Preparation and aluminum trichloride-induced cationic rearrangements of bicyclo[2.2.0]hexene carboxylic esters

Reaction between the tetramethyl4]annulene aluminum trichloride σ-complex and α, β-alkenic esters gave the corresponding 1,4,5,6-tetramethylbicyclo2.2.0]hex-5-one-2-endo-carboxylic esters. AlCl₃-induced cationic rearrangements of these bicyclic esters yielded a number of bicyclo3.1.0]hexene carboxylic esters and related lactones which were isolated and identified. The isomerization seems to proceed via stereospecific endo protonation and subsequent rearrangement to 2-methoxycarbonyl-1,4,5,6-exo-tetramethylbicyclo(3.1.0]hex-4-yl carbocations.