REGIOSELECTIVITY IN INTRAMOLECULAR CYCLOADDITION OF DOUBLE-BONDS TO TRIPLET ACYLBENZENES .2. EFFECTS OF SUBSTITUENTS META TO THE TETHER

The photoinduced intramolecular [2 + 2] cycloaddition of several 2-sub stituted 4-(3-methyl-3-buten-1-oxy)acetophenones was studied. With 2-m ethyl, methoxy, and trifluoromethyl substitution, the reaction is 100% regioselective syn, addition of the remote double bond occurring only at the 3,4-positions of the benzene ring, not the 4,5. The 2-fluoro d erivative alone provides both anti and syn regioisomers in a 15:85 rat io. The 2-cyano derivative did not react. The high regioselectivity ap pears to reflect a strong inductive effect on the first, biradical-for ming step of the reaction, possibly influenced by the dipole moment of the 1-carbonyl group.