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SummaryCopper-catalyzed addition of organomagnesium halides to 2-(2,2-diethoxyethy1)oxirane (1) affords aldol acetals 2 which upon acid treatment undergo hydrolysis and dehydration to give a,[)-unsaturated aldehydes 7 with high yields.The problem of synthesizing a,a-unsaturated aldehydes (enals) by chain extension appears to be solved. An exhaustive literature survey would produce an almost infinite variety of methods. A closer look, however, reveals some important restrictions: in general, the building blocks used for the chain extension do not have more than three carbon atoms and in most cases these are introduced as nucleophiles.A particularly simple approach is based on ylid reactions. An a$-unsaturated aldehyde can easily be converted into its next higher homolog by condensation with (tripheny1phosphonio)methoxymethanide [2] and hydrolysis of the resulting diene-ether. (Tripheny1phosphonio)formylmethanide [3] and its acetals [4] confer two C-atoms, (triphenylphosphonio)3-methoxyallylide [5] three C-atoms together with the proper functionality. Instead of ylids, organolithium reagents may serve the same purpose. Dichloromethyllithium [6] allows submission of carbonyl compounds to a Darzens-related reaction sequence leading to enals. Synthetic equivalents of the impractible acetaldehyde a-anion such as a-deprotonated Sch$f bases [7] or (Z)-2-ethoxyvinyllithium [8] offer the possibility of performing component-controlled ('directed') aldol condensations. Despite some difficulties in achieving regiocontrol, metalated ally1 ethers [9-121 or allenyl ethers [13] may be combined with aldehydes or, respectively, alkyl halides to afford again precursors to enals.For reasons that become clear later, we envisaged a novel CC-linking pattern. We wanted to elongate organometallic reagents by an electrophilic jhur-carbon unit carrying all the required functionality to generate ultimately the enal group. The readily accessible 2-(2,2-diethoxyethyl)oxirane (1) was devised as the key reagent.