Tributyltin hydride-mediated free radical cyclization of allene-tethered oxime ethers, oxime esters and hydrazones. Influence of the substituents borne by the C=N double bond on the course of the reaction

A set of allenic derivatives 1 bearing a C=N bond in β position and diversely substituted in R³ and Z was prepared. The behaviour of compounds 1 towards the Bu₃Sn^• radical was then studied. Depending on the nature of R³ and Z, products 2, 3, and 4 were obtained in diverse ratios. When the steric hindrance on the C-atom of the C=N bond was low (R³ = H), the product ‘normally’ obtained was 2. When the steric hindrance of R³ was higher, the formation of 3 and 4 became competitive. In this case, the ratio between 2, 3, and 4 can be explained on the basis of the polarity of R³ and Z. The higher the electroattractive effect of R³ and Z, the higher the quantity of 2 and 4 in comparison with 3.