Homologation, aldehyde Darzens reaction

When the epoxide is made from a carbonyl compound, e.g. (29) (Chapter 30), the result is chain extension to the homologous aldehyde. This procedure can be carried out with sulphur ylids (30) or by the Darzens reaction with decarboxylation of the intermediate acid (31). 

Darzens glycidic ester condensation generally involves the condensation of an aldehyde or ketone 2 with the enolate of an a-halo ester 1 which leads to an a,P-epoxy ester (a glycidic ester) (3). Thus the reaction adds two carbons to the electrophile however, the reaction has been primarily developed as a one-carbon homologation method. That is, subsequent to the condensation, the ester is saponified and decarboxylation ensues to give the corresponding aldehyde or ketone 5.2... 

In the past, Darzens methodology was primarily used for the synthesis of aldehydes and ketones, as a homologation reaction without any consideration of stereocontrol in the epoxide formation. For this sequence, saponification of the a,P-epoxy ester followed by decarboxylation gives the substituted carbonyl compound ... 

---