANE RING FORMATION DURING META PHOTOCYCLOADDITION OF ETHENES TO THE BENZENE-RING

The 3-trifluoromethyl derivatives of benzyl alcohol, benzyltrimethylsi lane and phenoxytrimethylsilane undergo 2,6-photocycloaddition to cycl opentene to give the tuted-tetracyclo[6.3.0.0(2,11).0(3,7)]undec-9-ene s 7, 10, 11 and 12. This specificity in isomer formation is considered to originate from an intramolecular attractive interaction between th e 1,3-substituents on the arenes during the addition of the ethene and this causes an asymmetric distortion of the C-6 ring which then contr ols the cyclopropane ring formation in the adduct. Similar reaction of 3-trifluoromethyl-phenol, -benzyl alcohol and -benzyltrimethylsilane, with vinyl acetate yields the hyl-endo-6-acetoxytricyclo[3.3.0.0(2,8) ]oct-3-enes 16, 17 and 18 respectively, showing that the directing inf luence of substituents on the arene dominates any effect of the acetox y group on the photocycloaddition process. The control by Si ... F int eraction is not, however, significant during the intramolecular photoc ycloaddition of the bichromophores 19 and 20 and only the 1,6-bridged dihydrosemibullvalene 24 and products derived from ortho cycloaddition are respectively formed.