Trihaloacetaldehyde N,O-acetals: useful building blocks for dihalomethylene compounds

The reaction of trifluoroacetaldehyde N,O-acetals with more than 2 equiv of alkyllithiums at −78 °C resulted in regiospecific defluorinative alkylation with unusual regioselectivity to give α,α-difluoroketone N,O-acetals in excellent yield. In contrast, under similar conditions, trichloroacetaldehyde N,O-acetals gave simple mono-dechlorinated product without the alkyl transfer reaction from alkyllithiums to the generated intermediates.