Disclosed is a novel .beta.-lactam compound, a 7-substituted-2-oxa-7-azabicyclo[3.2.0]heptan-6-one represented by the general formula ##STR1## in which R is an alkyl, cycloalkyl, aryl, halogen-substituted aryl or alkaryl group, having usefulness as an intermediate for the synthesis of various biologically active compounds. The compound can be prepared by the reaction of an isocyanate compound RNCO, R being the same as above, and 2,3-dihydrofuran, preferably, under pressurization up to 2000 atmospheres or higher.