Isoxazolin-5-one (III) was synthesized from ethyl propiolate (I) via ethyl malonaldehyde oxime (II) by a known method. Treatment of N-Boc-L-serine (IV) with N,N-dimethylformamide di-tert-butylacetal (V) in refluxing benzene produced Boc-serine tert-butyl ester (VI) along with a small amount of O-formyl derivative, which was separated by column chromatography. Mitsunobu coupling of isoxazolinone (III) with serine derivative (VI) employing diisopropyl azodicarboxylate (DIAD) and triphenylphosphine afforded the protected O-isoxazolyl serine (VII). The protecting groups of (VII) were finally removed by treatment with trifluoroacetic acid.