Interconversion of 5-arylbicyclo[2,2,1]hept-2-en-7-ones and 6-arylbicyclo[3,2,0]hept-3-en-2-ones by acid and by light

Abstract

Treatment of 1,2,3,4-tetrachloro-7,7-dimethoxy-5-endo-phenylbicyclo[2,2,1]hept-2-ene (VIIIa) with sulphuric acid results not only in hydrolysis of the ketal to give the 7-ketone (VIIa), as had been assumed, but also in rearrangement to 1,3,4,5-tetrachloro-6-phenylbicyclo[3,2,0]hept-3-en-2-one (Xa). Ultraviolet irradiation of this rearranged ketone gives rise to 1,2,3,4-tetrachloro-5-exo-phenylbicyclo[2,2,1]hept-2-en-7-one (XIVa), the genuine endo-isomer of which can be made by hydrolysis of the endo-dihenzyl ketal (XVIIa). At high temperatures in neutral solution both rearranged (Xa) and bridged exo-ketones (XIVa) lose carbon monoxide; however, the latter reacts much faster and the decarbonylation of the rearranged ketone (Xa) seems to involve prior rearrangement back to the bicyclo[2,2,1]hept-2-en-7-one system.

The exact relationship of these rearrangements to the Woodward–Katz rearrangement of oxo-dicyclopentadienes is not clear, but it seems likely that the acid-catalysed rearrangements involve carbonium ions (e.g. XIa) and the thermal ones diradicals.