CYCLOBUTENONE-BASED SYNTHESES OF POLYQUINANES AND BICYCLO[6.3.0]UNDECANES BY TANDEM ANIONIC OXY-COPE REACTIONS - TOTAL SYNTHESIS OF (+ -)-PRECAPNELLADIENE/

The addition of ethenyllithium derivatives to the carbonyl of dialkyl squarate-derived bicycloheptenones, e.g., 1a and 6a, initiates a low-t emperature anion-accelerated oxy-Cope rearrangement to provide polyqui nanes by a transannular aldol reaction of the intermediate bicyclo[6.3 .0]undecadienone 4. Additional functionality is introduced by alkylati on of the enolate 3 resulting from the oxy-Cope rearrangement. Phospho rylation or triflation of enolate 3 provides an entry into the bicyclo [6.3.0]undecane ring system. An application of this new methodology is demonstrated by the total synthesis of the sesquiterpene natural prod uct (+/-)-precapnelladiene from diisopropyl squarate (10 steps, 12%).