(4S)-4-[(butylsulfonyl)amino]-5-ethoxy-5-oxopentanoic acid

5-(tert-butyl) 1-ethyl (2S)-2-[(butylsulfonyl)amino]pentandioate

(4S)-4-{[(3,5-dimethyl-4-isoxazolyl)sulfonyl]amino}-5-ethoxy-5-oxopentanoic acid

5-(tert-butyl)-1-ethyl-(2S)-2-{[(3,5-dimethyl-4-isoxazolyl) sulfonyl]amino}pentanedioate

ethyl 2(S)-2-{[(benzyloxy)carbonyl]amino}-5-oxo-5-[9-(4-pyridyl)-3,9-diazaspiro [5.5]undec-3-yl]pentanoate