Question: Draw the product formed when each triene undergoes electrocyclic reaction under [1] thermal conditions, [2] photochemical conditions

The stereochemistry of the product of an electrocyclic reaction depends on the number of pi bonds present in the system and the reaction condition (thermal/photochemical).

The Woodward-Hoffman rules for electrocyclic reactions are as follows:

$\begin{array}{ccc}\mathrm{Number}\mathrm{of}\mathrm{\pi }\mathrm{bonds}& \mathrm{Thermal}\mathrm{reaction}& \mathrm{Photochemical}\mathrm{reaction}\\ \mathrm{Even}& \mathrm{Conrotatory}& \mathrm{Disrotatory}\\ \mathrm{Odd}& \mathrm{Disrotatory}& \mathrm{Conrotatory}\end{array}$

It describes the kind of rotation the orbitals undergo under different conditions.

The triene (a) undergoes disrotation, yielding a cis product.

Electrocyclic reaction of a under thermal conditions

The triene (b) undergoes disrotation, yielding a trans product.

Electrocyclic reaction of b under thermal conditions

The triene (a) undergoes conrotation, yielding a trans product.

Electrocyclic reaction of a under photochemical conditions

The triene (b) undergoes conrotation, yielding a cis product.

Electrocyclic reaction of b under photochemical conditions