Pd-II-catalyzed domino heterocyclization/cross-coupling of alpha-allenols and alpha-allenic esters: Efficient preparation of functionalized buta-1,3-dienyl dihydrofurans

A mild, palladium(II)-catalyzed reaction of alpha-allenols with alpha-allenic esters in a heterocyclization/cross-coupling sequence, applicable to a wide range of substitution patterns, has been developed for the preparation of 2,3,4,trifunctionalized 2,5-dihydrofurans. Our studies indicate high levels of chemo- and regiocontrol. The possibility of using optically active substrates as well as substrates of increased steric demand, such as tertiary alpha-allenols, makes this novel sequence of heterocyclization/cross-coupling an attractive method in organic synthesis. The cur-rent mechanistic hypothesis invokes a regiocontrolled palladium(II)-mediated intramolecular oxypalladation of the free allenol component, that then undergoes a cross-coupling reaction with the allenic ester partner, followed by a trans-beta-deacyloxypalladation with concomitant regeneration of the Pd-II species.