Pyrrole is heated with chloroform and sodium ethoxide (Ciamician-Dennstedt rearrangement). The main product is

Dichlorocarbene adds across the C2=C3 bond of pyrrole to give a dichlorocyclopropane fused to the ring. This opens with loss of $\mathrm{Cl}{}^{-}$ and ring expansion by one atom, giving the six-membered pyridine ring. The remaining chlorine ends up on the new ring carbon, which lies at position 3 of the pyridine.