(-)-(1S,5R,8S)-ethyl 8-hydroxy-3-oxo-2-oxa-6-azabicyclo<3.3.0>octane-6-carboxylate

(+)-(2S,3S,4S)-ethyl 2-(ethoxycarbonyl)methyl-3,4-dihydropyrrolidine-1-carboxylate

(1S,5R,8S)-ethyl 8-(imidazol-1-ylthiocarbonyloxy)-3-oxo-2-oxa-6-azabicyclo<3.3.0>octane-6-carboxylate