Synthesis of some bicyclo[2.2.2]oct-5-en-2-ones and bicyclo[2.2.2]octan-2-ones. Rearrangements accompanying oxidative decarboxylation with lead tetraacetate

Abstract

1-Methoxy-2-methyl-1,4-cyclohexadiene (3), 2-methoxy-1-methyl-1,3-cyclohexadiene (2), and 2-methoxy-1,5,5-trimethyl-1,3-cyclohexadiene (14) on heating with maleic anhydride give 1-methoxy-endo-7-methylbicyclo[2.2.2]oct-5-ene-syn-2,3-dicarboxylic acid anhydride (7) and its 6-methoxy-1-methyl (16a) and 6-methoxy-1,8,8-trimethyl (16b) analogues, respectively. On hydrolysis 16a and 16b give the corresponding keto dicarboxylic acids, 18a and 18b, via keto anhydrides 17a and 17b. Treatment of 18b with lead tetraacetate gives 1,8,8-trimethylbicyclo[2.2.2]oct-5-en-2-one (19) together with products in which rearrangement to a bicyclo[3.2.1]octane system has occurred. Treatment of 17b with bis(triphenylphosphino)nickel dicarbonyl gives only 19; similar treatment of 17a gives 1-methylbicyclo[2.2.2]oct-5-en-2-one (1). Reaction of bicyclo[2.2.2]octane-2,3-dione (27) with methyllithium gives 3-hydroxy-3-methylbicyclo[2.2.2]octan-2-one (28), its dimer 31, and a diol 30. Treatment of 5-exo-acetoxy-1, 5-endo-dimethyl-6-oxobicyclo[2.2.2]octane-anti-2,3-dicarboxylic acid (37) with lead tetraacetate gives 3-endo-acetoxy-1,3-exo-dimethyl-bicyclo[2.2.2]oct-5-en-2-one (33) as a minor product; the major product is derived by rearrangement to a bicyclo[3.2.1]octane system. It is proposed that this rearrangement, like that of 18b, involves oxidative decarboxylation of a single carboxylic acid group to give a carbonium ion that undergoes rearrangement via a 1,2-acyl migration.