The oxidative conversion of (E)-α-(Arylmethylene)benzeneacetates into substituted phenanthrenes: the propitious use of boron trifluoride with vanadium trifluoride oxide

Vanadium trifluoride oxide (VOF3)/boron trifluoride diethyl etherate is especially useful for effecting the cyclization of the (E)-α-(arylmethy1ene)benzeneacetates (4a,b,d) to the substituted phenanthrenes (5a,b,d). The products are obtained in 20% higher yield and under milder conditions than by employing the vanadium reagent with trifluoroacetic acid. A survey of the derivatives (4a-n) has shown that oxygen functionality is necessary at the 3- and 4- and/or the 3'- and 4'-positions of (4) for cyclization to occur.