New tandem ring enlargements of bicyclo[3.3.0]octanes to bicyclo[5.5.0]dodecanes and subsequent conversions into tricyclo[5.5.0.0.(2,9)]dodecane and tricyclo[5.5.0.0(2,10)]dodecane derivatives

Potential precursors of tetracyclo[5.5.0.0(4,10)]dodecatetraenes B were syn thesized by means of ring enlargements of bicyclo[3.3.0]octanes to bicyclo[ 5.5.0]dodecanes through enol ether/ketene additions and Michael-induced rea ctions, Base-catalyzed or oxidative ring closures of the highly functionali zed compounds that were obtained (most conspicuously 12 and 17) gave other polycyles (13, 14, 20). Unexpectedly, the reaction of bicyclooctanedione 15 with dimethyl butynedioate gave a bi(cyclohexadienyl) derivative 18 as wel l as the expected bicyclododecane derivative 17.