(+)-(4R,5R)-4-(tert-butoxycarbonyl)amino-5-[2-(methanesulfonyl)oxy]ethyl-dihydrofuran-2-one

(1R,5R)-6-tert-butoxycarbonyl-7-methoxy-2-oxa-6-azabicyclo[3.3.0]octan-3-one

(-)-(1R,5R)-6-tert-butoxycarbonyl-2,7-dioxa-6-azabicyclo[4.3.0]nonan-3-one

(+)-(4R,5R)-4-(tert-butoxycarbonyl)amino-5-(2-hydroxy)ethyl-dihydrofuran-2-one

((2R,3R)-2-Allyl-5-oxo-tetrahydro-furan-3-yl)-carbamic acid tert-butyl ester