Question

What product would you expect to obtain from the photochemical cyclizationof (2E,4Z,6E)-2,4,6-octatriene? Of (2E,4Z,6Z)-2,4,6-octatriene?

Short answer

The product will be trans – 5,6-Dimethyl-1,3- cyclohexadiene by photochemical cyclization of (2E,4Z,6E)-2,4,6-octatriene.

Step-by-step solution

In general cases.

This is a general idea picture of the whole conversion process during every case, that we consider both disrotatory and conrotatory.

The product obtained from the photochemical cyclization.

In case of photochemical cyclization, there is Huckel aromatic transition state. However photochemical flips symmetry of HOMO and their role inverses so dis rotatory will give you antiaromatic transition state.

So, the conrotatory will gives a Huckel aromatic transition state, this forms a stable product which is trans-5,6-Dimethyl-1,3-cyclohexadiene, this product can also upon heating form a 2E,4Z,6Z product.