A one-pot sequence of Stille and Heck couplings: Synthesis of various 1,3,5-hexatrienes and their subsequent 6 pi-electrocyclizations

Palladium-catalyzed cross-coupling reactions of 2-bromocyclohexl-enyl trifl ates 7 and 11 with a variety of alkenylstannanes occurred chemoselectively at the site of the triflate leaving group to give bromobutadienes which rea dily underwent Heck reactions with acrylates and styrene. Both steps could be performed in the same flask to give differentially functionalized hexatr ienes in up to 88% overall yield. With simple stannanes, the same catalyst precursor could be used for both coupling steps making it possible to perfo rm the whole sequence with only one portion of catalyst. For some of the fu nctionally substituted stannanes, specifically adjusted catalyst systems ha d to be used. The 1,3,5-hexatrienes obtained were further transformed, in p articular the methoxy-substituted compounds 14a-c were converted to bicyclo [4.4.0]decenones 30 (71-97%), bicyclo[4.3.0]nonenones 35 (74-93%), cyclodec ynone 37a (47%), and cyclononynone 39a (15%). Thermal electrocyclizations o f the other hexatrienes gave tetrahydronaphthalines 31 (60-61%), the tricyc lic lactone 32 (72-75%) and decahydrophenanthrene 33 (75%) in good yields.