Rearrangement of polyaryl-bicyclo[2,2,1]hept-2-en-7-ones and bicyclo-[3,2,0]hept-3-en-2-ones

Abstract

Sulphuric acid rearranges 5-aryl-1,4-dimethyl-2,3-diphenylbicyclo[2,2,1]hepten-7-ones into 6-aryl-1,3-dimethyl-4,5-diphenylbicyclo[3,2,0]hept-3-en-2-ones. Since the pure endo-(V) or exo-[2,2,1]-isomer (VI) gives a mixture of exo-(VII) and endo-[3,2,0]-isomers (VIII), the reaction cannot involve a concerted 1,3-allylic shift of the benzylic carbon atom but is thought to proceed through an open benzyl carbonium ion (X). This is confirmed by the isolation of products (XIII) of attack of the carbonium ion on the o-position of the 2-phenyl group. The 5,6-indeno-derivative (XXII), where inversion of the migrating carbon atom is prevented by the methylene bridge, yields only one isomer of the [3,2,0]-system (XXIII).

Treatment of the 5,6-cyclopenteno-derivative (XXV) with acid induces Woodward–Katz rearrangement to endo-2,4,5,6-tetraphenyltricyclo[5,2,1,0^2,6]-deca-4,8-dien-3-one (XXVI), also probably by way of the open carbonium ion. This rearrangement is reversed by heat.

Ultraviolet irradiation of the [3,2,0]-ketones, either endo- or exo-isomers, turns them back into a mixture of endo- and exo-[2,2,1]-isomers in the proportion of about 1 : 2. On the other hand the above tetraphenyltricyclodecadienone (XXVI) cyclises to the cage compound (XXVIII), like the parent system.