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Sigmatropic reactions involve the cleavage of the sigma bond in the reactant, the formation of a new sigma bond, and the rearrangement of the pi bond. The word 鈥渟igmatropic鈥� means 鈥渟igma-change.鈥�

The reaction given below is a [1,7] sigmatropic rearrangement.

[1,7] sigmatropic rearrangement

The reaction includes four pairs of electrons, and an antrafacial rearrangement takes place in it. Hence, when H migrates, as it is situated above the plane of the reactant, it ends up below the plane of the reactant molecule.

When D migrates, as it is below the plane of the reactant molecule, it ends up above the plane of the product molecule.

A and B in the product can be replaced by H and D appropriately, as shown below

Replacement of A and B with H and D