Reaction of dimethyl 3-ketoglutarate with 1,2-dicarbonyl compounds. 8. Selective base-catalyzed decarbomethoxylation of tetramethyl 3,7-dioxo-cis-bicyclo[3.3.0]octane-2,4,6,8-tetracarboxylate. Preparation of 2,6-dicarbomethoxy-cis-bicyclo[3.3.0]octane-3,7-dione