Pericyclic reactions 2: sigmatropic and electrocyclic reactions

This chapter explores two other classes of pericyclic reactions: sigmatropic rearrangements and electrocyclic reactions. The original sigmatropic rearrangement occurred when an aryl allyl ether was heated without solvent and an ortho-allyl phenol resulted. This is the Claisen rearrangement. This is a one-step mechanism without ionic intermediates or any charges, just like a cycloaddition. The difference between this and a cycloaddition is that one of the arrows starts on a σ bond instead of on a π bond. In an electrocyclic reaction, a ring is always broken or formed. Rings may, of course, be formed by cycloadditions as well, but the difference with electrocyclic reactions is that just one new σ bond is formed (or broken) across the ends of a single conjugated π system.