Efficient synthesis of 4-(2′-alkenyl)-2,5-dihydrofurans and 5,6-dihydro-2H-pyrans via the Pd-catalyzed cyclizative coupling reaction of 2,3-or 3,4-allenols with allylic halides

In the absence of a base, palladium(II) catalysts, such as PdCl2, PdCl2(CH3CN)(2), Pd(OAc)(2), and [(pi-C3H5)PdCl](2), can catalyze the cyclizative coupling reaction of 2,3- or 3,4-allenols with allylic halides in DMA at room temperature to provide 2,5-dihydrofurans and 5,6-dihydro-2H-pyrans, respectively, in moderate to good yields. Under similar reaction conditions, nonsubstituted 2,3-allenol 1s affords bimolecular cyclizative coupling product 5s as the major product. The scope of the reaction and its mechanism have been studied briefly. On the basis of the experimental results, the transformation was believed to proceed via a divalent palladium-catalyzed pathway.