The Darzens condensation is one of the older methods (1904) for the synthesis of three-membered heterocycles. It is most commonly the reaction of a 2-halo ester with a carbonyl derivative in the presence of base. The following examples of the Darzens condensation show how it is applied to the synthesis of oxacyclopropane and azacyclopropane rings. Suggest a reasonable mechanism for each of these reactions. 

(a)

(b)

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Cl KOC(CH),, (CH),CОН C,H3CHO + C,H;CHCOOCH,CH3 → H5C6- -C,H5 H COOCH,CH3