Tributyltin hydride-mediated free-radical cyclization of allene-tethered oxime ethers and hydrazones

In this work, we have shown that the tributyltin hydride-mediated cycloisomerization of allene-tethered oxime ethers or hydrazones is a convenient method for the preparation of (vinylstannyl)cyclopentylamine derivatives in terms of simplicity and chemical yields. As a result, the first and detailed analysis of the tributyltin hydride-mediated free-radical cyclization of alkyl-substituted allene-tethered oxime ethers and hydrazones is reported. The site-directed intermolecular attack of the tributyltin radical at the allene moiety and the final size of the ring after cyclization depends on the type of substitution in the substrate. Some general trends can be observed: (1) In crowded substrates having full substitution at C beta or at the terminal-trigonal carbon, the steric hindrance favors attack at the digonal carbon. (2) When different positions in the allene are free for attack, the kinetically more favored irreversible mode of cyclizations (5-exo > 6-exo > 6-endo) determines the ratio of isomers or the final size of the ring. Finally, after acid hydrolysis of the vinyltin products,the resulting O-methyl- or O-benzyl(hydroxylamino)cycloalkanes have been obtained in good yield.