The reaction product of pyrrolidin-2-one with hydrogen chloride and phosphorous pentachloride has been shown to be 2-(2,3-dichloro-Δ2-pyrrolin-1-yl)-Δ2-pyrroline, rather than 2-chloro-Δ1-pyrroline as previously reported, by a single crystal X-ray structure determination of the hydrobromide salt of the product.
