Diethyl (2,3-dibromopropyl)phosphonate → Diethyl 3-bromo-cis-1-propenylphosphonate

To a solution of diethyl 2,3-dibromopropylphosphonate (3.34 g.) in tert.-butanol (10 ml.) was added dropwise a solution of potassium tert.-butoxide (K: 430 mg., tert.--C4H9OH: 14 ml.) at ambient temperature in the course of 15 minutes. The reaction mixture was stirred at the same temperature for 30 minutes. The resultant mixture was concentrated under reduced pressure and then the residue was shaken with a mixture of ethyl acetate (50 ml.) and water (30 ml.). The ethyl acetate layer was separated, wshed with water (30 ml.), dried over magnesium sulfate and evaporated to dryness under reduced pressure to give an oily mixture of isomeric diethyl 3-bromo-propenylphosphonates (2.13 g.). An aliquot (1.86 g.) of which was fractionated by subjecting to a column chromatography on silica gel (developing solvent: chloroform) into two fractions (i.e. fraction A and fraction B). The fraction A was evaporated to dryness under reduced pressure to give oily diethyl 3-bromo-cis-1-propenylphosphonate (10 mg.). The fraction B was evaporated to dryness under reduced pressure to give oily mixture (1.65 g.) of diethyl 3-bromo-2-propenylphosphonate and diethyl 3-bromo-trans-1-propenylphosphonate (molar ratio: ca 1:1).