Baldwin’s Rules Classes of cyclization

Cyclization reactions can be classified by a simple system involving:

The ring size being formed

Whether the bond that breaks as the ring forms is inside (endo) or outside (exo) the new ring

Whether the electrophile is an sp (digonal), sp² (trigonal), or sp³ (tetrahedral) atom
The classes of cyclization reactions are important. This is because the class a reaction falls into determines whether or not it is likely to work, as not all cyclizations are successful even though they may look fine on paper.

^{Click the structures below to view the classes of three cyclization reactions:}

Compare 5- and 6-endo-trig cyclisation.

Compare 5-endo-trig and 5-exo-trig cyclisation.

K. Gilmore and I. V. Alabugin, Chem. Rev., 2011, 111, 6513–6556.