Electrocyclic – Disrotatory ring closure/opening

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For thermal electrocyclic reactions involving six pi electrons, it is disrotatory motion (terminal groups rotating in opposite directions), which is observed. However, the fundamental reason is the same: the phase of the terminal lobes of the HOMO specify a required sense of rotation for bonding overlap.

Richard Windsor – Undergraduate Final Year Project 2008

V. A. Pichko, B. Y. Simkin and V. I. Minkin, J. Org. Chem., 1992, 57, 7087–7092.