NOVELTY - Preparing pseudo dipeptide (R,R)-3-(3-tert-butoxy-2-(9H-fluoren-9-yl-methoxycarbonylamino)-propionyl)-2,2-dimethyl-oxazolidine-4-carboxylic acid (VIII), comprises reacting thionyl chloride with D-serine (I) and pulping, reacting obtained compound with isobutene (III), saponifying obtained compound in 10% sodium carbonate solution and further reacting with fluorenylmethyloxycarbonyl chloride, coupling obtained compound with D-serine to obtain N-fluorenylmethyloxycarbonyl-(O-tert-butyl-D-seryl)-D-serine (VI) and reacting (VI) with 2,2-dimethoxypropane (VII) under reduced pressure.    USE - The method is useful for preparing (R,R)-3-(3-tert-butoxy-2-(9H-fluoren-9-yl-methoxycarbonylamino)-propionyl)-2,2- dimethyl-oxazolidine-4-carboxylic acid (claimed).    ADVANTAGE - The method solves multi-step synthesis, long cycle, utilization of inflammable, explosive and more toxic raw materials, severe three-waste pollution and high cost technical problems in existing technology.    DETAILED DESCRIPTION - Preparing pseudo dipeptide (R,R)-3-(3-tert-butoxy-2-(9H-fluoren-9-yl-methoxycarbonylamino)-propionyl)-2,2-dimethyl-oxazolidine-4-carboxylic acid of formula (Fmoc-D-Ser(tert-butyl)-D-Ser-OH) (VIII), comprises (a) reacting thionyl chloride with D-serine of formula (I) in an anhydrous methanol solvent for 15-36 hours, evaporating the solvent and pulping by using diethyl ether or methyl tert-butyl ether to obtain D-serine methyl ester hydrochloride of formula (D-Ser-OMe.HCl) (II) as a white solid, (b) reacting (II) with isobutene of formula (III) in presence of p-toluenesulfonic acid catalyst in 1,4-dioxane or dichloromethane solvent for 48-96 hours to obtain (R)-4-(3-tert-butoxy-1-methoxy-1-oxopropan-2-ylamino)benzenesulfonic acid of formula (D-Ser(tert-butyl)-OMe.TsOH) (IV) as a white solid, (c) saponifying (IV) in 10% sodium carbonate solution for 20-25 hours and further reacting with fluorenylmethyloxycarbonyl chloride or N-(9-fluorenylmethoxycarbonyloxy)succinimide for 20-25 hours in 1,4-dioxane solvent to obtain (N-fluorenylmethyloxycarbonyl-O-tert-butyl)-D-serine of formula (Fmoc-D-Ser(tert-butyl)-OH) (V), (d) coupling (V) with D-serine in presence of N-hydroxy succinimide and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride in tetrahydrofuran to obtain N-fluorenylmethyloxycarbonyl-(O-tert-butyl-D-seryl)-D-serine of formula (Fmoc-D-Ser(tert-butyl)-D-Ser-OH) (VI) and (e) reacting (VI) with 2,2-dimethoxypropane of formula (VII) under reduced pressure to obtain (R,R)-3-(3-tert-butoxy-2-(9H-fluoren-9-yl-methoxycarbonylamino)-propionyl)-2,2-dimethyl-oxazolidine-4-carboxylic acid of formula (Fmoc-D-Ser(tert-butyl)-D-Ser-( psi Me, Me, pro)OH) (VIII) as white powder.