To a solution of 2,4-di-(benzyloxycarbonyl)butylphosphinic acid (6) (19.3 g, 49.4 mmol) in tetrahydrofuran was added benzyl alcohol (5.3 g, 49.3 mmol) and dimethylamino in tetrahydrofuran was added benzyl alcohol (5.3 g, 49.3 mmol) and dimethylamino pyridine (0.5 g). Dicylcohexylcarbodiimide (DCC, 12 g, 58 mmol) was added and a white precipitate formed. After 30 minutes the white suspension was filtered and the filtrate evaporated under reduced pressure. The clear and colorless oil was purified by flash chromatography and eluted with 1:1 Hex./EtOAc to give 2,4-di(benzyloxycarbonyl)butylbenzylphosphinic acid (7) (11.5 g, 47%) as a clear and colorless oil. Rf. 0.16 (1:1 Hex./EtOAc); 1H NMR (CD1C3): 7.3 ppm (m, 15H), 7.2 ppm (d, 1H), 5.0 ppm (m, 6H), 2.9 ppm (m, 1H), 2.2 ppm (m, 3H), 1.9 ppm (m, 3H).
