(2R,4S)-2-(tert-Butyl)-4-(1'-hydroxy-1'-methylethyl)oxazolidin-4-carbonsaeure-methylester

(3R,9aS)-3-tert-Butyl-5-oxo-tetrahydro-oxazolo[3,4-a]azepine-9a-carboxylic acid methyl ester

(2R,5S,6R)-2-tert-Butyl-7-oxo-6-phenyl-3-oxa-1-aza-bicyclo[3.2.0]heptane-5-carboxylic acid methyl ester

(2R,5R)-2-tert-Butyl-7-ethoxycarbonyl-6-hydroxy-8-oxo-1-aza-3-oxabicyclo[3.3.0]oct-6-ene

methyl (3R,6S,7S,7aR)-6-allyl-3-(tert-butyl)-7-hydroxy-5-oxodihydro-1H,3H-pyrrolo[1,2-c]oxazole-7a(5H)-carboxylate