Reaction of 4-methylthiobenzaldehyde (I) with acetone (A) affords in a 90% yield E-4-methylmercaptophenyl-3-buten-2-one (II), which is oxidized by sodium periodate to the sulfoxide (III) in a 55% yield without forming any sulfone as byproduct. The sulfoxide then reacts with 4-hydroxycumarin without solvent at 140 C to form methylsulfinylwarifarin. The compound is purified by extraction from ether with a 1% sodium hydroxide solution and then precipitated with 10% hydrochloric acid. The yield of the last step is 30% of theory.