To an ice chilled solution of 2-tert-butoxycarbonylamino-3-[2-(3,4-dimethoxy-phenyl)-2-oxo-ethylsulfanyl]-propionic acid methyl ester (2.00 g, 9.34 mmol, 1.0 equivalents) in dichloromethane (9 ml) was added trifluoroacetic acid (9 ml). The reaction solution was stirred on ice for 4 hours. The solution was concentrated, coevaporated with toluene. The residue was dissolved in dichloromethane (30 ml) and chilled on ice. To this was added sodium triacetoxyborohydride (2.97 g, 14.02 mmol, 1.5 equivalents) followed by dichloromethane (7 ml). The reaction was stirred 18 hours at room temperature. Saturated sodium bicarbonate was added, and the reaction mixture was extracted with dichloromethane. The dichloromethane extracts were dried with MgSO4, filtered and concentrated. 5-(3,4-Dimethoxy-phenyl)-thiomorpholine-3-carboxylic acid methyl ester was obtained as an oil (1.63 g, 59% yield). Analysis H1 NMR (400 MHz, CDCl3): 6.92 (2H, m), 6.83 (1H, d), 3.95 (3H, s), 3.93 (3H, s), 3.87 (2H, m), 3.85 (3H, s), 2.77 (3H, m), 2.42 (1H, m). MS (LC-MS): Calculated for C14H19NO4S 297.10, found 298.17.
