Sigmatropic rearrangements suprafacial allowed photochemically

A thermal [1,3] sigmatropic rearrangement is allowed only if one component is antarafacial, but a photochemical [1,3] sigmatropic rearrangement is expected to be allowed when it proceeds suprafacially with respect to both components. The stereochemical requirement changes because under photochemical conditions, the HOMO of the three-atom component is tf/2 (symmetric), not i/q (antisymmetric). 

A similar analysis of [1,5] sigmatropic rearrangements shows that in this case the thermal reaction must be suprafacial and the photochemical process antarafacial. For the general case, with odd-numbered /, we can say that [1,/] suprafacial migrations are allowed thermally when j is of the form 4n + 1, and photochemically when j has the form An - 1 the opposite is true for antarafacial migrations. 

The different Antarafacial and suprafacial sigmatropic shifts of H-atom or other groups which are photochemically allowed have been studied under photochemical rearrangements of 1, 5-diene. 

---