Cyclopentannulations Leading to the Synthesis of Bicyclic Conjugated Enediones

Base-induced reactions of 1-(phenylsulfonyl)-2-methylene-3-bromopropane (4) with 2-(phenylsulfonyl)-2-cycloalkenones 8a-d were investigated with the ultimate purpose to develop a route leading to bicyclic conjugated enediones. Low-temperature, fast-quenched reactions led generally to open-chain adducts, while increase of temperature and addition of HMPA resulted in subsequent ring closure by a tandem Michael—S_N2 process. The stereochemical features of the bicyclo[3.3.0]octanes 11 and 12, bicyclo[4.3.0]nonanes 18 and 19, bicyclo[5.3.0]decanes 22 and 23, and bicyclo[6.3.0]-undecanes 26—28 thus obtained have been determined. Ozonolysis and silica-induced elimination of the tertiary phenylsulfonyl group converted stereoselectively the above products into the desired enediones: pentalenedione 29, indenedione 30, azulenedione 32 and cyclopentacyclooctenedione 33, respectively.