Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 3 / 44 / b / iv

The Woodward-Hoffmann rules for a $4n+2$ electron electrocyclic reaction permit thermal disrotation and photochemical conrotation. Here six electrons mean $n=1$. For the ground-state six-electron chain, opposite terminal rotations match the terminal phases of its highest occupied orbital as the closing sigma bond forms. A one-electron excitation changes the relevant occupancy and reverses the allowed rotational mode. Thus A opens conrotatorily under light to the specified B, while B closes disrotatorily on heating. Reversing the rotational mode changes the adjacent methyl relationship from cis to trans, exactly as in C.