The thermal C2-C6/[2 + 2] cyclisation of enyne-allenes: reversible diradical formation

New enyne-allenes, structurally designed toward the thermal C(2)-C(6)/[2 + 2] cyclisation mode, were prepared and characterised, one of them even by X-ray crystallography. The mechanism of their transformation to formal [2 + 2] cycloadducts was interrogated by trapping experiments and DFT computations. The results support a stepwise mechanism that involves the reversible formation of the C(2)-C(6) diradical intermediate.