20 Common Reaction Types in Heterocyclic Chemistry
There is another category of cycloadditions important for heterocyclic chemistry, again involving redistribution of
six electrons, known as dipolar cycloadditions – [3 2] cycloadditions. These provide unique methods for the ring
synthesis of some fi ve-membered systems. The two reacting partners are a three-atom unit – the 1,3-dipole – and a
two-atom unit, the dipolarophile, with the latter having either a double or a triple bond. All 1,3-dipoles contain a het-
eroatom as the central atom of the trio, and in the common ones it is sp hybridised. Note: it is not possible to write a
neutral resonance form for 1,3-dipoles – azides (NKN-N-R) and nitrile oxides (R-CKN–O) are two examples.
The illustrations below show two dipolar cycloadditions, these being ring syntheses of a 4,5-dihydro-1,2,3-triazole
using an azide and that of a mixture of isoxazoles, using a nitrile oxide.