azetidin-2-one 2-azetidinone 930-21-2 328464_ALDRICH 11537_FLUKA 2-Azacyclobutanone Azetidinone Propiolactam 2-Azetdinone InChI=1/C3H5NO/c5-3-1-2-4-3/h1-2H2,(H,4,5 beta-Propiolactam ZINC04695140