Sigmatropic reaction Overview of Sigmatropic Shifts Woodward-Hoffman Sigmatropic Shift Nomenclature
A special notation is used to describe sigmatropic shifts. Each of the carbon atoms on the backbone of the molecule are assigned a position number. When the sum of i and j is an even number, this is an indication of the involvement of a neutral, all C atom chain. An odd number suggests that there is a charged C atom or of a heteroatom lone pair replacing a carbon-carbon double bond. Hydrogens are omitted in the third example for clarity.