Synthesis based on cyclohexadienes. Part 8. Synthesis of 1-methylbicyclo[2.2.2]oct-2-enecarboxylate derivatives

The 1,4-dihydrotoluic acids, obtained by the Birch reduction of 2-methyl- and 3-methyl-benzoic acids are isomerised to 2-methylcyclohexa-1,5-diene-1- carboxylic acid 10 and 5-methylcyclohexa-1,5-diene-1-carboxylic acid 15, respectively. These conjugated diene acids undergo facile cycloaddition with dienophiles resulting in bicyclo[2.2.2]octene derivatives having a bridgehead methyl group. While the cycloaddition of 10 with dienophiles produces regioisomeric mixture of adducts, the acid 15 affords regiospecific products.