Nitrogen functions have been introduced stereoselectively at the C-2 position in D-ribonolactone derivatives in order to prepare the title unnatural amino acids. The synthetic strategies lie in the inversion of configuration in S(N)2-type substitution reactions and stereospecific hydrogenation of conveniently substituted butenolides. The threo-isomer is the key precursor in the synthesis of the antibiotic clavalanine.