dickmann condensation/Intramolecular claisen ester condensation / cyclohexanone to cyclo pentanone conversion

when the two Easter functional group present in same molecule having one Alpha hydrogen when treated with any base for example sodium ethoxide it will give cycclization  reaction to from cyclic beta kero ester. 

This reaction is known as dickmann condensation or intramolecular claisen ester condensation.
To reduce carbon number in a cyclic compound we use this reaction.

Mechanism:

NaOEt is a strong base which captured one hydrogen from CH2 group and we get a nucleophilic centre  which further attack carbonyl group of other ester and we get beta keto ester.