Ring size effects in the C-2-C-6 biradical cyclisation of enyne-allenes and the relevance for neocarzinostatin

The regioselectivity of the thermal cyclisations of enyne-allenes 1 can be toggled as a function of the ring size of the cycloalkene. With a cyclopent ene as the ene moiety the Myers-Saito (C-2-C-7) cycloaromatisation product is formed, whereas with six- and seven-membered cycloalkenes the novel C-2- C-6 cyclisation is observed. DFT calculations are used to rationalise these changes. The implications of these findings for alternative thermal biradi cal cyclisations of neocarzinostatin are discussed.