of α-keto- or α- formyl acetals by Witting reaction with cyclopropylidenetriphenyl-phosphorane leads to α cyclopropylidene-acetals, easily deacetalized by moist silicagel into the corresponding α-cyclopropylidene-ketone and α-cyclopropylidene-aldehydes. The conversion of the latter to the former is also easily carried out by reaction with Grignardreagents followed by oxidation with active manganese