Tandem Cyclization of Allenenes to Tri- and Tetracyclic Heterocycles

Significance

Starting from amino allenenes, available from amino alcohol in five steps, a new method for the preparation of 5,6-fused hetero­cycles is presented. The reaction, proceeding in moderate to good yields, involves insertion of an arylpalladium(II) halide into the allenic moiety, followed by intramolecular carbopalladation, and aromatic C-H bond activation. Inhibition of the β-hydride elimination, either by β-methyl substitution or, as shown, by a steric effect of the α-phenyl group, was found to be essential in promoting the tandem cyclization. The scope of this reaction includes the C-H activation of heteroaromatic systems such as furan, indole and thiophene.