(3R)-3-(bromoacetoxy)-1-(2-pivaloyloxyethyl)pyrrolidine-2,5-dione

(1S,5S)-2-oxa-6-aza-6-(2-(trimethylacetoxy)ethyl)bicyclo<3.3.0>octane-3,7-dione

(1R)-6-(2-pivaloyloxyethyl)-2-oxa-6-azabicyclo<3.3.0>oct-4-ene-3,7-dione

(4R,5R)-4-methyl-5-<(3,5-dinitrobenzoyloxy)methyl>-5-hexanolide