A suspension of FMOC-Ser(O-Bzl) (4.16 g), paraformaldahyde (2.0 g), and p-toluenesulfonic acid (0.2 g) in toluene (400 ml) was heated under reflux with azeotropic water removal for 45 min. The solution was cooled, diluted with ethyl acetate (250 ml) and washed three times with 5% aqueous NaHCO3, dried (Na2SO4), and concentrated in vacuo. The residue was purified by silica gel column chromatography eluting with (8:2) hexane:ethyl acetate to give FMOC-Ser(O-Bzl)-oxazolidin-4-one as a crystalline product, m.p. 108°-109° C. Fab Mass spec. m/e 430(M+H)+. FMOC-Ser-(O-Bzl)-oxazolidin-4-one (3.14 g) was dissolved in chloroform (40 ml) and trifluoroacetic acid (40 ml) and triethylsilane (2.55 g) was added. The solution was stirred at room temperature for 22 hours, then concentrated in vacuo. The residue was purified by silica gel column chromatography eluting with (95:5) methylene chloride:methanol to give FMOC-N-Me-Ser(O-Bzl)-OH as a colorless oil. Fab Mass spec., m/e 432 (M+H)+.
