The reaction of 4-methylpent-3-en-2-one (mesityl oxide) or 4-methylpent-4-en-2-one (isomesityl oxide) with sodium chloroplatinate(II), and the reaction of dry acetone with platinum tetrachloride give the same product which has been shown by chemical and spectroscopic methods to be cis-[PtCl₂(MeC(:O)路CH₂路CMe:CH₂)]. This compound reacts with phosphines to give 4-methylpent-4-en-2-one in high purity and yield, This overall two-stage conversion of acetone constitutes a convenient synthesis.