Benzylphosphinic acid (2.3 g, 15 mmol) in 150 mL of dry dichloromethane was cooled to 0° C. under a nitrogen atmosphere. Triethylamine (6.5 g, 65 mmol) was added followed by trimethylsilyl chloride (5.8 g, 54 mmol) while the reaction temperature was maintained at 0° C. After 30 minutes dibenzyl 2-methylenepentanediote (2) in 20 mL of dichloromethane was added over 5 minutes. The reaction mixture was left to warm to room temperature and stirred overnight. The clear solution was cooled to 0° C. and quenched with 5% hydrochloric acid and with brine, dried (MgSO4) and evaporated to give a clear yellow liquid. Purification by flash chromatography and elution with 1:1 hexane/ethyl acetate containing 10% acetic acid yielded 2.0 g (28%) of benzyl[2,4-di(benzyloxycarbonyl)butyl]phosphinic acid (3, R=CH2Ph, R1+H) as a clear light yellow oil. Rf 0.37 (1:1 Hex./EtOAc, 10% AcOH).
