An interesting aspect of the application of the Woodward-Hoffmann rules to electrocyclic reactions is that many involved ring-systems, including CHD, are not planar. CHD, for example, has C2 symmetry with one methylene carbon above and one below the plane of the conjugated bond system (see e.g., the CHD structure in Fig. ). Thus, the mirror plane, which is assumed to be preserved during the thermally allowed ring-opening of CHD (see Fig. ), does not actually exist at the starting point of the reaction. Due to the lack of C2v symmetry, there is a clear steric preference for one of the two possible conrotatory motions in the photochemical pathway, the one which requires less rotation to make the methylene groups coplanar with the conjugated π system of the molecule (see also the ranges of f1 and f2 that enable the conrotatory motion in Fig. ). To observe this preference in the photoproducts, the high symmetry of CHD must be broken by unsymmetrical substitution of the methylene groups (see Fig. , bottom). For example, one of the methylene units of α-phellandrene (αPh) has a bulky isopropyl substituent, which can be either in pseudo-equatorial or pseudo-axial orientation, as illustrated in Figure . The resulting two conformers each yield photochemical ring-opening reaction products which are due to conrotatory motion in the pathway requiring less rotation (108; 109). Such steric control of conrotatory motion has also been observed for thermally allowed electrocyclic ring closure reactions of substituted dienes to cyclobutenes (110).