Many reactions in organic chemistry can occur in either an intramolecular or intermolecular senses.  Some reactions are by definition intramolecular or are only practiced intramolecularly, e.g., 
* Dieckmann condensation of diesters is the intramolecular version of aldol condensation.
* Madelung synthesis of indoles
* Smiles rearrangement
* Hydroacylation is almost invariably practiced intramolecularly to produce ketones.
:RCHO  +  CH=CHR   →   RC(O)CHCHR
* Nazarov cyclization reaction for the synthesis of cyclopentenones
*The Wurtz reaction, involving reductive coupling of alkyl halides, essentially is only useful when conducted intramolecularly.  Its utility is illustrated with the synthesis of strained rings:  
Some transformations that are enabled or enhanced intramolecularly.  For example, the acyloin condensation of diesters almost uniquely produces 10-membered carbocycles, which are difficult to construct otherwise.  Another example is the 2+2 cycloaddition of norbornadiene to give quadricyclane.