A Simple Method for Replacement of the OH Group in Free Carboxylic Acids by the Phosphonium Ylide Group.-In a one-pot reaction, the methylenetriphenylphosphorane (I) and mono-and dicarboxylic acids lead, besides formation of hexamethyldisiloxane, to acyl ylides such as (III) and (V) (nearly 20 examples). It is demonstrated that amino acids (e.g. (IIc)) also undergo this reaction. The method allows, for the first time, access to α-aminoacyl ylides which can be used as synthetic potentials in subsequent reactions. -(BESTMANN, H. J.; DOSTALEK, R.; BAUROTH, B.; Angew.