The regioselectivity of the thermal cyclisations of enyneâallenes 1 can be toggled as a function of the ring size of the cycloalkene. With a cyclopentene as the ene moiety the MyersâSaito (C2âC7) cycloaromatisation product is formed, whereas with six- and seven-membered cycloalkenes the novel C2âC6 cyclisation is observed. DFT calculations are used to rationalise these changes. The implications of these findings for alternative thermal biradical cyclisations of neocarzinostatin are discussed.