91Ó°ÊÓ

In electrocyclic closure of the ring, if the system of the open-chain partner contains k electrons, the corresponding cyclic partner contains (k - 2) electrons and one additional sigma bond.

During this reaction, the carbon at each end of the conjugated system should rotate in a concerted manner so that the p orbitals can overlap to form the sigma bond that closes the ring.

Mechanism: The HOMO of the compound has symmetrical end orbitals; therefore, they rotate in opposite directions (disrotatory) to form a new sigma bond.

Both the hydrogens are pushed on the same side in disrotatory ring closure, making them cis-products.

Thus, triene N undergoes thermal dis-rotation ring closure, giving the cis-product.

(b)

Mechanism:The HOMO of the compound has asymmetrical end orbitals; therefore, they rotate in the same direction to form a new sigma bond.

One hydrogen atom is pushed up, and another atom is pushed down in conrotatory ring closure, making them trans-product.

Thus, triene N undergoes photochemical con-rotation ring closure, giving the trans-product.

(c)

• Reaction (a) is thermally disrotatory allowed
• Reaction (b) is photochemically conrotatory allowed