The reaction of (−)-and (+)-O-ethyl ethylphosphonothioic acid (I) with phosphorus pentachloride has been shown to give optically active O-ethyl ethylphosphonochloridothionate (II) in high yield. For establishing the relative configuration and degree of stereospecificity of (II) use was made of the pyrophosphonothionates, prepared from I and II. The mechanism of the reaction is discussed.
