Aldol condensation intramolecular, regioselectivity

Scheme 2.10 illustrates intramolecular aldol condensations. Entries 1 and 2 are cases of formation of five-membered rings, with aldehyde groups serving as the electrophilic center. The regioselectivity in Entry 1 is due to the potential for dehydration of only one of the cyclic aldol adducts. 

Regioselective intramolecular aldolic condensation of 36 with piperidinium acetate gave only the compound 37 with the desired 2-hydroxy-3-oxohicyclo[4.3.0]non-4-ene-type skeleton. 

In Summary Intramolecular aldol condensation succeeds with both aldehydes and ketones. It can be highly regioselective and gives the least strained cycloalkenones. 

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