There are 4 possible enolates that may form, and they are numbered above.  The most easily formed of all structures is enolate 2.  Remember that the pka of an enolate with one carbonyl present for resonance stabilization is approximately 17-22.  When there are two carbonyl groups present to help with stabilization, the pka is lowered to 9 (very acidic).  However, this enolate reacts to form a 4-membered ring only, which is very unstable (25 kcal/mol ring strain).  As a result, although this enolate forms quickly, it will reverse to the carbonyl just as quickly.  Enolates 1, 3, and 4 have similar pka values and all form at approximately the same rate.  Enolate 1 cyclizes to give a 4-membered ring, which is very unstable and quickly reverses.  Enolate 3 does cyclize to give a 6-membered ring.  We learned that 6-membered rings have no ring strain and are nature’s favorites, however, this 6-membered ring is part of a bicylic skeletal structure and some ring strain is introduced as a result. 

Enolate 4 also cyclizes to form a 6-membered ring.  This ring structure is most stable of all possibilities.  As a result this pathway will reverse much more slowly than the other pathways.  Over time, product arising from enolate 4 reactivity will build in solution and product isolated from the final solution will arise from this mechanistic pathway.  The Robinson Annelation is a good example of using reversible enolate conditions to form a desired product efficiently and in high yield.

This reaction will also work nicely under acidic conditions, and indeed acidic conditions are often favored for intramolecular cyclization reactions.  The general equation for acid catalyzed cyclization (Robinson Annelation) is shown below.  The mechanism is left to the students to conduct as an exercise.

Listed below are some more examples of intramolecular Aldol condensations:

Note that 5- and 6-memberred rings are always favored in intramolecular Aldol Condensations.