STEREOCONTROLLED LACTONIZATION REACTIONS VIA PALLADIUM-CATALYZED 1,4-ADDITION TO CONJUGATED DIENES

Stereocontrolled palladium(II)-catalyzed 1,4-chloro- and 1,4-acetoxyla ctonizations of conjugated cyclic dienes have been developed to give s tereodefined fused lactones. The stereochemistry of the 1,4-acetoxylac tonization was controlled by the ligand on the metal catalyst, and in this way either a cis-or trans-acetoxylactonization was obtained. This dual stereoselectivity is explained by a stereo-controlled acetate at tack (trans or cis, respectively) on the allyl group in the catalytic (pi-allyl)-palladium intermediate. To further strengthen the mechanism the intermediate (pi-allyl)palladium complex was isolated and fully c haracterized. A stereospecific synthesis of cis- and trans-2-[6-(benzy loxy)-2,4-heptadien-1-yl]acetic acid (cis- and trans-9) followed by st ereoselective Pd(II]-catalyzed chloro- and acetoxylactonization in ace tone/acetic acid resulted in highly functionalized fused lactones with control of the relative stereochemistry at four different carbons.