The cycloisomerization of allene–enol ethers under Bi(OTf)3catalysis was developed as a novel “atom-economic” tool for accessing interesting functionalized cyclopentene rings. Bi(OTf)3 was shown to promote selectively the activation of the enol ether moiety of the substrate. This catalytic methodology was further extended to the synthesis of dihydrofuran and oxaspirocycle derivatives.
reaction of allene-tethered dithiosemicarbazides 4 is a convenient method for the preparation of five-membered unsaturated nitrogen heterocycles. The sulfur-directed intermolecular attack of the tin radical at the semicarbazide moiety leads to an allene-tethered iminyl radical, which then undergoes a 5-exo-dig cyclization leading to both the 3H-pyrroles 5 and the alkylidene pyrrolines 6; thermal isomerization
β-allenic O-methyl oximes undergo a free radical hydrostannylation reaction to afford cyclopentenes bearing a protected amine group and a vinyl stannyl fonction. These compounds were destannylated to yield the corresponding cyclopentenes.