A solution of N-(2-hydroxy-2-methylpropyl)morpholine (9.8 g, 62 mmol), N-benzyloxycarbonyl-L-methionine (12.6 g, 44 mmol), bromo-tris-pyrrolidino-phosphonium hexafluorophosphate (25 g, 54 mmol), DMAP (5.4 g, 44 mmole) in dichloromethane (250 ml) was stirred, in the presence of 4 Å molecular sieves (5 g), at ambient temperature for 48 h. After evaporation to dryness, the mixture was taken up in diethyl ether and the insoluble material eliminated by filtration. The organic layer was washed with a saturated aqueous solution of sodium hydrogen carbonate and a saturated solution of brine, evaporated to dryness and purified by flash column chromatography eluting with dichloromethane/ethanol (98:2) to give 2-(morpholinomethyl)prop-2-yl (2S)-2-(benzyloxycarbonylamino)-4-methylsulfanylbutyrate (40%).
