Tandem ring enlargement by two carbon units from the bicyclo[3.3.0]octane to the bicyclo[5.5.0]dodecane system

Reaction of the bis(enamines) 2b and 3b with acetylenedicarboxylic ester ga ve stable tetracyclic compounds 4b and 5b. The related compound 3a, however , led to bicyclo[5.5.0]dodecatetraenes 6a, 6b, and 7a/7b directly. 7a/7b ar e interrelated by a rapid [1,5]-H shift, whereas 6a and 6b do not rearrange . Hydrolysis of 6b yielded the novel cage compound 10 by dual intramolecula r Michael addition.