methyl (2S)-2-{(tert-butoxy)-N-[(tert-butyl)oxycarbonyl]carbonylamino}-4-oxobutanoate

dimethyl (2S)-2-{bis[(tert-butyloxy)carbonyl]amino}butane-1,4-dioate

(S)-(-)-methyl 2-[bis(tert-butoxycarbonyl)amino]-4-iodobutanoate