(3R,4S)-1-tert-butoxycarbonyl-4-hydroxy-3,8,8-trimethyl-2-oxo-1-aza-7,9-dioxaspiro[4,5]decane

(S)-ethyl 2-isobutyryl-5-oxopyrrolidine-2-carboxylate

ethyl 2-((1S)-1-hydroxy-2-methylpropyl)-5-oxo-D-prolinate

ethyl (1S)-1-[(tert-butoxycarbonyl)amino]-2-isopropylcyclopent-2-ene-1-carboxylate

1-tert-butyl 2-ethyl (2S)-2-[(1S)-1-(acetyloxy)-2-methylpropyl]-5-oxo-2,5-dihydro-1H-pyrrole-1,2-dicarboxylate